meselson stahl experiment replikation

• Biology Article
• Dna Replication Experiment

DNA Replication and Meselson And Stahl's Experiment

Literally, replication means the process of duplication. In molecular biology, DNA replication is the primary stage of inheritance. Central dogma explains how the DNA makes its own copies through DNA replication, which then codes for the RNA in transcription and further, RNA codes for the proteins by the translation.

Let’s go through Meselson and Stahl Experiment and DNA replication.

Meselson and Stahl Experiment

Meselson and Stahl Experiment was an experimental proof for semiconservative DNA replication. In 1958, Matthew Meselson and Franklin Stahl conducted an experiment on E.coli which divides in 20 minutes, to study the replication of DNA.

Semi conservative DNA Replication through Meselson and Stahl’s Experiment

15 N (heavy) and 14 N (normal) are two isotopes of nitrogen, which can be distinguished based on their densities by centrifugation in Ca,esium chloride (CsCl). Meselson and Stahl cultured E.coli in a medium constituting 15 NH 4 Cl over many generations. As a result, 15 N was integrated into the bacterial DNA. Later, they revised the 15 NH 4 Cl medium to normal 14 NH 4 Cl. At a regular interval of time, they took the sample and checked for the density of DNA.

Observation

Sample no. 1 (after 20 minutes): The sample had bacterial DNA with an intermediate density. Sample no. 2 (after 40 minutes): The sample contained DNA with both intermediate and light densities in the same proportion.

Based on observations and experimental results, Meselson and Stahl concluded that DNA molecules can replicate semi-conservatively. Investigation of semi-conservative nature of replication of DNA or the copying of the  cells , DNA didn’t end there. Followed by Meselson and Stahl experiment, Taylor and colleagues conducted another experiment on Vicia faba (fava beans) which again proved that replication of DNA is semi-conservative.

Also Read:  DNA Structure

DNA Replication

DNA is the genetic material in the majority of the organisms.  Structurally, it is a double-stranded helical structure which can replicate.

DNA replication is the process by which the DNA makes multiple copies of itself. It was originally proposed by Watson and Crick. DNA replication proceeds as follows:

• Primarily during this process, two DNA strands will open and separate.
• As the strands are separated, the enzymes start synthesizing the complementary sequence in each of the strands. That is, each parental strand will act as a template for the newly synthesized daughter strands.

Since the new DNA strands thus formed have one strand of the parent DNA and the other is newly synthesized, the process is called semiconservative DNA replication.

DNA Replication Fork

Also Read:  DNA Replication

Frequently Asked Questions

Which mode of replication did the messelson and stahl’s experiment support.

Messelson and Stahl’s experiment supported the semi-conservative mode of replication. The DNA was first replicated in 14N medium which produced a band of 14N and 15N hybrid DNA. This eliminated the conservative mode of replication.

What are the different modes of replication of DNA?

The different modes of replication of DNA are:

• Semiconservative
• Conservative

How are semi-conservative and conservative modes of replication different?

Semi-conservative mode of replication produces two copies, each containing one original strand and one new strand. On the contrary, conservative replication produces two new strands and would leave two original template DNA strands in a double helix.

What is the result of DNA replication?

The result of DNA replication is one original strand and one new strand of nucleotides.

What happens if DNA replication goes wrong?

If DNA replication goes wrong, a mutation occurs. However, if any mismatch happens, it can be corrected during proofreading by DNA Polymerase.

Leave a Comment Cancel reply

Your Mobile number and Email id will not be published. Required fields are marked *

Request OTP on Voice Call

Post My Comment

I FOUND IT VERY USEFUL AND THE WAY EVERYTHING IS EXPLAINED IS VERY HELPFUL. THANK YOU

This is awesome and straight to the point

Register with BYJU'S & Download Free PDFs

Register with byju's & watch live videos.

An official website of the United States government

Official websites use .gov A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS A lock ( Lock Locked padlock icon ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

• Publications
• Account settings
• Advanced Search
• Journal List

Meselson and Stahl: The art of DNA replication

Tinsley h davis.

• Author information
• Article notes
• Copyright and License information

Series information

Issue date 2004 Dec 28.

In 2003, the scientific community celebrated the 50th anniversary of James Watson and Francis Crick's landmark 1953 paper on the structure of DNA ( 1 ). The double helix, whose form has become the icon of biological research, was not an instant hit however. The model did not gain wide acceptance until the publication of another paper 5 years later.

