meselson and stahl experiment ib biology

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Biology archive

Course: biology archive   >   unit 17.

• DNA replication and RNA transcription and translation
• Leading and lagging strands in DNA replication
• Speed and precision of DNA replication
• Molecular structure of DNA
• Molecular mechanism of DNA replication

Mode of DNA replication: Meselson-Stahl experiment

• DNA proofreading and repair
• Telomeres and telomerase
• DNA replication

Key points:

• There were three models for how organisms might replicate their DNA: semi-conservative, conservative, and dispersive.
• The semi-conservative model, in which each strand of DNA serves as a template to make a new, complementary strand, seemed most likely based on DNA's structure.
• The models were tested by Meselson and Stahl, who labeled the DNA of bacteria across generations using isotopes of nitrogen.
• From the patterns of DNA labeling they saw, Meselson and Stahl confirmed that DNA is replicated semi-conservatively.

Mode of DNA replication

The three models for dna replication.

• Semi-conservative replication. In this model, the two strands of DNA unwind from each other, and each acts as a template for synthesis of a new, complementary strand. This results in two DNA molecules with one original strand and one new strand.
• Conservative replication. In this model, DNA replication results in one molecule that consists of both original DNA strands (identical to the original DNA molecule) and another molecule that consists of two new strands (with exactly the same sequences as the original molecule).
• Dispersive replication. In the dispersive model, DNA replication results in two DNA molecules that are mixtures, or “hybrids,” of parental and daughter DNA. In this model, each individual strand is a patchwork of original and new DNA.

Meselson and Stahl cracked the puzzle

The meselson-stahl experiment, results of the experiment, generation 0, generation 1, generation 2, generations 3 and 4, attribution:, works cited:.

• Watson, J. D. and Crick, F. H. C. (1953). A structure for deoxyribose nucleic acid. Nature , 171 (4356), 737-738. Retrieved from http://www.nature.com/nature/dna50/watsoncrick.pdf .
• Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). The basic principle: Base pairing to a template strand. In Campbell biology (10th ed.). San Francisco, CA: Pearson, 318-319.
• American Institute of Biological Sciences. (2003). Biology's most beautiful. http://www.aibs.org/about-aibs/030712_take_the_bioscience_challenge.html .
• Watson, J. D., and Crick, F. H. C. (1953). Genetical implications of the structure of deoxyribonucleic acid. Nature , 171 , 740-741.
• Davis, T. H. (2004). Meselson and Stahl: The art of DNA replication. PNAS , 101 (52), 17895-17896. http://dx.doi.org/10.1073/pnas.0407540101 .

References:

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5.6: The Meselson - Stahl Experiment

• Last updated
• Save as PDF
• Page ID 4745

• John W. Kimball
• Tufts University & Harvard

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DNA Replication is Semiconservative

The structure of DNA suggested to Watson and Crick the mechanism by which DNA — hence genes — could be copied faithfully. They proposed that when the time came for DNA to be replicated, the two strands of the molecule

• separated from each other but
• remained intact as each served as the template for the synthesis of
• a complementary strand.

As this interpretative figure indicates, their results show that DNA molecules are not degraded and reformed from free nucleotides between cell divisions, but instead, each original strand remains intact as it builds a complementary strand from the nucleotides available to it. This is called semiconservative replication because each daughter DNA molecule is one-half "old" and one-half "new".

Immortal strands. Note that the "old" strand (the red one in the top half of the figure) is immortal because — barring mutations or genetic recombination — it will continue to serve as an unchanging template down through the generations.

E. coli is a bacterium, but semiconservative replication of DNA also occurs in eukaryotes. And because each DNA molecule in a eukaryote is incorporated in one chromosome , the replication of entire chromosomes is semiconservative as well. This also means that the eukaryotic chromosome contains one "immortal strand" of DNA.

IB DP Biology Topic 2: Molecular biology 2.7 DNA replication, transcription and translation Study Notes

• IB Style Question Banks with Solution
• IB DP Biology SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
• IB DP Biology HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1 
• IB DP Biology SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
• IB DP Biology HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2

2.7  DNA Replication, Transcription & Translation

Essential Idea: Genetic information in DNA can be accurately copied and can be translated to make the proteins needed by the cell

Understandings:

• The replication of DNA is semi-conservative and depends on complementary base pairing
• Helicase unwinds the double helix and separates the two strands by breaking hydrogen bonds
• DNA polymerase links nucleotides together to form a new strand, using the pre-existing strand as a template
• Transcription is the synthesis of mRNA copied from the DNA base sequences by RNA polymerase
• Translation is the synthesis of polypeptides on ribosomes
• The amino acid sequence of polypeptides is determined by mRNA according to the genetic code
• Codons of three bases on mRNA correspond to one amino acid in a polypeptide
• Translation depends on complementary base pairing between codons on mRNA and anticodons on tRNA

Applications:

• Use of Taq DNA polymerase to produce multiple copies of DNA rapidly by the polymerase chain reaction (PCR)
• Production of human insulin in bacteria as an example of the universality of the genetic code allowing for gene transfer between species
• Use a table of the genetic code to deduce which codon(s) correspond to which amino acid
• Analysis of Meselson and Stahl’s results to obtain support for the theory of semi-conservative replication of DNA
• Use a table of mRNA codons and their corresponding amino acids to deduce the sequence of amino acids coded by a short mRNA strand of known base sequence
• Deducing the DNA base sequence for the mRNA strand

Topic 2.7: dna replication, transcription and translation

In the DNA Replication, Transcription and Translation unit you will learn the details of how and why DNA Replicates. You will also learn how the DNA codes for specific amino acids and how this information is transcribed from the DNA to make proteins.

The unit is planned to take 3 school days.

Essential Idea:

• Genetic information in DNA can be accurately copied and can be translated to make the proteins needed by the cell.

​Nature of science:

• Describe the procedure of the Meselson and Stahl experiment.
• Explain how the Meselson and Stahl experiment demonstrated semi-conservative DNA replication

2.7.U1 The replication of DNA is semi-conservative and depends on complementary base pairing. 

• Describe the meaning of “semiconservative” in relation to DNA replication.
• Explain the role of complementary base pairing in DNA replication.

DNA replication is a semi-conservative process, because when a new double-stranded DNA molecule is formed:

• One strand will be from the original template molecule
• One strand will be newly synthesisd
• When a cell is preparing to divide, the two strands of the double helix separate. The new strands are used as a guide or template for the creation of a new strand.
• New strands are formed by adding nucleotides, one by one, and linking them together
• The results of this process is 2 DNA molecules, both made up of the original strand and a newly synthesized strand.
• Therefore DNA replication is referred to as being semi-conservative.

The base sequence on the template strand determines the base sequence on the new strand, only a nucleotide carrying a base that is complementary to the next base on the template strand can successfully be added to the new strand. Since complementary bases form hydrogen bonds with each, stabilizing the structure, if a nucleotide with the wrong base started to be inserted, the hydrogen bond would not happen and the nucleotide would not be added to the chain Rule – one base always pairs with another is called complementary base pairing.This makes sure that the two DNA molecules that are created by DNA replication are identical in their base sequences to the parent molecule that was replicated.

2.7.U2 Helicase unwinds the double helix and separates the two strands by breaking hydrogen bonds. 

• State why DNA strands must be separated prior to replication.
• Outline two functions of helicase.
• State the role of the origin of replication in DNA replication.
• Contrast the number of origins in prokaryotic cells to the number in eukaryotic cells.

DNA replication is a semi-conservative process whereby pre-existing strands act as templates for newly synthesised strands. The process of DNA replication is coordinated by two key enzymes – helicase and DNA polymerase. To separate the two strands of molecules, this separation is carried out by helicases

• Helixcases is a group of enzymes that use energy from ATP, the energy is required for breaking hydrogen bonds between complementary bases
• Contains six golbular polypeptides arranged in a donuts shape, the polypeptides assemble with one strand of the DNA molecule passing through the center of the donut and the other outside it.
• Energy from ATP is used to help move the helicase along the DNA molecule breaking the hydrogen bonds between the bases and parting the two strands.
• Double stranded DNA can’t be split into two strands while it is till helical therefore helicase causes unwinding of the helix at the same time as it separates the strand

2.7.U3 DNA polymerase links nucleotides together to form a new strand, using the pre-existing strand as a template. 

• Describe the movement of DNA polymerase along the DNA template strand.
• Describe the action of DNA polymerase III in pairing nucleotides during DNA replication.

The creation of new strands is carried out by enzyme DNA polymerase

• DNA polymerase move along the template strand in the same direction, adding one nucleotide at a time
• Free nucleotides with each of the four possible bases are available in the area where DNA is being replicated
• Every time a nucleotide is added to the new strand only one of the four types of nucleotide has the base that can pair with the base at the position reached on the template strand.
• DNA polymerase brings nucleotides into the position where hydrogen bonds could be formed but unless this happens and a complementary base pair is formed, the nucleotide break away again
• Nucleotide is finally as the correct base and has been brought into position and hydrogen bonds have been formed between the two bases, DNA polymerase links it to the end of the new strand
• This is done with Covalent bonds between the phosphate group of the free nucleotide and the sugar of the nucleotide at the existing end of the new strand
• Pentose sugar is 3 terminal and the phosphate is the 5 terminal, DNA polymerase adds on the 5 terminal of the free nucleotide to the 3 terminal of the existing
• DNA polymerase continues to move along the template strand creating new strands with a base sequence complementary to the template strand – it does this with a very high degree of fidelity (very few mistakes made)

2.7.U4 Transcription is the synthesis of mRNA copied from the DNA base sequences by RNA polymerase.

• Define transcription.
• Outline the process of transcription, including the role of RNA polymerase and complementary base pairing.
• Identify the sense and antisense strands of DNA given a diagram of translation.​

Transcription is the synthesis of mRNA copied from the DNA base sequences by RNA polymerase. Sequence of bases in a gene does not, in itself, give any observable characteristic in an organism. Function of most genes is to specify the sequence of amino acids in a particular polypeptide – it is proteins that are often directly or indirectly determine the observable characteristics of an individual. Two processes are needed to produce a specific polypeptide, using the base sequence of a gene

​Transcription – the synthesis of RNA, using DNA as a template, because RNA is a single-stranded, transcription only occurs along one of the two strands of DNA

• The enzyme RNA polymerase binds to a site on the DNA at the start of the gene
• RNA polymerase moves along the gene separating DNA into single strands and pairing up RNA nucleotides with complementary bases on one strand of the DNA – [no thymine in RNA so uracil pairs in a complementary fashion with adenine]
• RNA polymerase forms covalent bonds between the RNA nucleotides
• RNA separates from the DNA and the double heliz reforms
• Transcription stops at the end of the gene and the completed RNA molecule is release
• Product of transcription is molecule of RNA with a base sequence that is complementary to the template strand of DNA
• RNA has a base sequence that is identical to the other strand with one exception there is uracil in place of thymine- to make an RNA copy of the base sequence of one strand of a DNA molecule, the other strand is transcribed.
• DNA with the same base sequence as the RNA is called the SENSE STRAND
• other strand that acts as the template and has a complementary base sequence to both the RNA and the sense strand is called the antisense strand

2.7.U5 ​Translation is the synthesis of polypeptides on ribosomes.

• Define translation.
• State the location of translation in the cell.​

Translation is synthesis of polypeptides on ribosomes. This is the second of the two processes needed to produce a specific polypeptide

• Synthesis of polypeptide with an amino acid sequence chosen by the base sequence of a molecule of RNA
• It takes place on the cell structure in the cytoplasm known as ribosomes – they are complex structures that consist of a small and a large subunit, with binding sites for each of the molecules that take place in the translation-

Messenger RNA and the genetic code

• RNA that carries information needed to synthesize a polypeptide is called mRNA
• Length of mRNA depends on the amount of amino acids in the polypeptide
• Genome – many different genes that carry the information needed to make polypeptide with a specific amino acid sequence
• Certain genes are transcribed when anytime a cell will only need to make some of these polypeptides/ only some will be available for the translation in the cytoplasm

Translation Mnemonic 

The key components of translation are:

• Messenger RNA  (goes to…)
• Ribosome  (reads sequence in …)
• Codons  (recognised by …)
• Anticodons  (found on …)
• Transfer RNA  (which carries …)
• Amino acids  (which join via …)
• Peptide bonds  (to form …)
• Polypeptides

Mnemonic:  Mr Cat App

2.7.U6 The amino acid sequence of polypeptides is determined by mRNA according to the genetic code.

• Outline the role of messenger RNA in translation.​

Condons help the cellular machinery to convert the base sequence on the mRNA into an amino acid sequence is called the genetic code.

