rosalind franklin x ray crystallography experiment

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DNA (deoxyribonucleic acid): molecular instructions that guide how all living things develop and function... more

Double Helix: nucleic acid double helix is a biology term used to describe the molecule of DNA and RNA... more

Helix: a smooth spiral curve. Helices (plural) can be either right-handed or left-handed. Springs are an example of a helix... more

Nobel Prize: a set of awards given each year in Physics, Chemistry, Physiology or Medicine, Literature, and Peace. Named after Alfred Nobel who on his death gave most of his fortune to establish the prize... more

Posthumous: an honor or award given after someone has died.

RNA: an acid found in all living things that carries messages from DNA to the rest of the cell to be made into protein.

Scavenger hunt: a game where players search and gather items on a list.

Virus: a super tiny germ that you can only see with a microscope. Viruses need a host in order to reproduce... more

Rosalind Franklin and the DNA Scavenger Hunt

In the early 1950s biologists were searching for the answers to some of the most important science questions left unanswered. How is information stored inside living cells? Could there be only one way these instructions were packaged? If there is, what does it look like? How did it work? All of these questions were an important part of biology and many scientists were racing to find the answers.

This portrait of Franklin was taken during her second visit to the United States. (National Library of Medicine NIH)

The answer came from a group of scientists who were working on their own projects as well as a few who were on a giant scientific scavenger hunt. James Watson and Francis Crick were two researchers who spent their time piecing together information that other scientists had published. They also spent time talking with scientists who were busy in their labs running experiments. One of these scientists was Rosalind Franklin (25 July 1920 – 16 April 1958). She was an expert in a technique called X-ray crystallography. Her work would hold the key to discovering the structure of DNA , the blueprint of life.

A Scientist from a Young Age

At the age of 15, Rosalind Franklin decided she wanted to become a scientist. Her father did not like this at all, because it was not considered to be appropriate. Yet she was determined and stuck to her plan. It was not always easy though. From 1951 to 1953, Franklin worked at King’s College in London. Her gender and her upper-class background made life difficult. It seems that some of her colleagues sneered at the way she spoke. On top of this women were not even allowed to enter the senior common room. This made her very angry, because many male colleagues had lunch there. However, none of this stopped Rosalind Franklin from making crucial contributions to science.

Contributions to Science

Rosalind Franklin used a technique called X-ray crystallography to find out the 3D shape of molecules. She applied this technique to different samples. Early in her career she worked on carbon and coal. Later she started working on biological subjects. She made major contributions to the discovery of the shape of DNA. After her work on this molecule, she also gave new insights into the first virus that was ever discovered: the Tobacco Mosaic Virus. She thought the virus might be hollow and only consist of one strand of RNA. Although no proof existed at that time, she turned out to be right. Unfortunately, this was not confirmed until after her death.

Two views of a tobacco mosaic virus. The side view (left) shows the helical shape of the virus. The top view (right) shows the opening in the center of the helix. Click on the image to see it larger and read more.

In 1962, James Watson, Francis Crick and Maurice Wilkins got the Nobel Prize for the discovery of the shape of DNA. Photo 51 was an X-ray diffraction image that gave them some crucial pieces of information. It was only after seeing this photo that Watson and Crick realized that DNA must have a double helical structure.

The problem was that Photo 51 was actually made by Rosalind Franklin. Maurice Wilkins, a colleague, had shown this picture to Watson and Crick without even letting her know. This added to the tension at the time of the discovery of DNA. Unlike her colleagues, Franklin was not awarded a Nobel Prize for her contributions to this important discovery. She died in 1958 and the Nobel Prize cannot be obtained posthumously.

While a lot of Rosalind Franklin's work used X-ray crystallography she also used other X-ray diffraction techniques. Her famous image of DNA called Photo 51 was made using a X-ray technique that did not require the sample to be in crystal form. She used this method since DNA, like some other big molecules, does not like to form a crystal. Instead, DNA prefers to form organized fibers. Photo 51 still shows the classic diffraction pattern, but in this case the sample still contained water and was not a crystal.