Matthew Meselson and Franklin Stahl's experiments on the replication of DNA, published in PNAS in 1958 ( 2 ), helped cement the concept of the double helix. Meselson, a graduate student, and Stahl, a postdoctoral researcher, both at the California Institute of Technology (Pasadena), gave validity to a model that many scientists saw as speculation: how two intertwined and tangled strands of a helix could physically code for the material of inheritance. The Perspective by Philip Hanawalt of Stanford University (Stanford, CA), in this issue of PNAS ( 3 ), reviews the scientific Revolution of this crowning achievement and outlines its subsequent impact on four decades of DNA replication, recombination, and repair research. The two men behind the laborious steps in discovering the semiconservative replication of DNA credit much of their success to timing, hard work, and serendipity.

A Partnership Begins

During his third year of graduate school at the University of Rochester (Rochester, NY), one of Stahl's advisors suggested that he take a physiology course and sent him to the Marine Biological Laboratory in Woods Hole, MA. “I partied my way through that course,” Stahl confesses. “During the partying, I met Meselson,” who was also temporarily at Woods Hole, working as a teaching assistant. During that summer of 1954, the double helix model had been well received but was only truly accepted by an enthusiastic minority of scientists. “Matt had the idea that one ought to be able to use density labels to test Watson's hypothesis,” said Stahl. Although at Woods Hole, Meselson was a graduate student with Linus Pauling at the time at Caltech. There, Meselson had heard Jacques Monod speak about the nature of chemical bonds and enzyme synthesis, which gave Meselson a new technique idea for working with β-galactosidase in bacterial protein synthesis and measuring changes in protein density.

To explore the project, Pauling, whose work centered on x-ray crystallography, sent Meselson to another Caltech professor, Max Delbrück, to learn about the biological aspects of the necessary experiments. Meselson credits Delbrück with giving him the information that would change the nature of the project. As he thrust the Watson and Crick papers toward the young scientist, “He said, `Read these and don't come back until you have,”' Meselson recalls. Up to that point, Meselson admits that he had not been aware of Watson and Crick's work or their DNA structure model.

Stahl planned to go to Caltech for his postdoctoral work, and at Woods Hole he and Meselson decided to collaborate on the density label project in their spare time. “Caltech is a cozy community. It's ruled by ideas, not by walls,” says Stahl. When he arrived at Caltech, Stahl began a bacteriophage project that did not end well after he inadvertently switched the labels on some culture plates. “In the midst of this gloom and doom, Matt came in,” Stahl says. Meselson had finished his main research project and was ready to tackle Watson and Crick's hypothesis. Thus, Stahl changed his focus from bacteriophages to DNA replication.

Not as Simple as It Seems

Meselson and Stahl faced a tangled problem. The Watson and Crick double helix seemed to suggest that the two strands dissociated, each giving rise to a new, complementary strand. The two daughter molecules would thus contain one strand each from the parent molecule, in a semiconservative replication fashion. If replication were conservative, however, the intertwined strands would be replicated as a whole. This would produce one daughter molecule with all original information and one with all new information. The third model, termed dispersive replication, considered that each strand of the daughter molecule could consist of DNA that had been shuffled around so each strand was a hybrid of old and new.

According to Meselson, “There were 2 years of things that didn't work” followed by a year of successful experiments. Jan Drake, then a postdoctoral researcher at Woods Hole, reflected on the years he shared a rented house with Meselson and Stahl and recalls that they all worked the same hard hours kept by many graduate students and postdoctoral researchers today. They would often discuss their work over dinner before returning to the laboratory in the evening. Despite the long hours, results were not immediately forthcoming. Yet perseverance prevailed, and Meselson and Stahl finally designed a successful experiment that would help distinguish new daughter strands from the parent strand.

Hanawalt's Perspective ( 3 ) outlines the intricacies of the differential nitrogen ( 14 N and 15 N) labeling and subsequent separation of the DNA. The experiments demonstrated that Watson and Crick's model of the double helix could replicate itself in a concerted, semiconservative fashion, and the results were published in PNAS after being communicated by Delbrück.