• Four different bases and 20 amino acids – so one base can’t code for one amino acids
• 16 combos for 2 bases = still  too few therefore living organisms use a triplet code
• Sequence of three bases is called codon – each codon codes for a specific amino acid to be added to the polypeptide
• Amino acids are carried on another kind of RNA called tRNA, each has a specific ( has three base anticondon complementary to the mRNA codon for the particular amino acid

The genetic code is the set of rules by which information encoded within mRNA sequences is converted into amino acid sequences (polypeptides) by living cells. The genetic code identifies the corresponding amino acid for each codon combination. As there are four possible bases in a nucleotide sequence, and three bases per codon, there are 64 codon possibilities (43). The coding region of an mRNA sequence always begins with a START codon (AUG) and terminates with a STOP codon

2.7.U7  Codons of three bases on mRNA correspond to one amino acid in a polypeptide .

• Define codon, redundant and degenerate as related to the genetic code.
• Explain how using a 4 letters nucleic acid “language” can code for a “language” of 20 amino acid letters in proteins.

The base sequence in a DNA molecule, represented by the letters A T C G, make up the genetic code. The bases hydrogen bond together in a complementary manner between strands. A will always go with T (U in RNA) and G will always go with C.

This code determines the type of amino acids and the order in which they are joined together to make a specific protein. The sequence of amino acids in a protein determines its structure and function. The DNA code is a triplet code. Each triplet, a group of three bases, codes for a specific amino acid:

• the triplet of bases on the DNA and mRNA is known as a codon
• the triplet of bases on the tRNA is known as an anti-codon

2.7.U8 Translation depends on complementary base pairing between codons on mRNA and anticodons on tRNA.

• Outline the role of complementary base pairing between mRNA and tRNA in translation.

Translation is the process of protein synthesis in which the genetic information encoded in mRNA is translated into a sequence of amino acids on a polypeptide chain

• Ribosomes bind to mRNA in the cytoplasm and move along the molecule in a 5’ – 3’ direction until it reaches a start codon (AUG)
• Anticodons on tRNA molecules align opposite appropriate codons according to complementary base pairing (e.g. AUG = UAC)
• Each tRNA molecule carries a specific amino acid (according to the genetic code)
• Ribosomes catalyse the formation of peptide bonds between adjacent amino acids (via condensation reactions)
• The ribosome moves along the mRNA molecule synthesising a polypeptide chain until it reaches a stop codon
• At this point translation ceases and the polypeptide chain is released

2.7.A1  Use of Taq DNA polymerase to produce multiple copies of DNA rapidly by the polymerase chain reaction (PCR). 

• Outline the process of the PCR.
• Explain the use of Taq DNA polymerase in the PCR.
• Denaturation – DNA sample is heated (~90ºC) to separate the two strands
• Annealing – Sample is cooled (~55ºC) to allow primers to anneal (primers designate sequence to be copied)
• Elongation – Sample is heated to the optimal temperature for a heat-tolerant polymerase (Taq) to function (~75ºC)
• Repeatedly doubles the quantity of the selected DNA, involves double-stranded DNA being separated into two single strands at one stage of the cycle and single strands combining to form double-stranded DNA at another stage.
• Reannealing – DNA is heated to a high temperature causing hydrogen bonds to break and the two strands separate. DNA is then cooled hydrogen bonds can form, so the strands pair up again
• PCR machine separates DNA strands by heating them to 95 C for 15 seconds then cooling the DNA quickly to 54 C
• This process allows the reannealing of parent strands to form double-stranded DNA
• A large amount of short sections of single-stranded DNA called primers is present. These primers bind rapidly to target sequences and as a large excess of primers is present, they prevent the re-annealing of the parent strands – causing the copying of the single parent strands then starts from the primers
• Next stage – is synthesis of double stranded DNA,using the single strands with primers as templates – enzyme Taq DNA polymerase is used to do this
• It was taken from a bacterium, Thermus aquaticus, the DNA polymerase are very adapted to be very heat-stable to resist denaturation
• Taq DNA polymerase is used because it can resist the brief period at 95 C used to separate the DNA

2.7.A2 Production of human insulin in bacteria as an example of the universality of the genetic code allowing gene transfer between species.

• Outline the source and use of pharmaceutical insulin prior to the use of gene transfer technology.
• Outline the benefits of using gene transfer technology in the production of pharmaceutical insulin.

The set of DNA and RNA sequences that determine the amino acid sequences used in the synthesis of an organism’s proteins. It is the biochemical basis of heredity and nearly universal in all organisms. The same genetic code appears to operate in all living things, but exceptions to this universality are known.

Since the same codons code for the same amino acids in all living things, genetic information is transferrable between species. The ability to transfer genes between species has been utilised to produce human insulin in bacteria (for mass production)

• The gene responsible for insulin production is extracted from a human cell
• It is spliced into a plasmid vector (for autonomous replication and expression) before being inserted into a bacterial cell
• The transgenic bacteria (typically E. coli) are then selected and cultured in a fermentation tank (to increase bacterial numbers)
• The bacteria now produce human insulin, which is harvested, purified and packaged for human use (i.e. by diabetics)

DNA Cloning Animation

2.7.S1 Use a table of the genetic code to deduce which codon(s) corresponds to which amino acid.

• Use a genetic code table to deduce the mRNA codon(s) given the name of an amino acid.​

2.7.S2 Analysis of Meselson and Stahl’s results to obtain support for the theory of semi-conservative replication of DNA.   

• Compare dispersive, conservative and semi-conservative replication.
• Predict experimental results in the Meselson and Stahl experiment if DNA replication was dispersive, conservative or semi-conservative.

The theory that DNA replication was semi-conservative was confirmed by the Meselson-Stahl experiment in 1958

Prior to this experiment, three hypotheses had been proposed for the method of replication of DNA:

• Conservative Model – An entirely new molecule is synthesised from a DNA template (which remains unaltered)
• Semi-Conservative Model – Each new molecule consists of one newly synthesised strand and one template strand
• Dispersive Model – New molecules are made of segments of new and old DNA

S 2.7.3 Use a table of mRNA codons and their corresponding amino acids to deduce the sequence of amino acids coded by a short mRNA strand of known base sequence.

• Use a genetic code table to determine the amino acid sequence coded for by a given antisense DNA sequence or an mRNA sequence.​

In order to translate an mRNA sequence into a polypeptide chain, it is important to establish the correct sequence The mRNA transcript is organised into triplets of bases called codons, and as such three different reading sequences exists

An open sequence will always start with AUG and will continue in triplets to a termination codon. A blocked sequence may be interrupted by termination codons. Once the start codon (AUG) has been located and sequence established, the corresponding amino acid sequence can be determined by using the genetic code

​2.7.S4 Deducing the DNA base sequence for the mRNA strand.

• Deduce the antisense DNA base sequence that was transcribed to produce a given mRNA sequence.​

mRNA is a complementary copy of a DNA segment (gene) and consequently can be used to deduce the gene sequence. For converting a sequence from mRNA to the original DNA code, apply the rules of complementary base pairing:

• Cytosine (C) is replaced with Guanine (G) – and vice versa
• Uracil (U) is replaced by Adenine (A)
• Adenine (A) is replaced by Thymine (T)

DNA replication

3.4.1 explain dna replication in terms of unwinding the double helix and separation of the strands by helicase, followed by formation of the new complementary strands by dna polymerase..

DNA replication is semi-conservative as both of the DNA molecules produced are formed from an old strand and a new one. The first stage of DNA replication involves the unwinding of the double strand of DNA (DNA double helix) and separating them by breaking the hydrogen bonds between the bases. This is done by the enzyme helicase. Each separated strand now is a template for the new strands. There are many free nucleotides around the replication fork which then bond to the template strands. The free nucleotides form hydrogen bonds with their complimentary base pairs on the template strand. Adenine will pair up with thymine and guanine will pair up with cytosine. DNA polymerase is the enzyme responsible for this. The new DNA strands then rewind to form a double helix. The replication process has produced a new DNA molecule which is identical to the initial one.

3.4.2 Explain the significance of complementary base pairing in the conservation of the base sequence of DNA.

Complementary base pairing is very important in the conservation of the base sequence of DNA. This is because adenine always pairs up with thymine and guanine always pairs up with cytosine. As DNA replication is semi-conservative (one old strand an d one new strand make up the new DNA molecules), this complementary base pairing allows the two DNA molecules to be identical to each other as they have the same base sequence. The new strands formed are complementary to their template strands but also identical to the other template. Therefore, complementary base pairing has a big role in the conservation of the base sequence of DNA.

3.4.3 State that DNA replication is semi- conservative.

DNA replication is semi-conservative.

Transcription & translation

3.5.1 compare the structure of rna and dna..

DNA and RNA both consist of nucleotides which contain a sugar, a base and a phosphate group. However there are a few differences. Firstly, DNA is composed of a double strand forming a helix whereas RNA is only composed of one strand. Also the sugar in DNA is deoxyribose whereas in RNA it is ribose. Finally, both DNA and RNA have the bases adenine, guanine and cytosine. However DNA also contains thymine which is replaced by uracil in RNA.

3.5.2 Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase.

DNA transcription is the formation of an RNA strand which is complementary to the DNA strand. The first stage of transcription is the uncoiling of the DNA double helix. Then, the free RNA nucleotides start to form an RNA strand by using one of the DNA strands as a template. This is done through complementary base pairing, however in the RNA chain, the base thymine is replaced by uracil. RNA polymerase is the enzyme involved in the formation of the RNA strand and the uncoiling of the double helix. The RNA strand then elongates and then separates from the DNA template. The DNA strands then reform a double helix. The strand of RNA formed is called messenger RNA.

3.5.3 Describe the genetic code in terms of codons composed of triplets of bases.

A triplet of bases (3 bases) forms a codon. Each codon codes for a particular amino acid. Amino acids in turn link to form proteins. Therefore DNA and RNA regulate protein synthesis. The genetic code is the codons within DNA and RNA, composed of triplets of bases which eventually lead to protein synthesis.

3.5.4 Explain the process of translation, leading to polypeptide formation.

Translation is the process through which proteins are synthesized. It uses ribosomes, messenger RNA which is composed of codons and transfer RNA which has a triplet of bases called the anticodon. The first stage of translation is the binding of messenger RNA to the small subunit of the ribosome. The transfer RNA’s have a specific amino acid attached to them which corresponds to their anticodons. A transfer RNA molecule will bind to the ribosome however it’s anticodon must match the codon on the messenger RNA. This is done through complementary base pairing. These two form a hydrogen bond together. Another transfer RNA molecule then bonds. Two transfer RNA molecules can bind at once. Then the two amino acids on the two transfer RNA molecules form a peptide bond. The first transfer RNA then detaches from the ribosome and the second one takes it’s place.The ribosome moves along the messenger RNA to the next codon so that another transfer RNA can bind. Again, a peptide bond is formed between the amino acids and this process continues. This forms a polypeptide chain and is the basis of protein synthesis.

3.5.5 Discuss the relationship between one gene and one polypeptide.

A polypeptide is formed by amino acids liking together through peptide bonds. There are 20 different amino acids so a wide range of polypeptides are possible. Genes store the information required for making polypeptides. The information is stored in a coded form by the use of triplets of bases which form codons. The sequence of bases in a gene codes for the sequence of amino acids in a polypeptide. The information in the genes is decoded during transcription and translation leading to protein synthesis.

Related Links

• IB DP Biology SL&HL: Question Bank
• IB Biology Study Guide and Notes for SL/HL
• IB DP Physics SL&HL: Question Bank
• IB Physics Study Guide and Notes for SL/HL

Pulse Chase Primer: The Meselson-Stahl Experiment

• DNA & RNA
• Experimental Design

Resource Type

Description.

This activity can be used in conjunction with the short film The Double Helix . It introduces students to the classic experiment by Matthew Meselson and Franklin Stahl, which revealed that DNA replication follows the semiconservative model.

In 1958, Meselson and Stahl published the results of a pulse-chase experiment to determine how cells replicate their DNA. Students will first read about how the experiment was conducted and describe the predicted results based on three possible models of DNA replication. They then evaluate the actual experimental results.

Student Learning Targets

Interpret experimental evidence to distinguish between different models of DNA replication.

Describe the semiconservative model of DNA replication.

Estimated Time

conservative replication, dispersive replication, experiment, nucleotide, radioactive, replication, semiconservative replication

Primary Literature

Meselson, Matthew, and Franklin W. Stahl. “The Replication of DNA in Escherichia coli .” Proceedings of the National Academies of Science 44, 7 (1958): 671–682. https://doi.org/10.1073/pnas.44.7.671 .