Picture of the famous Franklin X-ray : Sodium deoxyribose nucleate from calf thymus, Structure B, Photo 51, taken by Rosalind E. Franklin and R.G. Gosling. Linus Pauling's holographic annotations are to the right of the photo. May 2, 1952.

References:

Janus, The Papers of Rosalind Franklin. Retrieved May 2012 from Janus

Merry Maisel and Laura Smart, Science Women, Rosalind Elsie Franklin, (1997). Retrieved May 2012 from https://www.sdsc.edu/ScienceWomen/franklin.html

David Ardell, Biotech Chronicles , Rosalind Franklin (1920-195), (October 25, 2006). Retrieved May 2012 from accessexcellence.org/RC/AB/BC/Rosalind_Franklin.html

Martha Keyes , Contributions of 20th century Women to Physics, Rosalind Franklin, (May 16, 1997). Retrieved May 2012 but now at http://cwp.library.ucla.edu/Phase2/Franklin,[email protected]

David Goodsell. Molecule of the Month. January 2009. Retrieved August 30, 2012 from https://pdb101.rcsb.org/motm/109

Photograph of Rosalind Franklin and Photo 51 : Ask A Biologist tries to ensure proper permissions before posting items on this website. For these images we have not been able to identify or contact the current copyright owner. If you have information regarding the copyright owner, please contact Ask A Biologist using the feedback link in the gold box to the right.

Chicago Manual of Style

Martine Oudenhoven. "Rosalind Franklin - DNA". ASU - Ask A Biologist. 19 August, 2012. https://askabiologist.asu.edu/Rosalind-Franklin-DNA

MLA 2017 Style

Martine Oudenhoven. "Rosalind Franklin - DNA". ASU - Ask A Biologist. 19 Aug 2012. ASU - Ask A Biologist, Web. 12 Jun 2024. https://askabiologist.asu.edu/Rosalind-Franklin-DNA

A collection of protein and virus crystals including the satellite tobacco mosaic virus. All were grown in space.

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Photograph 51, by Rosalind Franklin (1952)

On 6 May 1952, at King´s College London in London, England, Rosalind Franklin photographed her fifty-first X-ray diffraction pattern of deoxyribosenucleic acid, or DNA. Photograph 51, or Photo 51, revealed information about DNA´s three-dimensional structure by displaying the way a beam of X-rays scattered off a pure fiber of DNA. Franklin took Photo 51 after scientists confirmed that DNA contained genes. Maurice Wilkins, Franklin´s colleague showed James Watson and Francis Crick Photo 51 without Franklin´s knowledge. Watson and Crick used that image to develop their structural model of DNA. In 1962, after Franklin´s death, Watson, Crick, and Wilkins shared the Nobel Prize in Physiology or Medicine for their findings about DNA. Franklin´s Photo 51 helped scientists learn more about the three-dimensional structure of DNA and enabled scientists to understand DNA´s role in heredity.

X-ray crystallography, the technique Franklin used to produce Photo 51 of DNA, is a method scientists use to determine the three-dimensional structure of a crystal. Crystals are solids with regular, repeating units of atoms. Some biological macromolecules, such as DNA, can form fibers suitable for analysis using X-ray crystallography because their solid forms consist of atoms arranged in a regular pattern. Photo 51 used DNA fibers, DNA crystals first produced in the 1970s. To perform an X-ray crystallography, scientists mount a purified fiber or crystal in an X-ray tube. The X-ray tube generates X-rays that strike the purified material. X-rays are electromagnetic waves that have a shorter wavelength and higher energy than visible light. Because of their short wavelength, X-rays can pass through a crystal and interact with the electrons of the atoms within the crystal. When X-rays interact with electrons in a crystal the X-rays scatter, or diffract, at angles that indicate the arrangement of atoms in the crystal, or its structure. When the X-rays scatter, they strike a film mounted behind the crystal and leave a pattern of dark marks. The pattern of dark marks on the film gives scientists information about the structure of the crystal.