The Legacy of Elegant Peaks

Now, more than 45 years later, the paper is still held aloft for its clarity. Looking back, though, Meselson says the paper has “one thing I wish weren't there.” At the time, published research from an established scientist, Paul Doty ( 4 ), seemed to show that salmon sperm DNA did not come apart when heated. Meselson and Stahl's research could then have two implications: either Doty was incorrect or Escherichia coli DNA actually had four strands. Hence, Meselson and Stahl were cautious with their wording and used the term “subunit” instead of “strand.” “We were little graduate students,” Meselson says. He and Stahl were wary of contradicting an established scientist. “I wish we had had the courage. You should believe in your convictions,” says Meselson. Doty's conclusions were later found to be incorrect because the instruments used were not sensitive enough to detect the DNA molecular weight changes.

Stahl credits the beauty and success of their paper to two things. First, the “delightfully clean data” were serendipitous. The clean data peaks they observed resulted from the DNA fragmenting during handling; unfragmented DNA would not have separated as nicely. Stahl likens pipetting DNA to “throwing spaghetti over Niagara Falls.” The stress of the pipetting caused tremendous shearing of the DNA, although they did not realize this at the time, nor did they realize how critical this would be to obtaining clean peaks. In addition, Meselson was a “stickler for clarity,” said Stahl. “Every single word in that paper was discussed several times before being allowed to keep its position in the sentence.”

Such clean data and clear writing, in addition to the significance of the paper for the field of molecular biology, have placed Meselson and Stahl's experiment on the pages of many a syllabus. At the Massachusetts Institute of Technology (Cambridge, MA), Professor of Biology Tania Baker says the experiment is part of a course required of all molecular biology graduate students. “It is a very nice test of a model of replication,” she says. “Conceptually, it's a very important technique.”

Today, the “little graduate students” stay in touch. Stahl is a professor at the University of Oregon (Eugene, OR), and Meselson is a professor at Harvard University (Cambridge, MA). As definitive as the 1958 paper may appear in its elegance and simplicity, its greater legacy is the subsequent research it has fostered. Cold Spring Harbor Laboratory (Cold Spring Harbor, NY) hosts a meeting for scientists in the field of DNA replication every other year. President and CEO Bruce Stillman acknowledges that it is not a large field—the attendees can fit into a single auditorium—but states that it is a very active one. Stillman says, “Forty-five years after Meselson and Stahl, we've still got work to do.”

• 1. Watson, J. D. & Crick, F. H. C. (1953) Nature 171 , 964–967. [ DOI ] [ PubMed ] [ Google Scholar ]
• 2. Meselson, M. & Stahl, F. W. (1958) Proc. Natl. Acad. Sci. USA 44 , 671–682. [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ]
• 3. Hanawalt, P. C. (2004) Proc. Natl. Acad. Sci. USA 101 , 17889–17894. [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ]
• 4. Rice, S. A. & Doty, P. (1957) J. Am. Chem. Soc. 79 , 3937–3947. [ Google Scholar ]
• View on publisher site
• PDF (91.8 KB)
• Collections

Similar articles

Cited by other articles, links to ncbi databases.

• Download .nbib .nbib
• Format: AMA APA MLA NLM

Add to Collections

The Meselson-Stahl Experiment (1957–1958), by Matthew Meselson and Franklin Stahl

In an experiment later named for them, Matthew Stanley Meselson and Franklin William Stahl in the US demonstrated during the 1950s the semi-conservative replication of DNA, such that each daughter DNA molecule contains one new daughter subunit and one subunit conserved from the parental DNA molecule. The researchers conducted the experiment at California Institute of Technology (Caltech) in Pasadena, California, from October 1957 to January 1958. The experiment verified James Watson and Francis Crick´s model for the structure of DNA, which represented DNA as two helical strands wound together in a double helix that replicated semi-conservatively. The Watson-Crick Model for DNA later became the universally accepted DNA model. The Meselson-Stahl experiment enabled researchers to explain how DNA replicates, thereby providing a physical basis for the genetic phenomena of heredity and diseases.

The Meselson-Stahl experiment stemmed from a debate in the 1950s among scientists about how DNA replicated, or copied, itself. The debate began when James Watson and Francis Crick at the University of Cambridge in Cambridge, England, published a paper on the genetic implications of their proposed structure of DNA in May 1953. The Watson-Crick model represented DNA as two helical strands, each its own molecule, wound tightly together in a double helix. The scientists claimed that the two strands were complementary, which meant certain components of one strand matched with certain components of the other strand in the double helix.