Terms of Use

The resource is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license . No rights are granted to use HHMI’s or BioInteractive’s names or logos independent from this Resource or in any derivative works.

Version History

Curriculum connections, ngss (2013).

HS-LS1-1, HS-LS3-1; SEP2, SEP4

AP Biology (2019)

IST-1.M; SP2, SP3, SP4

IB Biology (2016)

Common core (2010).

ELA-RST.9–12.7

Vision and Change (2009)

CC2, CC3; DP1, DP3

Educator Tips

Double Helix and Pulse-Chase Experiment

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How DNA Replicates

Matthew Meselson Franklin W. Stahl

Matthew Meselson had a passion for physics and chemistry throughout his early life, often conducting science experiments in his family's garage. At the age of 16, he enrolled at the University of Chicago, beginning an academic career that led to doctoral studies at the California Institute of Technology under Linus Pauling. In addition to his widely known work demonstrating semi-conservative replication of DNA with Frank Stahl, Meselson has made many key discoveries in the molecular biology. He is also known for his work in limiting the proliferation of chemical and biological weapons. Meselson is a member of the National Academy of Science and a recipient of the Lasker Award. He continues to serve as a member of the faculty at Harvard University, where he has taught and conducted research since 1960.

Following a sheltered life in a Boston suburb (Needham), Frank stumbled his way through college (Harvard, 1951) before fleeing to a graduate school in biology (U Rochester) to avoid the military draft. While in the graduate school, Frank took a course taught by A. H. (Gus) Doermann, and, for the first time in his life, he had a goal. With Gus, he studied genetic recombination in phage. To meet a departmental requirement, Frank took a summer course at Woods Hole, where he met Matt Meselson and began the work described in this Key Experiment. In 1959, Frank joined the faculty at the University of Oregon, Eugene, in their new Institute of Molecular Biology. He has been there ever since. Frank is now an emeritus faculty member who still enjoys teaching as well as family life and the natural wonders of Oregon.

What's the Big Deal?

Some experiments have proven so influential that they have been christened with the names of the scientists who performed them. The "Meselson–Stahl experiment" is one of those. It has also been called "the most beautiful experiment in biology," a title that has seemed to stick over the years. Why was the Meselson and Stahl experiment so important? Their experiment provided the first critical test of the Watson–Crick models for the structure of DNA and its replication, which were not universally accepted at the time. The convincing results of the Meselson–Stahl experiment, however, dispelled all doubts. DNA was no longer just an imaginary model; it was a real molecule, and its replication could be followed in the form of visually compelling bands in an ultracentrifuge. Meselson and Stahl found that these DNA bands behaved in the ultracentrifuge exactly as Watson and Crick postulated they should. Why was the Meselson–Stahl experiment "beautiful"? Because it was conceptually simple and yet sufficiently powerful to differentiate between several competing hypotheses for how DNA might replicate. Taken together, the Watson–Crick model and the Meselson–Stahl experiment marked the transition to the modern era of molecular biology, a turning point as impactful as the theory of evolution. The story of the Meselson–Stahl experiment, as told here by its protagonists, also reveals how friendship and overcoming obstacles are as crucial to the scientific process as ideas themselves.

Learning Overview —

Big concepts.

Faithful replication of the genetic material (DNA) is the foundation of all life on earth. The experiment by Meselson and Stahl established that DNA replicates through a semi-conservative mechanism, as predicted by Watson and Crick, in which each strand of the double helix acts as a template for a new strand with which it remains associated, until the next replication.

Bio-Dictionary Terms Used

Bacteriophage (phage) , base , base pairing , chromosome , DNA , Hershey–Chase experiment , eukaryote , mutation , nucleotides , Prokaryote (bacteria) , recombination , RNA , ultraviolet light

Terms and Concepts Explained

Equilibrium density-gradient centrifugation , DNA replication , isotope , semi-conservative DNA replication

Introduction

Matthew Meselson and Franklin Stahl (both 24 years old) met at the Marine Biological Laboratory in Woods Hole in Massachusetts and decided to test the Watson–Crick model for DNA replication, which was unproven at the time.

What Events Preceded the Experiment?

Watson and Crick proposed a "Semi-Conservative" model for DNA replication in 1953, which derived from their model of the DNA double helix. In this proposal, the strands of the duplex separate and each strand serves as a template for the synthesis of a new complementary strand. Watson's and Crick's idea for DNA replication was a model, and they did not have data to support it. Some prominent scientists had doubts.

Two other models, "Conservative" and "Dispersive", for DNA replication were proposed.

Setting Up the Experiment

A method was needed to detect a difference between the parental and daughter (newly replicated) DNA strands. Then, one could follow the parent DNA molecule in the progeny. Meselson thought to distinguish between parental and newly synthesized DNA using a density difference in the building blocks (nucleotides) used to construct the DNA. The three models for DNA replication would predict different outcomes for the density of the replicated DNA in the first- and second-generation daughter cells.

The general experimental idea was first to grow bacteria in a chemical medium to make high-density DNA and then abruptly shift the bacteria to a low-density medium so that the bacteria would now synthesize lower density DNA during upcoming rounds of replication. The old and newly synthesized DNA would be distinguished by their density.

To measure a density difference in the DNA, Meselson and Stahl invented a method called equilibrium density gradient centrifugation. In this method, the DNA is centrifuged in a tube with a solution of cesium chloride. When centrifuged, the cesium chloride, being denser than water, forms a density gradient, reaching a stable equilibrium after a few hours. The DNA migrates to a point in the gradient where its density matches the density of the CsCl solution. Heavy and light DNA would come to different resting points and thus physically separated.

Doing the Key Experiment

Meselson and Stahl first decided to study the replication of DNA from a bacteriophage, a virus that replicates inside of bacteria, and used a density difference between two forms of the nucleobase thymine (normal thymine and 5-bromouracil). These experiments did not work.

The investigators changed their plans. They studied replication of the bacterial genome and used two isotopes of nitrogen (15N (heavy) and 14N (light)) to mark the parental and newly synthesized DNA.

When the population of bacteria doubled, Meselson and Stahl noted that the DNA was of an intermediate density, half-way between the dense and light DNA in the gradient. After two doublings, half of the DNA was fully light and the other half was of intermediate density. These results were predicted by the Semi-Conservative Model and are inconsistent with the Conservative and Dispersive Models.

Meselson and Stahl did another experiment in which they used heat to separate the two strands of the daughter DNA after one round of replication. They found that one strand was all heavy DNA and the other all light. This result was consistent with the Semi-Conservative model and provided additional evidence against the Dispersive Model.

Overall, the results provided proof of Semi-Conservative replication, consistent with the model proposed by Watson and Crick.

What Happened Next?

Within a couple of weeks after their key experiment, Meselson wrote a letter to Jim Watson to share news of their result (letter included).

Max Delbruck, the Caltech physicist and biologist who had proposed the dispersive model, was elated by the results, even though Meselson and Stahl disproved his replication hypothesis, and urged the young scientists to write up their results for publication and announce the important result to the world (1958).

Scientists now know a great deal about the protein machinery responsible for DNA replication.

Closing Thoughts

The Meselson–Stahl experiment had a powerful psychological effect on the field of genetics and molecular biology. It was the first experimental test of the Watson and Crick model, and the results clearly showed that DNA was behaving in cells exactly as Watson and Crick predicted.

In addition to having a good idea, the behind-the-scenes tour of the Meselson–Stahl experiment reveals that friendship and persistence in overcoming initial failures play important roles in the scientific discovery process. Also important was an atmosphere of freedom that allowed Meselson and Stahl, then very junior, to pursue their own ideas.

Guided Paper

Meselson, M. and Stahl, F.W. (1958). The replication of DNA in Escherichia coli. Proceedings of the National Academy of Sciences U.S.A., 44: 672–682.

This classic paper provides experimental evidence that the strands of the DNA double helix serve as templates to create a new copy of DNA. These results provide experimental evidence of The Watson and Crick model of DNA replication (‘semi-conservative replication’) demonstrating that genetic information is passed from one cell or organism to its progeny.

The first conversation between Matt Meselson and Frank Stahl, in the summer of 1954, began a collaboration that led to their Key Experiment on DNA replication and marked the beginning of a lifelong friendship. Matt and Frank describe the circumstances that brought them together below.

It was 1954, the year after Jim Watson and Francis Crick published their two great papers describing their double helical model of DNA and its implications for how it might replicate, mutate, and carry genetic information. Jim Watson (26 years old) and I (a 24-year-old first-year graduate student) were both at Caltech and living at the Athenaeum, the Caltech faculty club. We often talked while waiting for dinner. One day, Jim asked me to join him for the summer as his teaching assistant in the Physiology Course at the Marine Biological Laboratory (MBL) at Woods Hole, Massachusetts. He mainly wanted me to do experiments to see if RNA was a double helix. (As a side note, those experiments showed that RNA is not a double helix.) So in June 1954, I drove my 1941 Chevrolet coupe across the country from Cal Tech in Pasadena, California, to the MBL at Woods Hole, Massachusetts.

One day in Woods Hole, while planning student assignments for the Physiology Course, Jim went to the window of the course office upstairs in the MBL Lillie building and pointed towards a student sitting on the grass under a tree serving gin and tonics. That student was Frank Stahl. Jim said let's give him a really hard experiment to do all by himself – the Hershey–Chase Experiment; then, we'll see how good he really is. Two years earlier, Alfred Hershey and Martha Chase published an influential experiment that provided evidence implicating DNA, and not protein, as the substance conferring genetic inheritance in bacteriophage (see the whiteboard animation video on the Hershey–Chase experiment ).

I thought that this guy serving gin and tonics must be an interesting fellow, so I went downstairs to meet him and let him know what was being planned for him. Frank was then a biology graduate student at the University of Rochester. I sat down on the grass under the tree, and we hit it off right away. We found that we had much to discuss. I was very impressed with Frank's knowledge of phage genetics, a subject I knew nothing about, coming from the laboratory of Linus Pauling where I was doing X-ray crystallography. Frank can tell you more about our conversation under the "gin and tonic tree."

1954 was an exciting time for molecular biology. One year earlier, Watson and Crick published their model for the double helix structure of DNA (see the Narrative on DNA Structure by Vale ), which aroused much excitement as well as some serious disbelief. With Watson as an instructor and Crick and Sydney Brenner as visitors and several other founders of molecular biology, Woods Hole that summer became an epicenter for discussion of the great questions in molecular biology. Could the double helix model, as Watson and Crick proposed, explain the replication of the genetic information? What is the "code" for reading out the nucleotide sequence of DNA and turning that into the sequence of amino acids in protein? Would RNA have a similar structure to DNA? However, at the time of my arrival, I had no idea that Woods Hole was hosting anyone, but me, interested in the big problems of modern, i.e., molecular, biology.

I was a 24-year-old biology graduate student at the University of Rochester and had come to the MBL to take the Physiology Course. To beat the heat and, perchance, to meet someone interesting, I invested in a bottle of gin and a 6-pack of tonic, found some ice, a thermos jug, and a tree and sat myself down in the shade. Matt Meselson was one of my first catches.

Our conversation under the "gin and tonic tree" was life-changing. After Matt warned me of Watson's planned test of my experimental aptitude, we talked about the work we were doing. I explained to him the problem in phage genetics on which I was working. Eventually our conversation turned to DNA replication. Neither of us was working on the problem at the time, although we were both keenly interested in it. It was perhaps the most important contemporary question in molecular biology.

At Caltech, Matt had already come up with an idea for how the mechanism of DNA replication might be studied by density labeling. But, as a physical chemist, he was unfamiliar with the methods of phage and bacterial biology that would be needed to conduct the actual experiments. So we decided to collaborate. I was planning to be a postdoctoral fellow at Caltech starting that September. If we could develop a method for measuring the density of DNA molecules and successfully apply it to the problem of DNA replication, we could establish whether the Watson–Crick model for DNA replication, and even the model of the structure itself, was correct or not.

We did not begin our collaboration immediately because Matt needed to finish his X-ray crystallography and I had made plans to do my postdoctoral work on bacterial genetics with Joe Bertani.

You can also hear Matt Meselson describing the Meselson–Stahl experiment in Video 1 .

The genetic material of eukaryotic cells is organized in the form of chromosomes , each a single linear, double-stranded DNA molecule ( Figure 1 ). Most prokaryotes (bacteria) have a single, circular chromosome ( Figure 1 ). All forms of life must replicate their DNA and, except for recombination and infrequent mutations, pass identical copies of their genetic material to their progeny. (See also the Whiteboard Video on Keeping Track of Your DNA .)