Scientists began collecting X-ray diffraction patterns of DNA in the 1930s before they confirmed that DNA contained genes. William Thomas Astbury, a crystallographer working at the University of Leeds in Leeds, England, gathered the first diffraction patterns of DNA in 1937. However, Astbury´’s diffraction patterns were blurry and difficult to interpret. At the time of Astbury´s experiments, scientists had determined the chemical composition of DNA. However, at that time scientists generally agreed that DNA merely provided structural support for cells and that protein must be genetic material. In 1944 Oswald Avery, Colin MacLeod and Maclyn McCarty published an experiment that isolated DNA as the material that contained genes.

Maurice Wilkins, a scientist working at King´s College London, collected X-ray diffraction patterns of DNA in 1950. Wilkins and his graduate student, Raymond Gosling, later Franklin´s graduate student, collected X-ray diffraction patterns of DNA purified in a way that produced longer fibers than those accessible to Astbury. When mounting the DNA fibers for viewing, Wilkins and Gosling were able to bundle many of the thin fibers together and pull them tight to provide a larger sample to better diffract X-rays. Furthermore, the two researchers kept the DNA fibers wet with water by keeping them in a humid environment. The resulting X-ray diffraction pattern of DNA was of a higher quality than any patterns collected prior.

Franklin, a specialist in X-ray crystallography, continued previous X-ray crystallography experiments on DNA with Gosling when she joined the King´s College London lab in 1951. Before joining the lab, Franklin conducted X-ray diffraction experiments on carbon compounds at a government lab in Paris, France, and published several papers on X-ray crystallography of coal and coal compounds. Throughout Franklin´s early work at King´s College London, she found that DNA fibers with a higher water content produced a different diffraction pattern than DNA fibers with a lower water content, indicating that wet and dry DNA adopted different three-dimensional conformations. Franklin later defined the drier DNA conformation as the A-Form DNA and the wetter DNA conformation as B-Form DNA. As of 2018, scientists continue to use the A Form and B Form designations for the two conformations of DNA. In addition to identifying the two forms of DNA, Franklin determined that Asbury´s diffraction patterns of DNA came from a mixture of A and B-Forms of DNA.

By improving her methods of collecting DNA X-ray diffraction images, Franklin obtained Photo 51 from an X-ray crystallography experiment she conducted on 6 May 1952. First, she minimized how much the X-rays scattered off the air surrounding the crystal by pumping hydrogen gas around the crystal. Because hydrogen only has one electron, it does not scatter X-rays well. She pumped hydrogen gas through a salt solution to maintain the targeted hydration of the DNA fibers. Franklin tuned the salt concentration of the solution and the humidity surrounding the crystal to keep DNA entirely in the B-Form. After exposing the DNA fibers to X-rays for a total of sixty-two hours, Franklin collected the resulting diffraction pattern and labeled it Number 51 that became Photo 51.

Photo 51 presents a clear diffraction pattern for B-Form DNA. The outermost edge of the diffraction pattern consists of a black diamond shape. The diamond has rounded corners with the darkest corners situated at the top and bottom of the film. The diamond shape of the DNA diffraction pattern is not made of fine, definite lines, but rather thick, fuzzy boarders that vary in darkness such that the boarders fade on the left and right hand sides of the film. Inside the diamond is a cross shape like the letter "X." The X shape is not made of continuous lines. Instead, along each line of the X are four horizontal dashes, called spots that become darker moving closer to the center of the film. There is a hole at the center of the film, with dark spots lining the outside of the center hole.

Researchers could interpret an X-ray diffraction pattern of DNA with knowledge about DNA´s composition, which scientists had at the time Franklin collected photo 51. Years prior to Franklin´s work, scientists determined that DNA consists of a chain of repeating units called nucleotides. Each nucleotide has three key features. Each nucleotide consists of a center sugar ring called deoxyribose. Attached to one end of the deoxyribose ring is a negatively charged phosphate group consisting of phosphorus and oxygen atoms. Attached to the other end of the deoxyribose ring is a molecule called a base consisting of either single or double rings of carbon and nitrogen. There are four types of bases in DNA.