With that model of DNA, scientists aimed to explain how organisms preserved and transferred the genetic information of DNA to their offspring. Watson and Crick suggested a method of self-replication for the movement of genetic information, later termed semi-conservative replication, in which DNA strands unwound and separated, so that each strand could serve as a template for a newly replicated strand. According to Watson and Crick, after DNA replicated itself, each new double helix contained one parent strand and one new daughter strand of DNA, thereby conserving one strand of the original double helix. While Watson and Crick proposed the semi-conservative model in 1953, the Meselson-Stahl experiment confirmed the model in 1957.

In 1954, Max Delbrück at Caltech published a paper that challenged the Watson-Crick Model for DNA replication. In his paper, Delbrück argued that the replication process suggested by Watson and Crick was unlikely because of the difficulty associated with unwinding the tightly-wound DNA structure. As an alternative, Delbrück proposed that instead of the entire structure breaking apart or unwinding, small segments of DNA broke from the parent helix. New DNA, Delbrück claimed, formed using the small segments as templates, and the segments then rejoined to form a new hybrid double helix, with parent and daughter segments interspersed throughout the structure.

After the release of Delbrück´s paper, many scientists sought to determine experimentally the mechanism of DNA replication, which yielded a variety of theories on the subject by 1956. Delbrück and Gunther Stent, a professor at the University of California, Berkeley, in Berkeley, California, presented a paper in June 1956 at a symposium at Johns Hopkins University in Baltimore, Maryland, which named and summarized the three prevailing theories regarding DNA replication at the time: semi-conservative, dispersive, and conservative. Delbrück and Stent defined conservative replication as a replication mechanism in which a completely new double helix replicated from the parent helix, with no part of the parent double helix incorporated into the daughter double helix. They described the semi-conservative process as Watson and Crick suggested, with half of the parental DNA molecule conserved in the daughter molecule. Lastly, Delbrück and Stent summarized Delbrück´s dispersive model, in which parental DNA segments distribute throughout the daughter DNA molecule. Delbrück and Stent´s paper provided the background for the Meselson-Stahl experiment.

In 1954, prior to publication of Delbrück´s initial challenge of the Watson-Crick model, Matthew Meselson and Franklin Stahl had joined the DNA replication discussion. During the spring of 1954, Meselson, a graduate student studying chemistry at Caltech, visited Delbrück´s office to discuss DNA replication. According to historian of science Frederic Holmes, during that meeting Meselson began brainstorming ways to determine how DNA replicated. In the summer of 1954, Meselson met Stahl at the Marine Biological Laboratory in Woods Hole, Massachusetts. Stahl, a graduate student studying biology at the University of Rochester in Rochester, New York, agreed to study DNA replication with Meselson the following year at Caltech.

Meselson and Stahl began their collaboration in late 1956. By that time, Stahl had completed his PhD and Meselson had completed the experiments for his PhD, which he received in 1957. They worked on a variety of projects, including DNA replication. All of their projects, however, involved a method first devised by Meselson in 1954, called density-gradient centrifugation. Density-gradient centrifugation separates molecules based on their densities, which depend on the molecular weights of the molecules.

Meselson and Stahl used density-gradient centrifugation to separate different molecules in a solution, a method they later used to separate DNA molecules in a solution. In density gradient centrifugation, a solution is placed in an ultracentrifuge, a machine that spins the samples very fast on the order of 140,000 times the force of gravity or 44,770 revolutions per minute (rpm). As the samples spin, denser substances are pushed toward the bottom, while less dense substances distribute according to their weight in the centrifuge tube. By the end of centrifugation, the molecules reach a position called equilibrium, in which the molecules stop moving and remain in a gradient. The position of the molecules at equilibrium is dependent on the density of the molecule. Meselson and Stahl measured the areas in which DNA was at the highest concentration. Higher concentrations were represented by darker bands of DNA in the centrifuged sample. Stahl represented those bands on a graph, so that the peaks represented locations in the gradient where there was the highest concentration of molecules. Multiple peaks meant that molecules of different densities separated out of the solution.

To describe how DNA replicated, Meselson and Stahl needed to distinguish between parental and daughter DNA. They achieved that by modifying the molecules so each kind had a different density. Then Meselson and Stahl could separate the molecules using density-gradient centrifugation and analyze how much parental DNA was in the new daughter helices after every replication cycle. First they tried to alter the density of parental DNA by substituting a one nucleotide base, thymidine, with a heaver but similar DNA nucleotide base, 5-bromouriacil (5-BU). However, Meselson and Stahl struggled to substitute enough units of 5-BU into the DNA molecules to make the parental DNA significantly denser than normal DNA.