Based upon their model for the structure of DNA, Watson and Crick proposed that DNA replicates in a "Semi-Conservative" manner ( Figure 2 ). In this model, the two strands of the DNA double helix unwind and separate, and each "parent" strand serves as a template for the synthesis of a new "daughter" strand. The Watson–Crick base pairing (see the Narrative on DNA Structure by Vale ) of adenine with thymine and guanine with cytosine ensures that, except for rare copying errors, mutations, the information of the original double-stranded DNA molecule is preserved during the synthesis of the daughter strands. In the Semi-Conservative model, each daughter cell in the first generation would inherit one of the original DNA strands from the parent and a recently synthesized DNA strand. In the second generation, two of the granddaughters would be composed of all newly synthesized DNA and two granddaughters would have hybrid DNA (one parental strand and one newly synthesized strand).

While (spoiler alert) the Semi-Conservative Model turned out to be correct, it was far from a foregone conclusion. Before our experiment, several leading scientists questioned the Semi-Conservative Model (see Dig Deeper 1 for more information on their reservations) and proposed alternate models discussed below.

Explorer's Question: What do you imagine are the pros and cons of this model?

Answer: The beauty of this model is that it provides a clear explanation of how a daughter strand is made from the template strand ( Figure 3 ). However, the model requires that the two long parental DNA strands unwind to become single-strand templates. This was seen as a weakness by many scientists at the time (see Dig Deeper 1 for more information).

The unwinding of the DNA helix and keeping the daughter and parental strands from becoming tangled posed problems for the Semi-Conservative model in the minds of many scientists. As a result, other models for DNA replication were imagined. One was a "Conservative Replication" model ( Figure 4 ). In this model, the parental double helix forms a template for a completely new double helix. The two original strands remain together, no unwinding occurs, and the daughter DNA is formed from newly synthesized DNA. In this model, in the first generation, one daughter DNA would inherit the original DNA double helix from the parent DNA and the other daughter DNA would inherit the newly synthesized DNA double helix. In the second generation, one of the four granddaughters would have the original parental DNA and the other three granddaughters would all be composed of newly synthesized DNA. While Conservative Replication was a logical possibility, it was not elaborated by any specific mechanism.

Answer: This model did not call for unwinding of the DNA strands, as in the Semi-Conservative Model, thus solving the concern about DNA unwinding. However, it was unclear how the copying machinery would read out the nucleotide sequence information buried in the core of the DNA double helix.

A third possibility was a model proposed by Max Delbrück (later called "Dispersive Replication") ( Figure 5 ). Delbrück doubted that the two strands of the double helix could be unwound or pulled apart to undergo Semi-Conservative replication and instead suggested that DNA strands broke at every half-turn of the helix during replication (discussed in more depth below and in Dig Deeper 1 ). According to Delbrück's Dispersive Replication Model, each helix of the replicated DNA consists of alternating parental and daughter DNA. Unlike the other two models, the progeny in subsequent generations would be indistinguishable with regard to their compositions of parental and newly synthesized DNA.

Answer: Fragmentation would create shorter templates for replication, which would minimize any unwinding or untangling problems faced by one very long DNA molecule. However, the reassembly of the fragments again into the intact chromosome could be problematic, especially if the breaks occur at every half-turn of the helix.

Explorer's Question: In the first generation, which model(s) would predict that the two daughter cells would receive approximately equal amounts of the original and newly synthesized DNA?

Answer: The Semi-Conservative Model and the Dispersive Model. However, differences in daughter composition arise in the second generation in the two models.

Matt and Frank learned about the models for DNA replication prior to their first meeting at the Physiology Course at the Marine Biological Laboratory through circumstances described below.

Sometime in 1953, while I was a graduate student of the great chemist Linus Pauling, I went to see Max Delbrück, a physicist and founder of the "phage group" who had become deeply interested in genetics and the basis of life (Max Delbrück, and his work with Salvador Luria, is featured in the Narrative on Mutations by Koshland ). I wanted to learn what problems in biology he thought were most important and what advice he might have for me about getting into biology. Almost as soon as I sat down in his office, he asked what I thought about the two papers by Watson and Crick that had been published in Nature earlier that year. I confessed that I had never heard of them.

Exasperated, Delbrück flung a little heap of reprints of the Watson–Crick papers at me and shouted "Get out and don't come back until you have read them." What I heard was "come back." So I did, but only after reading the papers.

When I came back, Delbrück said he did not believe in the Semi-Conservative mechanism of DNA replication proposed by Watson and Crick. Max had imagined that if replication is semi-conservative, the two daughter duplexes would become wound around each other turn-for-turn as the two chains of the mother molecule became unwound. To get around the supposed problem of untangling the daughter molecules, he proposed a model in which breaks are made to prevent interlocking when separating, and then joined back together (see Figure DD1 in Dig Deeper 1 ). This mechanism required rotation of only short lengths of duplex DNA, after which the chains would be rejoined. In the rejoining process, sections of the new chain would be fused to sections of the old chain, making all four of the chains mosaics of new and old DNA. Delbrück, in 1954, published a paper that questioned the Watson–Crick model and presented this new model (later referred to as "Dispersive Replication" as shown in Figure 5 and Dig Deeper 1 ). In some ways, the idea of Delbrück was ahead of its time. Transient breaks are now known to be made by an enzyme called topoisomerase, but in a manner that leaves the individual chains of the parent duplex intact (see Dig Deeper 1 ).

In addition to Max's reservations and model, several other scientists also posed their own concerns and solutions to the "unwinding problem" or alternatives to the Watson–Crick DNA structure itself (see Dig Deeper 1 ).

What I gathered from my conversations with Max and others was that not everyone believed the DNA replication model of Watson and Crick. It was only a hypothesis with no experimental evidence to support it. The key to solve this problem was to follow the parental DNA in the progeny. But how?

I was working on something very different for my PhD thesis with Linus Pauling, but earlier that year, I had an idea for labeling protein molecules with deuterium, a heavy isotope (2H) of hydrogen (1H) and separating them from unlabeled protein molecules in a centrifuge according to their density as a means to solve a quite different problem (see Dig Deeper 2 ). After that second meeting with Max, it occurred to me that density labeling and centrifugal separation might be used to solve the DNA replication problem. When I told this to Pauling, he urged me to get my X-ray crystallography done first. And when I proposed the density approach to Watson, one of those evenings waiting for dinner at the Athenaeum, he said I should go to Sweden to do it – where the ultracentrifuge had been invented (which I never did).

My entry point to thinking about DNA replication came when I was trying to understand how bacteriophage (viruses that infect bacteria) exchange pieces of DNA with one another. This process of DNA exchange between chromosomes is called recombination (see the whiteboard video on the experiments by Morgan and Sturtevant ). When did this recombination process occur? Did it occur when DNA replicates? Or perhaps recombination was an event that stimulated DNA replication? My intuition was that the processes of recombination and replication were somehow related. However, little was known about the mechanisms of either DNA replication or recombination at the time. Furthermore, I did not know how to pursue these questions in 1954. My ideas for experiments were lame and would have led to un-interpretable data.

Like many interesting questions in biology, often one has to be patient until either the right idea or technology emerges that allows one to answer them properly. In 1954, my awareness of a possible connection between replication and recombination primed my interest for the first gin and tonic conversation with Matt. However, it was several decades before I was sure that, in bacteriophage, DNA replication and recombination, in a large degree, depended upon each other. The convincing experiments were based on variations of a technique pioneered by Matt and Jean Weigle at Caltech. In this method, density-labeled, genetically marked parental phage infect the same bacteria. The densities and genetic makeup of progeny phage are determined by bioassay of the individual drops collected from a density gradient.

Matt and Frank

To distinguish between the Semi-Conservative, Conservative, and Dispersive Models of DNA replication described above, we needed a method that could tell the difference between the parental and daughter DNA strands. Figures 2 , 4 , and 5 illustrate the parent and newly synthesized strands with different colors. However, we needed to find a real physical difference that would serve the same function of distinguishing between the old and newly synthesized DNA. Matt had the idea of distinguishing old and new DNA by having the bacteria synthesize them with different isotopes and separating them in a centrifuge according to their density. If the original and the newly synthesized DNAs could be made of different density materials, then we could perhaps measure this physical difference. We will discuss the chemicals that were used to make the DNA heavier or lighter in the next section.

Our experimental idea was to grow an organism in a chemical medium that would make its DNA heavy. Then, while it was growing, we would switch to a new medium in which the newly synthesized DNA would be made of lighter material ( Figure 6 ). The density difference between the original and the newly replicated DNA could allow us to distinguish between models for DNA replication.

To separate DNA of different density, we invented a method, called "equilibrium density gradient centrifugation," and published it, together with Jerome Vinograd, a Caltech Senior Research Fellow who had taught us how to use the ultracentrifuge in his lab and provided advice. In this method, as applied to DNA, a special tube that has quartz windows so that ultraviolet light photos can be taken while the centrifuge is running is filled with a solution of cesium chloride and the DNA to be examined. Upon centrifugation at high speed (~45,000 revolutions per minute or 140,000 times gravity), the CsCl gradually forms a density gradient, becoming most concentrated at the bottom of the tube ( Figure 7 ). The CsCl solution toward the top of the tube is less dense than the DNA, while the CsCl solution at the bottom is denser than DNA. Thus, when a mixture of DNA in a CsCl solution is centrifuged, the DNA will eventually come to a resting point where its density matches that of the CsCl solution ( Figure 7 ). The DNA absorbs UV light, and its position along the tube was recorded by using a special camera while the centrifuge is running.

The method now seems straightforward, but in reality, it took a couple of years to develop. For example, we did not come to cesium chloride immediately. We looked at a periodic table for a dense monovalent atom that would not react with DNA; rubidium chloride (molecular weight of 121) was available in the Chemistry Department stockroom and we initially tried to use that but found that even concentrated solutions were not dense enough to float DNA to a banding point. We then moved one level down in the periodic table to cesium (the molecular weight of cesium chloride is 168) and that worked (for more details, see Dig Deeper 3 ).

Frank and Matt

In the fall of 1954, we were reunited in Cal Tech and lived for about eight months in the same house across the street from the lab. We finally could begin doing experiments to test models of DNA replication. It should be noted that DNA replication was our "side" project; we also had our "main" projects under the supervision of our respective professors. However, faculty at Cal Tech was kind in allowing two young scientists to venture forward with their own ideas.

While the general experimental approach that took form under the "gin and tonic tree" was straightforward, choices had to be made in how exactly to do the experiment. What organism should we use? Would a chemical trick of making DNA heavier or lighter work and could we measure a small density difference between the two? It took us a while to get the conditions right, about two years.

We first decided to examine the replication of the bacteriophage T4 inside of the bacterium Escherichia coli. Bacteriophages are viruses that invade and replicate inside of a bacterium; when new viruses are made, they will burst the bacterium and then spread to new hosts. Bacteriophage have small genomes and are therefore the smallest replicating systems. Frank's PhD thesis was on T4, so he knew how to work with this phage. Max Delbrück and others at Cal Tech were also actively studying phage (see the Narrative on Mutations by Koshland ). Thus, T4 seemed the obvious choice. To create DNA of heavier density than normal DNA, we decided to use the analogue, 5-bromouracil, of the base thymine, in which a heavier bromine atom replaces a lighter hydrogen atom. During replication, 5-bromouracil could be incorporated into DNA, instead of thymine.

However, while this approach seemed reasonable, it did not work in practice. Although we did not appreciate it at the time, during phage growth, the DNA molecules undergo recombination, joining parental DNA to newly synthesized DNA in a manner that after several generations would give no clear-cut distribution of old DNA among progeny molecules. Also, we learned from a recent paper that 5-bromouracil was mutagenic and made a detour into studying mutagenesis before coming back to our main project.

We clearly needed a new strategy.

Instead of phage, we decided to study the replication of the bacterial genome. This was a good choice – the bacterial DNA gave a very sharp and clear band when centrifuged in a solution of cesium chloride (to learn more about why we used cesium chloride to create a density gradient and the general use of this technique; see Dig Deeper 3 ).

We also switched our density label. DNA is made up of several elements – carbon, nitrogen, oxygen, phosphorus, and hydrogen. Some of these elements come in different stable isotopes, with atomic variations of molecular weight based upon different numbers of neutrons. Nitrogen-15 (15N) is a heavier isotope of nitrogen (the most common isotope, 14N, has a molecular weight of 14 Daltons). We could easily buy 15N in the form of ammonium chloride (15NH4Cl), which was the only source of nitrogen in our growth medium. The 15N in the medium then found its way into the bacterial DNA (as well as other molecules) in a harmless manner and did not impair bacterial growth.