Using the available knowledge about DNA´s composition and mathematical techniques, Franklin learned of some key features regarding the structure of B-Form DNA from Photo 51. The presence of the X shape in the diffraction pattern indicated to Franklin that DNA strands were helical. Each dash of the X shape marks the repetition of atoms, or atomic repeats, in DNA. Therefore, based on the distances between the dashes, Franklin determined the distance between nucleotides, the smallest repeating units in DNA. The angles of the X shape revealed to Franklin the radius of DNA, or half the horizontal distance from one side of the molecule to the other. From the distance between the top and bottom of the outer diamond shape, Franklin found that there are ten nucleotides between each turn of the DNA molecule. Lastly, the lighter nature of the diamond on the top and bottom of the film showed Franklin that the DNA bases face the inside of the helix whereas the phosphate groups face outside. With knowledge of the density, mass per unit volume, of her DNA samples, Franklin also concluded that DNA contained two strands. While Franklin obtained Photo 51 in May 1952, she did not complete her analysis of Photo 51 until early 1953.

In January 1953, Watson visited King´s College London. While visiting, Wilkins showed Watson one of Franklin´s X-ray diffraction images of DNA, which historians claim was one of the clearest image of DNA, Photo 51, without Franklin´s knowledge. From the image, Watson concluded that DNA was helical. During his meeting with Wilkins, Watson also obtained necessary dimensions of DNA derived from Photo 51 that he and Crick later used to develop their proposed structure of DNA. Later, Watson and Crick received an internal King´s College London research report written by Franklin about her DNA diffraction images. From that report, Crick determined that DNA contains two strands, with each strand running in opposite directions.

Watson and Crick, two scientists at the University of Cambridge in Cambridge, England, relied on Franklin´s Photo 51 to propose a three-dimensional structure of DNA and in April 1953, they suggested a three-dimensional structure of DNA partly based on Photo 51. The model they suggested consisted of two helical strands of repeating nucleotides wound around each other making a double helix. The double helix had ten nucleotides between each turn. The phosphate groups faced outside the double helix and the DNA bases faced horizontally inward of the helix. The two strands held together through interactions between the bases of each strand. The DNA strands ran in opposite directions. As of 2019, Watson and Crick´s proposed DNA structure has remained the verified structure with a few variations of B-Form DNA, the major form of DNA in living cells.

Later, in May 1953, Watson and Crick proposed a replication mechanism for DNA using their DNA structure. Their replication mechanism, later called semi-conservative replication, described how to copy the DNA molecule that contained the genes and to pass the genes from cell to cell and from parent to offspring. Many features of B-Form DNA present in Photo 51 are necessary for semi-conservative replication, such as the DNA bases facing horizontally inward in the double helix. In addition, some aspects of B-Form DNA as indicated in Photo 51 posed challenges for semi-conservative replication. Watson and Crick proposed that the DNA strands needed to unwind and separate in order to replicate. However, because of the helical nature of DNA, as shown in the X-ray diffraction pattern of Photo 51, some scientists argued that the DNA strands would be too difficult to unwind and separate. Some years passed before scientists accepted semi-conservative replication due to the perceived difficulty of unwinding the helical strands.

For their findings related to DNA, Watson, Crick, and Wilkins received the 1962 Nobel Prize in Physiology or Medicine. Franklin also contributed to understanding DNA structure, especially through her collection of Photo 51. She also determined many important features about DNA´s structure independently using Photo 51. The award of the Nobel Prize is never posthumously and Franklin died in 1958 before the award of the 1962 Nobel Prize. Some controversy and speculation surrounds the 1962 Nobel Prize concerning Franklin and her contributions to Watson and Crick´s DNA model. Only after the publication of Watson´s, book The Double Helix: A Personal Account of the Discovery of the Structure of DNA in 1968 was the roll that Franklin played in the discovery of the structure of DNA realized.