By July 1957, Meselson and Stahl successfully incorporated the heavy substitution in parental DNA, but the type of DNA they used still caused problems. Meselson and Stahl first used DNA from a specific type of virus that infects bacteria, called a bacteriophage. However, bacteriophage DNA not only broke apart in solution during centrifugation, but also replicated too quickly for the distribution of DNA to be adequately measured after each cycle. Consequently, Meselson and Stahl struggled to see clear locations within the density gradient with the highest concentration of bacterial DNA. Therefore, in September 1957, Meselson and Stahl switched to using the DNA from the bacteria Escherichia coli (E. coli) . E. coli DNA formed clearer concentration peaks during density gradient centrifugation.

At around the same time, in addition to changing the source DNA, Meselson and Stahl also changed the type of density label they used, from substitution labels to isotope labels. An isotope of an element is an atom with the same number of positive charged nuclear particles or protons, and a different number of uncharged particles, called neutrons. A difference in neutrons, for the most part, does not affect the chemical properties of the atom, but it alters the weight of the atom, thereby altering the density. Meselson and Stahl incorporated non-radioactive isotopes of nitrogen with different weights into the DNA of E. coli . As DNA contains a large amount of nitrogen, so long as the bacteria grew in a medium containing nitrogen of a specified isotope, the bacteria would use that nitrogen to build DNA. Therefore, depending on the medium in which E. coli grew, daughter strands of newly replicated DNA would vary by weight, and could be separated by density-gradient centrifugation.

Starting in October 1957, Meselson and Stahl conducted what later researches called the Meselson-Stahl experiment. They grew E. coli in a medium containing only the heavy isotope of nitrogen ( 15 N) to give the parental DNA a higher than normal density. As bacteria grow, they duplicate, thereby replicating their DNA in the process. The researchers then added an excess of light isotopes of nitrogen ( 14 N) to the heavy nitrogen environment.

Meselson and Stahl grew E. coli in the 14 N isotope environment for all subsequent bacterial generations, so that any new DNA strands produced were of a lower density than the original parent DNA. Before adding 14 N nitrogen, and for intervals of several bacterial generations after adding light nitrogen, Meselson and Stahl pulled samples of E. coli out of the growth medium for testing. They centrifuged each sample for initial separation, and then they added salt to the bacteria so that the bacteria released its DNA contents, allowing Meselson and Stahl to analyze the samples.

Next, Meselson and Stahl conducted density gradient centrifugation for each DNA sample to see how the parental and daughter DNA distributed according to their densities over multiple replications. They added a small amount of each sample of bacterial DNA to a cesium chloride solution, which when centrifuged had densities within the range of the bacterial DNA densities so that the DNA separated by density. The researchers centrifuged the DNA in an ultracentrifuge for twenth hours until the DNA reached equilibrium. Using ultraviolet light (UV), the researchers photographed the resulting DNA bands, which represented peaks of DNA concentrations at different densities. The density of the DNA depended on the amount of 15 N or 14 N nitrogen present. The more 15 N nitrogen atoms present, the denser the DNA.

For the bacterial DNA collected before Meselson and Stahl added 14 N nitrogen, the UV photographs showed only one band for DNA with 15 N nitrogen isotopes. That result occurred because the DNA from the first sample grew in an environment with only 15 N nitrogen isotopes. For samples pulled during the first replication cycle, the UV photographs showed fainter the 15 N DNA bands, and a new DNA band formed, which represented half 15 N DNA nitrogen isotopes and half 14 N DNA nitrogen isotopes. By the end of the first replication cycle, the heavy DNA band disappeared, and only a dark half 15 N and half 14 N DNA band remained. The half 15 N half 14 N DNA contained one subunit of 15 N nitrogen DNA and one subunit of 14 N nitrogen DNA. The data from the first replication cycle indicated some distribution of parental DNA, therefore ruled out conservative replication, because only parental DNA contained 15 N nitrogen isotopes and only parental DNA could represent the 15 N nitrogen isotopes in daughter DNA.