We also had good luck in that Caltech bought a brand new type of ultracentrifuge called an analytical ultracentrifuge (Model E) developed by the Beckman Instrument Company. The Model E was a massive machine about the size of a small delivery truck (the current model is just a bit bigger than a dishwasher). Importantly, the Model E could shine a UV light beam on the tube while the centrifuge was spinning and detect and photograph the position of the DNA. The good news was that 15N-containing DNA and 14N-containing DNA could be clearly distinguished by their different density positions ( Figure 8 ).

Finally, we had everything in place to try our experiment properly. I decided to set up our first experiment in the following two ways:

1) Grow the bacteria in "light" nitrogen medium and then switch to "heavy."

2) Grow another culture of bacteria in "heavy" nitrogen for many generations and then switch to "light."

Frank was called to a job interview and could not perform this first experiment with me. But before leaving, he warned me – "Don't do the experiment in such a complicated way on your first try. You might mix up the tubes."

I ignored Frank's advice and did the experiment both ways.

In the first experiment after transferring bacteria grown in heavy nitrogen (15N) growth medium and then switched to "light" (14N) nitrogen medium, I saw three discrete bands corresponding to old, hybrid, and new DNA, as predicted by Semi-Conservative replication. Excited developing the photograph in the darkroom, I remember letting out a yelp that caused a young woman working nearby to leave in a hurry. But later I realized my mistake. Frank had been prophetic. I indeed had mixed tubes, combining two different samples, one taken before and the other taken after the first generation of bacterial growth in the light medium. As described for the correct experiment below, there is no time when old, hybrid, and fully new DNA are present at the same time.

When I came back from my trip, Matt and I performed what proved to be the decisive experiment. We grew bacteria in "heavy" nitrogen (15N; from 15NH4Cl) and then switched to "light" nitrogen (14N; 14NH4Cl) and, at different time points, collected the bacteria by centrifugation, added detergent to release the DNA, and combined this with concentrated CsCl solution to reach the desired density. After 20 hours of centrifugation and the final density positions of the DNAs had come into view, we knew that we had a clean answer ( Figure 9 ). The DNA from bacteria grown in heavy nitrogen formed a single band in the gradient. However, when the bacteria were shifted to a light nitrogen medium and then allowed to replicate their DNA and divide once (first generation), essentially all DNA had shifted to a new, "intermediate" density position in the gradient ( Figure 9 ). This intermediate position was half-way between the all heavy and all light DNA. At longer times of incubation in light nitrogen, after the cells had divided a second time (second generation), a DNA band at lighter density was seen and there were equal amounts of the intermediate and light DNA.

Explorer's Question: Which of the three models (Conservative, Semi-Conservative, or Dispersive) is most consistent with the results of this experiment?

Answer: The Semi-Conservative model. The Conservative model predicts a heavy and light band at the first generation, not an intermediate band. The Dispersive model predicts a single intermediate band at both the first and second generations (the band shifting toward lighter densities with more generation times).

Explorer's Question: Why are the two DNA bands at the 1.9 generation time point of approximately equal intensity?

Answer: After the first generation, each of the two heavy strands is partnered with a light strand. The bacterial DNA consists of one heavy strand and one light strand. When that heavy–light DNA replicates again in the light medium, the heavy strands are partnered with new light strands (intermediate density DNA) and the light strands are also partners with new light strands (creating all light density DNA).

The experiment that Frank described above took hardly any time at all (2 days) and yielded a clean result. We then repeated it without any problem. Once we knew how to set up the experiment, it was relatively easy. But it took us two years of trials before we got the experimental design and conditions right for the final ideal experiment.

The experiment clearly supported the Semi-Conservative Replication model for replication and, in doing so, also supported the double helical model of DNA itself. However, we wanted to do one more experiment that would examine whether the "intermediate" density band of DNA in the first generation was truly made of two and just two distinct subunits, as predicted by the Watson–Crick model. The model predicts that one complete strand of DNA is from the parent and should be heavy and the other complete DNA strand should be all newly replicated and therefore light ( Figure 10 ). We could test this hypothesis by separating the subunits with heat and then analyzing the density and molecular weight of the separated subunits by equilibrium density-gradient ultracentrifugation.

On the other hand, the Dispersive Model predicted that each DNA strand of first generation is an equal mixture of original and newly replicated DNAs ( Figure 11 ).

The results from the experiment were again clear ( Figure 12 ). The "intermediate density" DNA in the first generation split apart into a light and heavy component. From the width of the DNA band in the gradient (see Dig Deeper 3 ), we could also tell that the light and heavy DNA obtained after heating had each half of the molecular weight of the intermediate density DNA before heating. These results indicated that each parental strand remained intact during replication and produced a complete replica copy. This was decisive evidence against the Delbrück model for it predicted that both strands would be mosaics of heavy and light, not purely heavy and purely light. And the finding that the separated heavy and light subunits each had half the molecular weight of the intact molecule indicated that DNA was made up of two chains, as predicted by the Watson Crick model, and was not some multichain entity.

Based upon the results in Figure 9 and Figure 12 , we concluded that:

1) The nitrogen of a DNA molecule is divided equally between two subunits. The subunits remain intact through many generations.

2) Following replication, each daughter molecule receives one parental subunit and one newly synthesized subunit.

3) The replicative act results in a molecular doubling.

These conclusions precisely aligned with the Watson–Crick Semi-Conservative model for DNA replication. DNA, as a double-stranded helix, unwinds, and each strand serves as a template for the synthesis of a new strand.

When we had our result, Matt quickly shared the news with Jim Watson in a letter dated November 8, 1957 (available for the first time here ). It was common in those days to share results with colleagues through letters prior to a publication.

We also shared our results with Max Delbrück who took the news well that his Dispersive Replication model was incorrect. In fact, he wrote to a colleague that Meselson and Stahl had obtained a "world shaking result." But we were slow to get our work written up for publication. Once we knew the answer, we were keen to move onto new experiments rather than writing up our results.

Finally, Max had enough of our dallying and brought us down to the Caltech marine station at Corona del Mar. There, he quarantined us to a room in a tower, saying that we could not come out until we had written a draft of our paper. He was not being unkind, and we thought it great fun. Max's wife Manny Delbrück kindly came in occasionally to bring us delicious sandwiches, and Max also kept us company. We worked for 2 days straight and got him a draft.

Shortly thereafter, we completed our paper and Max communicated it in May 1958 to the Proceedings of the National Academies of Science, 4 years after our meeting at the Marine Biological Laboratory but less than a year after finally getting our experiments to work.

After our paper was published, we went separate ways in our lives. Frank got a job at the University of Missouri but soon thereafter moved to the University of Oregon in Eugene. Matt got promoted from a postdoctoral fellow to an assistant professor at Caltech and was teaching physical chemistry. However, the constant teaching limited time in the laboratory. Matt asked to be demoted from assistant professor back to senior postdoc, so he could get more work done in the lab. This is perhaps the only case in the history of Caltech in which a professor asked to move down the academic ladder. After a year as a senior postdoc, Matt then moved to Harvard to become an associate professor.

Decades have passed, and we now know much about the machinery that orchestrates DNA replication, including the unwinding of the strands and the synthesis of a new strand from the parental template. The details are beyond what can be discussed here, but you can view an animation of this process in Video 2 .

When we first discussed the use of density labeling to test models for DNA replication under the gin and tonic tree, we could not imagine the psychological effect our experiment would have on the field. Many scientists were not initially convinced by the Watson–Crick model for the structure of DNA or their proposal for its mode of replication. It was not clear if their model could explain heredity and the properties of genes. Some people seemed to think the model was too simple to be the gene. Others thought it too simple (meaning too beautiful) to be wrong!

However, after our experiment, the DNA model seemed very real. We could watch DNA with a camera; the visualization of DNA bands was simple and clear. Our results showed that the gene is made of two complementary halves, each a template for the other. Even the disbelievers, such as the deeply thoughtful Max Delbrück, acquiesced. DNA was no longer an imaginary molecule in the heads of Watson and Crick. It was a dynamic molecule; one could perform experiments on it, and it behaved in living cells as one might predict. Mendel's concept of a discrete "factor" that could determine a plant character and remain intact generation after generation and the physical reality of a gene as double-stranded DNA became intertwined from that moment on.

It is gratifying to think that our experiment, so simple by modern standards, is still valued and taught. But beyond the logic of how the experiment was performed, we hope that our story also conveys other important lessons about science.

• Every hypothesis needs to be rigorously tested with a clear experiment.

• An atmosphere of freedom is important. We were both very junior at the time of this experiment, but we were supported by senior scientists who encouraged us to pursue our own ideas.

• Success does not come immediately. Reading most scientific papers (including ours), everything seems straightforward and works right away. Our narrative shows that the so-called "most beautiful experiment in biology" had some unsuccessful excursions and two years of work to come to successful finish.

• Because success does not come immediately, it is valuable to be able to share difficult times with a friend. We kept each from getting discouraged. There was a certain gaiety in our work. We even had fun when things went wrong.

• Much of science is built upon collaboration and friendship. This Key Experiment could never have been the "Meselson Experiment" or the "Stahl Experiment." The "Meselson and Stahl Experiment" required both parties. We complemented each other scientifically and encouraged each other personally. Well more than a half-century has passed since this experiment was performed, and we remain good friends today.

Dig Deeper 1: Alternatives to Semi-Conservative replication

Max Delbrück, in his 1954 paper (PNAS 40: 783-788), said the following of the Watson and Crick Semi-Conservative replication mechanism:

"The principal difficulty of this mechanism lies in the fact that the two chains are wound around each other in a large number of turns and that, therefore, the daughter duplexes generated by the process just outlined are wound around each other with an equally large number of turns. There are three ways of separating the daughter duplexes: (a) by slipping them past each other longitudinally; (b) by unwinding the two duplexes from each other; (c) by breaks and reunions. We reject the first two possibilities as too inelegant to be efficient and propose to analyze the third possibility."

Max's solution was to break the single chains at regular intervals, allowing rotation about single bonds of the unbroken chain followed by joining in a way that dispersed short segments of parental DNA among the single chains of the daughter duplexes. This is the Dispersive Model presented in Figure 5 and presented in more detail in Figure DD1 . There was a germ of truth in Delbrück's idea of breakage. We now know of topoisomerases, enzymes that facilitate DNA replication by temporarily breaking, allowing unwinding, and then rejoining DNA. There are also enzymes that unwind DNA helixes called DNA helicases. Both enzymes use chemical energy derived from hydrolyzing adenosine triphosphate to perform work on the stable DNA double helix.

Another type of solution to the "unwinding problem," one that required no breaking and no entangling of the daughter molecules, was to imagine that the synthesis process would cause the entire parental duplex to rotate one turn for each turn of DNA synthesized. But this posed problems of its own – giving rise to a variety of long-forgotten proposed models, including evoking a motor at the growing point that would drive the rotation of the parent molecule, as proposed by John Cairns and Cedric Davern [J. Cellular Physiology, 70: S65–76 (1967)].

Alternative solutions questioned the DNA double helix model, but not semi-conservative replication. For example, one idea was to assume that the two chains are not wound around a common axis, but instead are simply pushed together (plectonemic coiling), which would require no unwinding and no rotation. This possibility, although it appeared remote, was not rigorously ruled out until much later in a paper by Crick, Wang, and Bauer in 1979 (J. Mol. Biol, 129: 449–461).

Dig Deeper 2: The idea for using density for separation

The idea for using density as a separation method came to me early in 1954 while I was a first-year graduate student at Caltech listening to a lecture by the great French scientist Jacques Monod. Monod was describing the problem of regulation of an enzyme called beta-galactosidase. If the bacteria were growing in a medium without lactose (a sugar), the enzyme activity was very low. When lactose or a chemical analogue of lactose was added, the enzyme activity was induced. The question was how? One model was that the enzyme was always there, but is inactive unless lactose is around. Another model (the correct one) was that the enzyme is synthesized de novo after the inducer is added. I thought that it might be possible to measure new enzyme and distinguish it from old enzyme if the enzyme was synthesized from heavier building blocks (amino acids). How could one make heavier building blocks? I thought that deuterium (a heavy isotope of hydrogen; 2 H) might be the answer. If one grew bacteria in heavy water ( 2 H 2 O) and switched to normal water ( 1 H 2 O) when one added inducer, then any newly synthesized beta-galactosidase would have had a greater density than the pre-existing beta-galactosidase. I never did the experiment, but the idea primed me for the DNA replication problem.

Dig Deeper 3: The role of the centrifuge in the Meselson–Stahl experiment

Matt describes briefly how this technique evolved

The first paper (see the reference list) that Frank and I wrote together (along with Jerome Vinograd) was on the method and theory of using centrifugation in an equilibrium density gradient, which showed that this method not only could separate molecules but could also be used as a tool to determine their molecular weights. This work was also part of my PhD thesis at Caltech. When I presented this work at my thesis defense, the great physicist Richard Feynman was on the examination committee, along with Pauling, Vinograd, and one of Pauling's post-docs who taught me X-ray crystallography. Feynman had not read the thesis but did so during the defense. I presented my rather long mathematical derivation showing that macromolecules in a density gradient in a centrifugal field would be distributed in a Gaussian manner about the position of neutral buoyancy with the width dependent upon the square root of the molecular weight. Feynman then went to the blackboard and, on the spot, produced a much shorter derivation of the same thing, modeled on the wave function for the quantum mechanical harmonic oscillator. Feynman writes about our experiment in his jolly book, Surely You're Joking, Mr. Feynman .

The way in which we found that CsCl forms a density gradient on its own was somewhat fortuitous. We initially thought that we needed to pour a CsCl gradient in the tube in advance. However, we found that just by centrifuging an initially homogeneous solution of cesium chloride produced a continuous gradient density on its own after several hours. From the width of the DNA band in the density gradient, we could also calculate the molecular weight of the DNA molecules in the gradient as 7 million Daltons. The chromosome of E. coli is much, much larger, but long molecules of DNA are fragile and had been broken up by shear forces while passing through the hypodermic needle with which we loaded the centrifuge cell. Subsequent to our result, CsCl equilibrium density-gradient centrifugation became a standard tool for isolating DNA from cells for decades and was used in important experiments such as the demonstration of messenger RNA by Brenner, Meselson, and Jacob and showing the mechanism of general recombination in phage lambda by Frank Stahl.

References and Resources

• Matthew Meselson’s letter to James Watson from November 8, 1957, describing the results of their experiments on DNA replication. Download .

This paper describes the use of the centrifuge and density gradient to analyze biological molecules, a technique that was used in their 1958 paper but also very broadly used for many applications in biology. See also Dig Deeper 3 .

An outstanding resource for those wanting a detailed, accurate description of the Meselson–Stahl experiment.

A nice 7:30 min video describing the Meselson–Stahl experiment and its conclusions.

This film documents the discovery of the structure and replication of DNA including interviews with James Watson who, along with Crick, proposed the double helix model of DNA.

This activity is often used in conjunction with the short film The Double Helix. It introduces students to Meselson and Stahl experiment and helps them understand the concepts generated via those experimental results.

This collection of resources from HHMI Biointeractive addresses many of the major concepts surrounding DNA and its production, reading, and replication.

Meselson and Stahl Experiment

Meselson and Stahl experiment gave the experimental evidence of DNA replication to be semi-conservative type . It was introduced by the Matthew Meselson and Franklin Stahl in the year 1958 . Matthew Meselson and Franklin Stahl have used E.coli as the “ Model organism ” to explain the semiconservative mode of replication.

There are three modes of replication introduced during the 1950s like conservative, semi-conservative and dispersive. The researchers were confused between these three that what could be the actual pattern of DNA replication. In 1958, Matthew Meselson and Franklin Stahl presented their research, where they concluded that the replication of DNA is semiconservative type .

Matthew Meselson and Franklin Stahl have conducted several experiments after the discovery of DNA structure (by the two scientists Watson and Crick ). Watson and Crick’s model is widely accepted to demonstrate the replicative model of DNA. We will discuss the definition, steps and observation of the Meselson and Stahl experiment along with the semi-conservative model of DNA.

Content: Meselson and Stahl Experiment

Semi conservation model of dna, meselson and stahl experiment steps, observation, definition of meselson and stahl experiment.

Meselson and Stahl Experiment gave us the theory of semi-conservative replication of DNA. They have taken E.coli as the model organism and two different isotopes, N-15 and N-14 . The N-15 is the heavier isotope, whereas N-14 is the lighter or common isotope of nitrogen. Meselson and Stahl performed their experiment by first growing the E.coli in the medium containing 15 NH 4 Cl for several generations. They observed that the heavy isotope has incorporated in the genome of E.coli and the cells become more substantial due to 15 N heavy isotope.

Meselson and Stahl then transferred the E.coli cells incorporated with 15 N isotope to the medium containing 14 NH 4 Cl for several generations. After every twenty minutes, the E.coli cells multiply. For the processing of DNA, the cells were centrifuged by the addition of Caesium chloride, resulting  in the formation of the concentration gradient. As a result, light, intermediate and heavy DNA strands will get separated.

After completing their experiment, Meselson and Stahl concluded that after each cell division, half of the DNA would be conserved for every next generation. Therefore, this experiment proves that the DNA replication obeys the semi-conservative mode of replication in which 50% of the DNA conserve for every next generation in a way like 100%, 50%, 25%, and 12.5% and so on.

It is the type of DNA replication. The term semi means “ Half ” and conservative means “ To store ”. The semi-conservative DNA replication results in the two daughter DNAs after the parent DNA replication.

In the two daughter DNA’s, each strand will contain a mixture of the parent DNA’s template strand, and the other with a newly synthesized strand (in F-1 gen ). When the parental DNA replicates, half of the 100%, i.e. 50% of the DNA is conserved by having parent strand and the remaining 50% will produce newly synthesized strands.

After the F-1 gen, the multiplication of the cell will get double, which will produce four DNA strands (in F-2 gen ). In F-2 gen half of 50%, i.e. only 25% of DNA is conserved by having parental strands, and the remaining 75% will produce newly synthesized strands.

• Growth of E.coli : First, the E.coli were grown in the medium containing 15 NH 4 Cl for several generations. NH 4 provides the nitrogen as well as a protein source for the growth of the E.coli. Here, the 15 N is the heavy isotope of nitrogen.
• Incorporation of 15 N : After several generations of E.coli, Meselson and Stahl observed that the 15 N heavy isotope has incorporated between the DNA nucleotides in E.coli.
• Transfer of E.coli cells : The DNA of E.coli labelled with 15 N isotope were transferred to the medium containing 14 NH 4 Cl . Here, the 14 N is the light isotope of nitrogen. The E.coli cells were again allowed to multiply for several generations. The E.coli cells will multiply every 20 minutes for several generations.
• Processing of DNA : For the processing or separation of DNA, the E.coli cells were transferred to the Eppendorf tubes. After that, caesium chloride is added, having a density of 1.71 g/cm 3 (the same of DNA). Finally, the tubes were subjected to high-speed centrifugation 140,000 X g for 20 hours.

The result, after two generations of E.coli, the following results were obtained:

In the F-1 generation : According to the actual observations, two DNA strands (with a mixture of both 15 N and 14 N isotopes) will produce in F-1 gen. The above diagram shows that the semiconservative and dispersive model obeys the pattern of growth explained by Meselson and Stahl.

Thus, it is clear that the DNA does not replicate via “Conservative mode”. According to the conservative model, the DNA replicates to produce one newly synthesized DNA and one parental DNA. Therefore, the conservative model was disapproved, as it does not produce hybrid DNA in the F-1 generation.

In the F-2 generation : According to the actual observation, four DNA strands ( two with hybrid and the remaining two with light DNA ) will produce in the F-2 generation. The hybrid DNA includes a mixture of 15 N and 14 N. The light DNA strands contain a pure 14 N. The diagram shows that only semi-conservative type of replication gave similar results conducted by Meselson and Stahl. Thus, both the conservative and dispersive modes of replication were disapproved.

Therefore, we can conclude that the type of replication in DNA is “ Semi conservative ”. The offsprings have a hybrid DNA containing a mixture of both template and newly synthesized DNA in the semi-conservative model. After each multiplication, the number of offspring will double, and half of the parental DNA will be conserved for the next generation.

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DNA Replication ( Edexcel A (SNAB) A Level Biology )

Revision note.

DNA Replication

• Before a (parent) cell divides, it needs to  copy the DNA  contained within it
• Doubling the DNA ensures that the two new (daughter) cells produced will both receive  full copies of the parental DNA
• The DNA is copied via a process known as  semi-conservative replication  (semi = half)
• The process is called this because in each new DNA molecule produced, one of the polynucleotide DNA strands (half of the new DNA molecule) is from the original DNA molecule being copied
• The other polynucleotide DNA strand (the other half of the new DNA molecule) has to be newly created by the cell
• Therefore, the new DNA molecule has conserved half of the original DNA and then used this to create a new strand

The importance of retaining one original DNA strand

• It   ensures   there is   genetic continuity   between generations of cells
• In other words, it ensures that the new cells produced during cell division   inherit all their genes   from their parent cells
• Replication of DNA and cell division also occurs during   growth

Semi conservative replication of DNA

Semi-conservative replication

• DNA replication occurs in preparation for mitosis , the number of DNA molecules in the parent cell must be   doubled   before mitosis takes place
• DNA replication occurs during the   S phase   of the cell cycle (which occurs during   interphase , when a cell is   not dividing )
• The enzyme   helicase   unwinds  the DNA double helix by breaking the   hydrogen bonds   between the base pairs on the two antiparallel polynucleotide DNA strands to form two single polynucleotide DNA strands
• Each of these single polynucleotide DNA strands acts as a   template   for the formation of a   new strand   made from free nucleotides that are attracted to the exposed DNA bases by   base pairing
• The new nucleotides are then   joined together   by the enzyme DNA polymerase
• The original strand and the new strand join together through hydrogen bonding between base pairs to form the new DNA molecule
• This method of replicating DNA is known as   semi-conservative replication   because   half of the original   DNA molecule is kept ( conserved ) in each of the two new DNA molecules

DNA Polymerase

• These nucleotides are known as   nucleoside triphosphates   or   ‘activated nucleotides’
• The extra phosphates activate the nucleotides, enabling them to take part in DNA replication
• The bases of the free nucleoside triphosphates   align   with their   complementary bases   on each of the   template   DNA strands
• The enzyme   DNA polymerase   synthesises new DNA strands   from the two template strands
• It does this by   catalysing condensation reactions   between the   deoxyribose sugar and phosphate groups   of adjacent nucleotides within the new strands, creating the   sugar-phosphate backbone of the new DNA strands
• DNA polymerase cleaves (breaks off) the two extra phosphates and uses the   energy released   to create the   phosphodiester bonds   (between adjacent nucleotides)
• Hydrogen bonds   then form between the   complementary base pairs   of the template and new DNA strands

Nucleotides are bonded together by DNA polymerase to create the new complementary DNA strands

Leading & lagging strands

• DNA polymerase   can only build the new strand in one direction (5’ to 3’ direction)
• As DNA is ‘unzipped’ from the 3’ towards the 5’ end, DNA polymerase will attach to the   3’ end of the original strand   and   move towards the replication fork   (the point at which the DNA molecule is splitting into two template strands)
• This means the DNA polymerase enzyme can synthesise the leading strand   continuously
• This template strand that the DNA polymerase attaches to is known as the   leading strand
• The other template strand created during DNA replication is known as the   lagging strand
• On this strand,   DNA polymerase moves away from the replication fork   (from the 5’ end to the 3’ end)
• This means the DNA polymerase enzyme can only synthesise the lagging DNA strand in   short segments   (called Okazaki fragments)
• A second enzyme known as   DNA ligase   is needed to   join these lagging strand segments together   to form a continuous complementary DNA strand
• DNA ligase does this by catalysing the formation of   phosphodiester bonds   between the segments to create a   continuous sugar-phosphate backbone

The synthesis of the complimentary strand occurs differently on the leading and lagging strands of DNA

Meselson and Stahl’s Experiment

• Scientists were unsure if DNA replication was conservative or semi-conservative
• Two scientists called Matthew Meselson  and  Franklin Stahl , showed that DNA replication was semi-conservative by experimenting with isotopes of nitrogen

Meselson and Stahl's Experiment

• DNA contains nitrogen in its bases
• As the bacteria replicated, they used nitrogen from the broth to make   new DNA nucleotides
• After some time, the culture of bacteria had DNA containing   only heavy ( 15 N)  nitrogen
• This showed that the DNA containing the heavy nitrogen settled near the bottom of the centrifuge tube
• If   conservative DNA replication   had occurred, the original template DNA molecules would only contain the heavier nitrogen and would settle at the bottom of the tube, whilst the new DNA molecules would only contain the lighter nitrogen and would settle at the top of the tube
• If   semi-conservative replication   had occurred,   all   the DNA molecules would now contain   both   the   heavy  15 N   and   light   14 N   nitrogen and would therefore settle in the   middle of the tube   (one strand of each DNA molecule would be from the original DNA containing the heavier nitrogen and the other (new) strand would be made using only the lighter nitrogen)
• The DNA from this second round of centrifugation settled in the middle of the tube, showing that each DNA molecule contained a   mixture   of the   heavier and lighter nitrogen isotopes
• If more rounds of replication were allowed to take place, the   ratio of   15 N: 14 N  would go from 1:1 after the first round of replication, to 3:1 after the second and 7:1 after the third

Meselson and Stahl's experiment that showed bacterial DNA replicated via semi-conservative DNA replication

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Meselson and stahl experiment.