The same trends continued in future DNA replication cycles. As the bacteria continued to replicate and the bacterial DNA replicated, UV photographs showed that the band representing half 15 N half 14 N DNA depleted. A new band, representing DNA containing only 14 N nitrogen isotopes or light DNA, became the prevalent DNA band in the sample. The depletion of the half 15 N half 14 N band occurred because Meselson and Stahl never re-introduced 15 N nitrogen, so the relative amount of 15 N nitrogen DNA decreased. Meselson and Stahl then mixed the samples pulled from different replication cycles and centrifuged them together. The UV photograph from that run showed three bands of DNA with the half 15 N half 14 N DNA band at the midpoint between the 15 N DNA band and 14 N DNA band, making it an intermediate band. The result indicated that the half 15 N half 14 N DNA band had a density exactly between the 15 N and 14 N nitrogen DNA, showing that the DNA in the central band contained half of the 15 N nitrogen and half of the 14 N nitrogen isotopes, just as predicted by the Watson and Crick model. The exact split between heavy and light nitrogen characterized semi-conservative DNA replication.

Meselson and Stahl made three conclusions based on their results. First, they concluded that the nitrogen in each DNA molecule divided evenly between the two subunits of DNA, and that the subunits stayed intact throughout the observed replication cycles. Meselson and Stahl made that conclusion because the intermediate band had a density halfway between the heavy and light DNA bands. That conclusion made by Meselson and Stahl challenged the dispersive mechanism suggested by Delbrück, which involved breaking the DNA subunits into smaller pieces.

Meselson´s and Stahl´s second conclusion stated that each new DNA double helix contained one parental subunit, which supported semi-conservative replication. Assuming that DNA consists of two subunits, if a parent passes on one subunit of DNA to its offspring, then half of the parental DNA is conserved in the offspring DNA, and half of the parental DNA is not. The researchers made that conclusion because if parental DNA did not replicate in that way, then after the first replication, some DNA double helices would have contained only parental heavy nitrogen subunits or only daughter light nitrogen subunits. That type of replication would have indicated that that some parental DNA subunits did not separate in the semi-conservative fashion, and instead would have supported conservative replication. The presence of one parental subunit for each daughter DNA double helix supported semi-conservative replication.

The third conclusion made by Meselson and Stahl stated that for every parental DNA molecule, two new molecules were made. Therefore, the amount of DNA after each replication increased by a factor of two. Meselson and Stahl related their findings to the structure of DNA and replication mechanism proposed by Watson and Crick.

Before Meselson and Stahl published their findings, word of the Meselson-Stahl results spread throughout Caltech and the scientific community. According to Holmes, Delbrück, who had strongly opposed the semi-conservative method of DNA replication, immediately accepted DNA replication as semi-conservative after seeing the results from the Meselson-Stahl experiment. Some experiments earlier that year had pointed towards semi-conservative replication, and the Meselson-Stahl experiment served to further support semi-conservative replication.

Despite the positive reception of the Meselson-Stahl experiment, years passed before scientists fully accepted the Watson-Crick Model for DNA based on the findings from the Meselson-Stahl experiment. The Meselson-Stahl experiment did not clearly identify the exact subunits that replicated in DNA. In the Watson and Crick model, DNA consisted of two one-stranded DNA subunits, but the Meselson-Stahl experiment also supported models of DNA as having more than two strands. In 1959, Liebe Cavalieri, a scientist at the Sloan-Kettering Institute for Cancer research in New York City, New York, and his research team had produced evidence supporting the theory that DNA consisted of two two-stranded subunits, making DNA a quadruple helix. Cavalieri´s proposal did not contradict the Meselson-Stahl experiment, because the Meselson-Stahl experiment did not define DNA subunits. However, later experiments performed by Meselson on bacteriophage DNA from 1959 to 1961, and experiments performed by John Cairns on E. coli DNA in 1962, settled the debate and showed that each subunit of DNA was a single strand.

As described by Holmes, many scientists highly regarded the Meselson-Stahl experiment. Scientists including John Cairns, Gunther Stent, and James Watson all described the experiment as beautiful in both its performance and simplicity. Holmes also described the academic paper published by Meselson and Stahl on their experiment as beautiful because of its concise descriptions, diagrams, and conclusions. The Meselson-Stahl experiment appeared in textbooks decades after Meselson and Stahl performed the experiment. In 2001, Holmes published Meselson, Stahl, and the Replication of DNA: A History of "The Most Beautiful Experiment in Biology," which told the history of the experiment.

The Meselson-Stahl experiment gave a physical explanation for the genetic observations made before it. According to Holmes, for scientists who already believed that DNA replicated semi-conservatively, the Meselson-Stahl experiment provided concrete evidence for that theory. Holmes stated that, for scientists who contested semi-conservative replication as proposed by Watson and Crick, the Meselson-Stahl experiment eventually changed their opinions. Either way, the experiment helped scientists´ explain inheritance by showing how DNA conserves genetic information throughout successive DNA replication cycles as a cell grows, develops, and reproduces.