November 18, 2020

Matt Meselson and Frank Stahl were in their mid-20s when they performed what is now recognized as one of the most beautiful experiments in modern biology. In this short film, Matt and Frank share how they devised the groundbreaking experiment that proved semiconservative DNA replication, what it was like to see the results for the first time, and how it felt to be at the forefront of molecular biology research in the 1950s. This film celebrates a lifelong friendship, a shared love of science, and the serendipity that can lead to foundational discoveries about the living world.

View the full talk with additional resources on our website

The most beautiful experiment: meselson and stahl.

Matt Meselson and Frank Stahl share the story of their groundbreaking experiment from 1958 that definitively showed semiconservative DNA replication. (Talk recorded in February 2020)

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The Meselson-Stahl Experiment: “the Most Beautiful Experiment in Biology”

• First Online: 24 November 2020

Cite this chapter

• Allan Franklin 3 &
• Ronald Laymon 4  

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In this chapter we will examine the Meselson-Stahl experiment which decided the issue between two models of DNA replication. The experimental result was held not to require replication. A second experiment of a rather different nature was used to eliminate a third possibility, and whereby contrast that experiment was further refined so as to yield a more accurate result. These experiments set the stage for the creation of new competing accounts of DNA replication all of which—given the framework established by the Meselson-Stahl experiments—were eliminated by experimental tests leaving only the Watson-Crick model in play.

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For an excellent history of this complex episode and complete references see Holmes ( 2001 ).

See Delbruck and Stent ( 1957 ) and Holmes ( 2001 , pp. 15–29, 393–394).

Often referred in textbooks as being N 14 labeled as opposed to unlabeled.

It should be noted that the consistent locations of the various bands as a function of their association with the labeling was determined and validated by the calibration procedure of superimposing the data images at generation points 0 and 1.9 as shown in the penultimate data row.

To see how this goes trace the replication paths indicated in Fig.  2.5 and note that at the end of the first generation there are now (as per the first part of the first conclusion) twice as many subunits are there were parental subunits to begin with. And then at the next generation there will again (as per the second part of the first conclusion) be twice as many progeny but where only one half will be half labeled. Thus, the number of half-labeled subunits at the end will be twice the number of parental subunits at the initial stage of the process.

In a later chapter on the discovery of the positron we’ll review another instance where the theoretical results were deduced from the experimental data (augmented by non-controversial theoretical assumptions) when Patrick Blackett made such a derivation in order to clearly demarcate what his experimental results demonstrated from their role in confirming Dirac’s hole theory of the production and nature of the positron.

Younger readers may need to be reminded that email was not always available.

For an extensive analysis of null experiments and the sense in which they satisfy demands for replication see Franklin and Laymon ( 2019 ).

There is, though, a caveat that we need to make. There emerged on the basis of further experimentation and theoretical development a realization that the sharpness of the separation bands reported by Meselson and Stahl was due in part to an unrecognized but uniformly consistent fragmentation of the DNA molecule that occurred during the preparation of the DNA samples. But the defect was not claimed in any way to lend experimental support for conservative replication, and moreover new experiments continued to support semi-conservative replication. For the details see Holmes ( 2001, pp. 395–397 ). As Thomas Kuhn would have described it, these further developments were part of normal science conducted in response to the Meselson and Stahl experiment understood as a paradigm (Kuhn 1962 , pp. 23–42).

There is an argument to be made even if it’s true that dispersive replication is not formally inconsistent with semi-conservative replication, the constraints imposed by semi-conservative replication on dispersive replication are so severe and restrictive that dispersive replication is effectively disconfirmed because of the evident difficulties that would be involved in creating a developed and specifically testable version of dispersive replication. Alternatively stated, once was enough for the Meselson and Stahl experiment to have effectively discouraged any further pursuit of dispersive replication as a promising approach to understanding the replication of DNA. Viewing dispersive replication as having been effectively disconfirmed in this way provides a charitable explanation why many of the textbooks claim that the Meselson and Stahl experiment disconfirmed not only conservative replication but also dispersive replication. See, for example (Lehninger 1975 , pp. 659–61) where it is claimed that the results of the Meselson and Stahl experiment “are exactly those expected from the hypothesis of semiconservative replication proposed by Watson and Crick; whereas they are not consistent with the alternative hypotheses of conservative or dispersive replication.”.

One can make the representation more realistic (in the sense of representing dispersion at multiple locations) by simply stacking the rectangular modules and making them of different height (and internal proportionality) to accommodate different dispersion locations. In which case, the argument for consistency with semi-conservative replication carries through as before.

Meselson and Stahl were here relying on a method of density and weight determination that they had developed along with Jerome Vinograd which was reported in Meselson et al. ( 1957 ).

For a brief review of this development see Holmes ( 2001, 394–395 ).

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Franklin, A., Laymon, R. (2021). The Meselson-Stahl Experiment: “the Most Beautiful Experiment in Biology”. In: Once Can Be Enough. Springer, Cham. https://doi.org/10.1007/978-3-030-62565-8_2

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Meselson–Stahl Experiment

Discovery and history, the background of replication of dna, semi-conservative replication , the dispersive hypothesis, the conservative hypothesis, the semi conservative,  procedure and protocol followed by meselson and stahl, results and observation , ruling out conservative hypothesis, ruling out dispersive hypothesis, importance and application of meselson and stahl experiment, what did meselson-stahl experiment show, why were 14 n and 15n used in meselson and stahl’s experiment, how did meselson and stahl turn e.coli dna into heavy dna, what is the semiconservative model of dna replication.

The Meselson-Stahl experiment was a groundbreaking scientific study conducted in 1958 by Matthew Meselson and Franklin Stahl. This experiment provided strong evidence supporting the theory of semi-conservative DNA replication, which was proposed by James Watson and Francis Crick.

Read more about an Introduction to DNA Replication

After the brilliant work of describing the DNA structure, Watson and Crick also proposed a hypothesis that the DNA replication process is semi-conservative. This hypothesis was strengthened by the experiment of Meselson and Stahl in which they elucidated the nature of replication of DNA.

The cell was first discovered to be dividing by Hugo Von Mohl in 1835. Later, cell division was filmed and captured by Kurt Michel in 1943, who was renowned for micro-cinematography. It became evident that when the cell divides the nucleus also divides, and whatever material is present in the nucleus also undergoes division. So, the DNA present in the nucleus must undergo division. This division of DNA was named DNA replication. There were a lot of other hypotheses explaining the nature of DNA replication, but the Meselson-Stahl Experiment stood out the most as it supported the already existing hypothesis of Watson and Crick.

DNA is the basic code of life. When the cell divides it also divides and is transferred to the daughter cells. Just like the production of daughter cells, there are production daughter DNA strands. This production is simply the copying of the hereditary material and is called DNA replication. The process begins at specific sites where there is a characteristic nucleotide sequence.

Several enzymes have been found to assist in this process most important of which is DNA polymerase. This enzyme is of a single type in the case of eukaryotes while there are three types (DNA polymerase I, II, and III) in the case of prokaryotes.

The DNA polymerase I is relatively a short sized enzyme that plays only a supportive role, but DNA polymerase III is the main enzyme that causes replication in E. coli. DNA polymerase III has a large size, almost 10 times larger than DNA polymerase I. It moves along the DNA strand, adding nucleotides at a very quick rate of 1000 per second. However, DNA polymerase cannot itself initiate DNA replication. It requires the enzyme primase to create RNA primer, a complementary sequence of 10 nucleotides. DNA polymerase III identifies this and starts further adding nucleotides to complete the new daughter strand. 

In this replication, it is cleared that when the DNA helix replicates it unwinds itself and turns out to create two strands. This unwinding is done by Helicase enzyme. One of the parent strands act as a template strand and on this, the complementary base pair attach to form a new stand. After the formation of this new strand or daughter strand, they wind together to form a supercoiled DNA double Helix with the help of topoisomerase enzymes. The same process happens with the other parent strand of DNA.

Thus, it was concluded that the new copies or replicas of the parent DNA helix consist of half (one of the strands) of the parent DNA. That is why it is called semi-conservative as it contains fifty percent of older or parent DNA. 

Hypotheses Regarding DNA replication

Three hypotheses were tried to elucidate the DNA replication process up to their own extent.

• The Dispersive hypothesis
• The Semi-conservative Hypothesis

This hypothesis was based on the model, which was put forward by Max Delbruck, a German – American biophysicist. He introduced the concepts of physics at the molecular level in biology. According to the dispersive hypothesis, the parent DNA is digested into around ten or more segments. These segments are used as templates to make new segments. The old and new segments then mix and coil to form the daughter DNA strands.

Simply, this model suggested that the parent DNA strand would disintegrate, and the new strands formed will be the blend of the newly formed nucleotides (made by polymerase enzymes) and older nucleotides (from the parent DNA). 

This hypothesis considers the DNA a template as a whole. The histone proteins bind to the DNA and revolve around it, producing a new DNA copy. In simple terms, the DNA duplex will remain the same. It will not be unwound and will create completely new copies.

As described earlier, it supports the hypothesis of Watson and Crick. Each of the parent DNA strands acts as a template and the new bases or nucleotides will form. This means it will be half new and half old i.e. semi-conservative replication. 

In all of the above hypotheses, there is a prognosis about how the parent DNA would distribute itself. In essence, the conservative hypothesis tells us that the parent DNA will not mix up with the daughter DNA, but it will retain its identity. However, the daughter DNA might be new, but it will have the same sequence as the parent DNA.

The semi conservative hypothesis tells us that after the replication process the new DNA duplex formed will have fifty percent of the parent material and fifty percent new formed or daughter material (nucleotides), as complete single strands.

Whereas, the dispersive theory explains that the whole DNA duplex will divide itself in about 10 nucleotides in size and the old fragments will join randomly with the newly formed fragment or more precisely the nucleotides.

Experimental Process and Protocol

All three hypotheses were assessed by Mathew Meselson and Franklin Stahl. They performed experiments in California. As their experiment was highly based on the studies and hypotheses of Watson and Crick. They realized that DNA is made up of nucleotides. These nucleotides are further comprised of a phosphate group, deoxyribose sugar and most importantly nitrogenous base. 

The nitrogenous bases are present in each nucleotide of DNA. All the bases present in those nucleotides have nitrogen. They realized it would be greatly helpful to tag the parent DNA.

The best way to tag it was to change the nitrogen atoms present in the parent DNA. They used a nitrogen isotope to make a difference between the daughter and parent strand. Due to this remarkable use of isotopes and biophysics, this experiment is regarded as the most beautiful experiment. 

The experiment began with the preparation of a culture media for the bacteria. The microorganism chosen was  E. coli . The culture media consisted of NH4CL and 15 N isotope of nitrogen. This isotope of nitrogen has a higher molecular weight than the normal existing nitrogen isotope 14 N. 

When the  E. coli  was allowed to grow in the culture containing high molecular weight isotope of nitrogen, the bacteria started to incorporate 15 N atoms of nitrogen into its DNA. So, after several generations, the DNA of the bacteria grown in 15 N culture becomes denser than the normal bacteria grown in normal culture.

The other  E. coli  ware grown in a normal culture media containing the common isotope of nitrogen, the 14 N nitrogen. Thus, the DNA of this bacterial culture was less dense than 15 N culture media.

After completing enough generations, they transferred the bacteria from the 15 N media to 14 N media. They allowed the bacteria containing 15 N in their DNA to grow in a culture containing 14 N nitrogen.

They obtained DNA from the mixture and dissolved it in solution containing cesium chloride. Later, they centrifuged this mixture at high speed i.e. using the ultracentrifugation technique. The high centrifugal forces cause the ions of CsCl to produce a density gradient. The gradient is produced as the ions migrate to the bottom of the tube due to ultra-centrifugation. A constant density is established throughout the solution. The DNA samples start to float in the solution. After keeping the tube at rest for a while, the different DNAs of different densities maintain a specific position according to their densities.

As the DNA with 15 N isotope is heavier, it finds its place at the bottom of the tube. On the other hand, the DNA with 14 N isotopes of nitrogen finds its place on an elevated level in the tube. However, a type of DNA is also found in between the elevated level and bottom level. 