• Cairns, John. "A Minimum Estimate for the Length of the DNA of Escherichia coli Obtained by Autoradiography." Journal of Molecular Biology 4 (1962): 407–9.
• Cavalieri, Liebe F., Barbara Hatch Rosenberg, and Joan F. Deutsch. "The Subunit of Deoxyribonucleic Acid." Biochemical and Biophysical Research Communications 1 (1959): 124–8.
• Davis, Tinsley H. "Meselson and Stahl: The Art of DNA Replication." Proceedings of the National Academy of Sciences 101 (2004): 17895–6. http://www.pnas.org/content/101/52/17895.long (Accessed April 18, 2017).
• Delbrück, Max. "On the Replication of Deoxyribonucleic Acid (DNA)." Proceedings of the National Academy of Sciences 40 (1954): 783–8. http://www.pnas.org/content/40/9/783.short (Accessed April 18, 2017).
• Delbrück, Max and Gunther S. Stent. "On the Mechanism of DNA Replication." In McCollum-Pratt Symposium on the Chemical Basis of Heredity , eds. William D. McElroy and Bentley Glass, 699–736. Baltimore: Johns Hopkins University Press, 1956.
• Holmes, Frederic L. Meselson, Stahl, and the Replication of DNA: a History of "The Most Beautiful Experiment in Biology." New Haven: Yale University Press, 2001.
• "Interview with Matthew Meselson." Bioessays 25 (2003): 1236–46.
• Judson, Horace Freeland. The Eighth Day of Creation: Makers of the Revolution in Biology . Cold Spring Harbor: Cold Spring Harbor Laboratory Press, 1996.
• Levinthal, Cyrus. "The Mechanism of DNA Replication and Genetic Recombination in Phage." Proceedings of the National Academy of Sciences 42 (1956): 394–404. http://www.pnas.org/content/42/7/394.short (Accessed April 18, 2017).
• Litman, Rose M. and Arthur B. Pardee. "Production of Bacteriophage Mutants by a Disturbance of Deoxyribonucleic Acid Metabolism." Nature 178 (1956): 529–31.
• Meselson, Matthew. "The Semi-Conservative Replication of DNA." iBioMagazine 5 (2011). https://www.ibiology.org/ibiomagazine/issue-5/matthew-meselson-the-semi-conservative-replication-of-dna.html (Accessed April 18, 2017).
• Meselson, Matthew, and Franklin W. Stahl. "The Replication of DNA in Escherichia Coli." Proceedings of the National Academy of Sciences 44 (1958): 671–82. http://www.pnas.org/content/44/7/671.long (Accessed April 18, 2017).
• Meselson, Matthew, and Jean Weigle. "Chromosome Breakage Accompanying Genetic Recombination in Bacteriophage." Proceedings of the National Academy of Sciences 47 (1961): 857–68. http://www.pnas.org/content/47/6/857.short (Accessed April 18, 2017).
• Meselson, Matthew, Franklin W. Stahl, and Jerome Vinograd. "Equilibrium Sedimentation of Macromolecules in Density Gradients." Proceedings of the National Academy of Sciences 43 (1957): 581–8. http://www.pnas.org/content/43/7/581.short (Accessed April 18, 2017).
• Taylor, J. Herbert, Philip S. Woods, and Walter L. Hughes. "The Organization and Duplication of Chromosomes as Revealed by Autoradiographic Studies Using Tritium-Labeled Thymidine." Proceedings of the National Academy of Sciences 43 (1957): 122–8. http://www.pnas.org/content/43/1/122.short (Accessed April 18, 2017).
• Watson, James D., and Francis H C Crick. "Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid." Nature 171 (1953): 737–8. https://profiles.nlm.nih.gov/ps/access/SCBBYW.pdf (Accessed April 18, 2017).
• Watson, James D., and Francis H C Crick. "Genetical Implications of the Structure of Deoxyribonucleic Acid." Nature 171 (1953): 964–7. https://profiles.nlm.nih.gov/ps/access/SCBBYX.pdf (Accessed April 18, 2017).
• Weigle, Jean, and Matthew Meselson. "Density Alterations Associated with Transducing Ability in the Bacteriophage Lambda." Journal of Molecular Biology 1 (1959): 379–86.