Both the scientists concluded that if conservative hypothesis were to be true, then there would be DNAs of only two densities. As the parent DNA would have maintained its integrity and gave birth to a new daughter strand. However, they found three types of densities regarding DNA in their experiment. 

According to the semi-conservative replication hypothesis, there would be a hybrid density DNA. As one strand will be of 14 N and one strand will be of 15 N isotope. However, concerning dispersive hypothesis the new DNA formed will be of intermediate density just like the semi-conservative. But, each of the strands from the double helix will not solely contain either 14 N or 15 N nitrogen, it will be a mixture of both isotopes. 

To rectify these confusions, Meselson and Stahl sampled the second replication of 15 N bacteria in the 14 N medium. They followed the same procedure and protocol and put the DNA in the test tube with Cesium Chloride and established a density gradient.

It was found that there were DNAs of two different densities, unlike the first time when there were three. One of the two had intermediate density just like in the previous experiment and the second one had the density of pure 14 N isotope.

This result did not support the dispersive hypothesis of replication as it would have given one DNA having a density lower than the intermediate density found in the experiment. 

This experiment provided modern biology with the knowledge of DNA replication. This knowledge granted the vision to peek into hereditary diseases and disorders. It is a simple yet provoking experiment that has been accepted by a lot of scientists.

Despite the affirmative result provided by the experiment of the Meselson-Stahl, it took a few years for acceptance by the scientific community. The Meselson and Stahl experiment did not only provide evidence for the semi-conservative theory which was put forward by Watson and Crick, this experiment also confirmed the Watson and Crick model of DNA structure. Thus, it strengthened the standpoint of Watson and Crick which was taking years to get accepted.

In 1952, a scientist studied the experiment by implementing it on the cancer researches. He did not negate the semi-conservative replication hypothesis. This further supported the experiment.

However, the only thing that fell short in this experiment was the proper explanation about the DNA subunits. This discovery of Meselson and Stahl allowed the scientists to discover the mode of transmission of DNA and follow up on the genetic disorders.

This experiment is simply based on tagging the DNA and separating them based on their densities relative to the solution created by using Cesium Chloride.

It was a known fact by then that DNA is made up of nucleotides and these nucleotides contained nitrogen.

Meselson and Stahl utilized the common nitrogen by replacing it with a heavier isotope so that they can identify the parent and daughter DNA in solution by mixing the DNAs of different densities.

They experimented using simple techniques as ultra-centrifugation and density grading.

This experiment is a great milestone in modern biology as those were early times when atomic physics was applied in biological studies. Meselson and Stahl made three major outcomes.

• DNA is made up of two strands, which was based on the Watson and Crick Model
• If the parent DNA has two strands, then each of the strand acts as a template and retains its integrity and forms a daughter strand. Thus, the semi-conservation is evident
• Each of the parent DNA strand is shared by two daughter DNA.

In any case, the investigation helped researchers to clarify legacy by indicating how DNA saves hereditary data through all the progressive DNA replication cycles as a cell grows, divides and repeats the cycle.

Frequently Asked Questions

Meselson-Stahl’s experiment confirmed that DNA replicates semi-conservatively and ruled out the other proposed mechanisms of DNA replication.

Nitrogen is one of the abundant elements in DNA structure. Meselson and Stahl used the isotopes of N14 and N15 in the experiment to incorporate them into the DNA of newly growing organisms and separate DNAs of different densities to observe the mode of DNA replication.

Meselson and Stahl cultured E.coli in 15NH4Cl containing medium over many generations. As a result, 15N was incorporated into the bacterial DNA, converting it into heavy DNA.

According to this model, both strands of DNA segregate in the replication process, and each strand acts as a template to synthesize a new daughter strand. Resultingly, daughter DNA contains one parent strand and one new strand.

• John Cairns to Horace F Judson, in The Eighth Day of Creation: Makers of the Revolution in Biology (1979). Touchstone Books,  ISBN   0-671-22540-5 . 2nd edition: Cold Spring Harbor Laboratory Press, 1996 paperback:  ISBN   0-87969-478-5 .
• Watson JD, Crick FH (1953). “The structure of DNA”.  Cold Spring Harb. Symp. Quant. Biol.   18 : 123–31.  doi : 10.1101/SQB.1953.018.01.020 .  PMID   13168976 .
• Bloch DP (December 1955).  “A Possible Mechanism for the Replication of the Helical Structure of Desoxyribonucleic Acid” .  Proc. Natl. Acad. Sci. U.S.A.   41  (12): 1058–64.  doi : 10.1073/pnas.41.12.1058 .  PMC   528197 .  PMID   16589796 .
• Delbrück M (September 1954).  “On the Replication of Desoxyribonucleic Acid (DNA)”  (PDF).  Proc. Natl. Acad. Sci. U.S.A.   40  (9): 783–8.  doi : 10.1073/pnas.40.9.783 .  PMC   534166 .  PMID   16589559 .

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• Book Review
• Published: 01 March 2002

Meselson, Stahl and the Replication of DNA: A History of “The Most Beautiful Experiment in Biology”

• Bruce Stillman 1  

Nature Medicine volume  8 ,  page 211 ( 2002 ) Cite this article

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Meselson, Stahl and the Replication of DNA

• Frederic Lawrence Holmes

Great experiments either prove a previous notion, or they reveal unexpected results that lead to new ideas. In science, ideas are propagated and the very best of them survive for many years, if not forever. Experiments are, by their very nature, often transitory and useful but for a moment in time. However, at least one notable experiment is an exception: the famous Meselson and Stahl experiment. In a recent labor of love, Frederic Lawrence Holmes delves into this experiment, telling us how it came about, how it was conceived, how it was executed and what it meant at the time. Along the way, one gets a glimpse of what it was like to do science in the earliest days of molecular biology and a sense of the social aspects of science in those heady times.

The second of the famous papers by Jim Watson and Francis Crick deals with the implications of the double-helix structure for inheritance and states that “each chain then acts as a template for the formation on to itself of a new companion chain so that eventually we shall have two pairs of chains, where we only had one before”. Therein, they proposed that the DNA unwound and each strand was a template for the synthesis of a complementary strand, begetting two identical helices. They suggested that DNA might replicate in a semi-conservative manner, rather than the alternative conservative mode whereby the parental double helix remained intact and the new double helix was identical to the parent, but composed of entirely new strands. The Meselson and Stahl experiment demonstrated that Watson and Crick were correct in their assumption.

It seems difficult these days to comprehend that there was ever any doubt about how DNA must replicate. But masterfully, and in great detail, Holmes takes us back to the discourse that emerged immediately after the double-helix revelation. Many were concerned about what the great Max Delbrück thought of the double helix, and although he enthusiastically spread the word about its structure, true to form, Max had a problem: the “untwiddling problem”. How could the two strands that were intertwined so many times separate during replication? He was not only concerned about the problem, as were Watson and Crick, but he proposed a complicated (and incorrect) solution in a Proceedings of the National Academy of Sciences paper in the Spring of 1954.

Holmes' well-written book describes every detail from thenceforth. The chance meeting of Matt Meselson and Frank Stahl at Woods Hole, the seminar by Monod that induced Meselson to think about density transfer, the trials of experimentation and of course the “beautiful experiment” itself. Although dense, the story is worth reading to understand what science was like in the 1950s and how a great experiment came about. It also describes the environment at Caltech during that era, scientifically exciting, but socially bleak. I assume the social environment in Pasadena has improved, but clearly the science there remains as strong as it was. Students who do science, or those who study the process will learn much from this book on how great science can be accomplished.

What struck me while reading this treatise was the remarkably open exchange of ideas between the early phage investigators, via letters and discussions at meetings. Scientists traveled (and reveled) more than I would have thought, a common thread that has emerged in other books I have read about the early phage days. For example, Holmes reports that Meselson and Stahl wrote many times to Jim Watson and others about the design and progress of their experiments. Obviously Watson had a more than passing interest in the matter, but more interestingly, Meselson and Stahl wrote to and visited Gunther Stent at Berkeley to discuss their progress. They did this even though Stent was working on the replication problem and favored the Delbrück proposal that DNA replication was not semi-conservative. We should learn from history, because unfortunately, in modern molecular biology where scientists are not as technique-limited as they once were, the free exchange of ideas is in danger of being lost.

The measure of a great technique is what it reveals and whether it lasts. The Meselson and Stahl experiment is still in wide use today. It has been used to demonstrate the distributive nature of histone deposition during chromosome replication and most recently to study the mechanism and timing of replication of the entire genome of the yeast Saccharomyces cerevisiae . Very few experimental methods have survived as long as the density-transfer idea. Thus, I expect that Holmes' book will be read for many years to come, and justifiably so.

Also reviewed by Sydney Brenner

Salk Institute for Biological Studies La Jolla, California, USA

In these days of high throughput science, when advances in technology have literally given us the power to make atom-by-atom descriptions of all living matter, it is refreshing to look back at an earlier time, when advances in science required both a good idea and the means to show it was true. We were like Houdinis, strapped in chairs with our hands tied behind our backs trying to escape from locked rooms. This book is the history of the Meselson–Stahl experiment—the most beautiful experiment in biology—and reconstructs both the background and the event itself in a most meticulous and admirable way. Although we learn about the revolution in biology consequent upon the discovery of the double helix, it is not history in the large but rather history on the minute scale of what actually happened in the creation and execution of the experiment. The author has had access both to the notebooks and the memories of the scientists as well as to others and he has marshaled all of this detail into a narrative that is interesting and informative.

When the double helical structure of DNA was proposed, the intertwining of the strands created an objection in the minds of some who became concerned that the strands would have to be unwound in order for them to be replicated. Max Delbruck, in particular, was most troubled by it. It was fortunate that, at the time, people did not know that there were DNA molecules that were closed circles, because they would have declared the replication model proposed by Watson and Crick impossible. Somewhere the book says that there was a small band of enthusiastic supporters who were not troubled by this difficulty. I was one of them and took the view that if it were a problem, biological systems would have found a way to solve it. Indeed, I think it was Leslie Orgel who said that nature would have invented an enzyme to do it, a most perceptive insight.

The consequences of the replication model were clear: after one replication step two molecules would be present, each with one old and one new strand. How could one prove this? I met Matt Meselson outside Blackford Hall in Cold Spring Harbor in September 1954 when he had already conceived of the idea of doing the experiment with heavy isotopes using some sort of density centrifugation to separate the molecules. Frank Stahl knew how to work with phages and the partnership was formed. However, Meselson had to complete his PhD thesis research in crystallography, and while making the transition from physical chemistry to biology, he kept detailed notes about what he was reading in a workbook. The evolution of his thinking can be followed from these books.

After spending time trying to do the transfer experiment with 5-bromouracil-labeled bacteriophage T2, density-gradient ultracentrifugation became possible and they switched to using bacteria and 15 N labeling. They were able to show that the difference in density between light 14 N- and heavy 15 N-labeled DNA was sufficient to allow a molecule of intermediate density to be resolved, whereupon Meselson decided to do a double-transfer experiment from heavy to light and light to heavy medium against the advice of Stahl who had to go to an interview in Missouri. Meselson also added several controls and labeled the tubes from this large series of experiments with a complicated code before proceeding to analyze them in the ultracentrifuge. His memory was that the experiment had worked, but an examination of the original films showed that his recollection of the result was wrong. None of the films showed the expected three bands that Meselson thought he saw when he rushed over to announce the result at a party being held at his house. Of course, later experiments gave the expected result.

It could be said that if historians have the benefit of hindsight, scientists have the advantage of foresight. Meselson had sketched the expected result before doing the experiments and I think he superposed in his mind the individual results of his experiments to generate an answer compatible with it. All experimentalists know you have to do an experiment four times. The first one is a complete mess and shows only a hint that it might have worked. The second one is better but still messy. Then you do it the third time for the book. This is when you forget to add a reagent, or mix up the tubes or the centrifuge leaks. That is why there is always a fourth time.

I urge every young scientist to read this book. In 1957, when the experiment was performed, Meselson was 27 and barely with a PhD in chemistry. Frank Stahl was 28 and a postdoctoral fellow at the California Institute of Technology. Both were doing an experiment that had nothing to do with their official programs of research. They simply went ahead and did it. They filled out no forms, made no applications, had no reviews. They only had the judgments of their real scientific peers.

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Stillman, B. Meselson, Stahl and the Replication of DNA: A History of “The Most Beautiful Experiment in Biology”. Nat Med 8 , 211 (2002). https://doi.org/10.1038/nm0302-211

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