How to cite

Articles rights and graphics.

Copyright Arizona Board of Regents Licensed as Creative Commons Attribution-NonCommercial-Share Alike 3.0 Unported (CC BY-NC-SA 3.0)  

Last modified

Share this page.

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

• View all journals
• Explore content
• About the journal
• Publish with us
• Sign up for alerts
• 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

4678 Accesses

2 Altmetric

Metrics details

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.

Author information

Authors and affiliations.

Cold Spring Harbor Laboratory, Cold Spring Harbor, New York, USA

Bruce Stillman

You can also search for this author in PubMed   Google Scholar

Rights and permissions

Reprints and permissions

About this article

Cite this article.

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

Download citation

Issue Date : 01 March 2002

DOI : https://doi.org/10.1038/nm0302-211

Share this article

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative

Quick links

• Explore articles by subject
• Guide to authors
• Editorial policies

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

• Illustrations
• Problem Sets
• Calculators

You are here

Meselson–stahl experiment.

In their second paper on the structure of DNA * , Watson and Crick (pdf) described how DNA's structure suggests a pattern for replication:

"…prior to duplication the hydrogen bonds are broken, and the two chains unwind and separate. Each chain then acts as a template for the formation onto itself of a new companion chain, so that eventually we shall have two pairs of chains, where we only had one before." - Watson and Crick, 1953

This is called semiconservative replication .

Today we know that this is the pattern used by living cells, but the experimental evidence in support of semiconservative replication was not published until 1958 . In the 5 years between Watson and Crick's suggestion and the definitive experiment, semiconservative replication was controversial and other patterns were considered.

Three hypothesized patterns were proposed:

• Semiconservative - The original double strand of DNA separates and each strand acts as a template for the synthesis of a complimentary strand.
• Conservative replication - the original double strand of DNA remains intact and is used as a template to create a new double stranded molecule.
• Dispersive replication - similar to conservative replication in that the original double strand is used as a template without being separated, but prior to cell division, the strands recombine such that each daughter cell gets a mix of new and old DNA. With each round of replication, the original DNA gets cut up and dispersed evenly between each copy.

The methods Meselson and Stahl developed allowed them to distinguish existing DNA from newly synthesized DNA and to track new and old DNA over several rounds of replication.

They accomplished this by labeling cells with different stable isotopes of nitrogen. First, a culture of bacterial cell were grown for several generations in a media containing only 15 N ( a stable, heavy isotope of Nitrogen). After this period * of growth, all of the DNA in the cells contained 15 N. These cells were then rinsed and put into a media containing only the more common, lighter isotope of nitrogen ( 14 N). As the cells grew and divided in this fresh media, all newly synthesized DNA would contain only the lighter nitrogen isotope, while DNA from the original cells would still contain 15 N. In this illustration above, 15 N labeled DNA is shown in orange and 14 N labeled in green.

The 15 N and 14 N labeled DNA was then tracked using high speed centrifugation and a density * gradient created with cesium chloride (CsCl).

During centrifugation in a CsCl gradient, DNA accumulates in bands along the gradient based on its density. Since 15 N is more dense than 14 N, 15 N enriched DNA accumulates lower down in the centrifuge tube than the 14 N DNA. DNA containing a mixture of 15 N and 14 N ends up in an intermediate position between the two extremes.

By spinning DNA extracted at different times during the experiment, Meselson and Stahl were able to see how new and old DNA interacted during each round of replication.

The beauty of this experiment was that it allowed them to distinguish between the three different hypothesized replication patterns. The key result occurs at the second generation when all three proposed replication patterns give different results in the CsCl gradient.

That Meselson and Stahl's experiment showed the pattern predicted by the semiconservative hypothesis provided the definitive experimental evidence in support of the process proposed by Watson and Crick.

Related Content

• Complementary Nucleotide Bases
• DNA Polymerase
• Semiconservative Replication
• Nucleotides in DNA practice problems
• Complementary nucleotide bases concept questions
• DNA Polymerase concept questions
• DNA replication

Subject tag:

• Printer-friendly version

Search form

Latest illustrations.

• Ocean Gyres and Geostrophic Flow
• Langmuir Circulation
• Test sensitivity - specificity calculator
• How earthquakes show us the inside of the Earth
• Surface currents, the Ekman spiral, and Ekman transport

© Andrew Staroscik 2011-2022