Photo 51, a clear X-ray diffraction pattern of DNA, showed structural features of DNA necessary for scientific understanding of DNA´s three-dimensional structure. By understanding DNA structure, scientists could learn about how DNA functioned as genetic material. The DNA structure revealed in Photo 51 related the essential functions of a gene how its information is preserved and carried from cells to cell and from parent to offspring.

Asbury, William Thomas, Sylvia Dickinson, and Kenneth Bailey. "The X-ray Interpretation of Denaturation and the Structure of the Seed Globulins." Biochemical Journal 10 (1935): 2351–60. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1266766/
Avery, Oswald, Theodore, Colin Munro MacLeod, and Maclyn McCarty. "Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types: Induction of Transformation by Deoxyribonucleic Acid Fraction Isolated from Pneumococcus Type III." Journal of Experimental Medicine 79 (1944): 134–58. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2135445/pdf/137.pdf (Accessed March 8, 2018).
Franklin, Rosalind E., "Influence of the Bonding Electrons on the Scattering of X-Rays by Carbon." Nature 165 (1950): 71–2.
Franklin, Rosalind E. and Raymond G. Gosling. "Molecular Configuration in Sodium Thymonucleate." Nature 171 (1953): 740–1.
Hamilton, Leonard D.,”DNA: Models and Reality.” Nature , 18 (1968): 633–7
Judson, Horace Freeland. The Eighth Day of Creation. New York: Cold Spring Harbor: Cold Spring Harbor Laboratory Press, 1996.
Klug, Aaron. "Rosalind Franklin and the Discovery of the Structure of DNA." Nature 219 (1968): 808–10 and 843–4.
Klug, Aaron. "Rosalind Franklin and the Double Helix." Nature 248 (1974): 787–8.
Lucas, Amand A. "A-DNA and B-DNA: Comparing Their Historical X-ray Fiber Diffraction Images." Journal of Chemical Education 85 (2008): 737.
Lucas, Amand. A. Phillippe Lambin, Richard Mairesse, and Michel Mathot. "Revealing the Backbone Structure of B-DNA from Laser Optical Simulations of Its X-ray Diffraction Diagram." Journal of Chemical Education 76 (1999): 378.
Maddox, Brenda. Rosalind Franklin: The Dark Lady of DNA. London: HarperCollins Publishers, 2002.
Maddox, Brenda. "The Double Helix and the ´Wronged Heroine´." Nature 421 (2003): 407–8.
Marsh, Richard E. "Biographical Memoir of Robert Brainard Corey." National Academy of Sciences, 72 (1997) 51–69. https://www.nap.edu/read/5859/chapter/5 (Accessed January 21, 2019).
Sayre, Anne. Rosalind Franklin and DNA. New York: W. W. Norton & Company, 1975.
Watson, James D. The Double Helix: A Personal Account of the Discovery of the Structure of DNA. New York: Athenaeum Press, 1968.
Watson, James D., and Francis H.C. Crick. “Molecular Structure of Nucleic Acids.” Nature 171 (1953): 737–8. https://www.genome.gov/edkit/pdfs/1953.pdf (Accessed January 21, 2019).
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 January 21, 2019).

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Biology archive

Course: biology archive > unit 17.

DNA as the "transforming principle"
Hershey and Chase: DNA is the genetic material
Classic experiments: DNA as the genetic material
The discovery of the double helix structure of DNA

Discovery of the structure of DNA

Discovery of DNA

Introduction

The components of dna, chargaff's rules.

A, T, C, and G were not found in equal quantities (as some models at the time would have predicted)
The amounts of the bases varied among species, but not between individuals of the same species
The amount of A always equalled the amount of T, and the amount of C always equalled the amount of G (A = T and G = C)

Watson, Crick, and Rosalind Franklin

Watson and crick's model of dna, antiparallel orientation, right-handed helix, base pairing, the impact of the double helix, explore outside of khan academy, attribution:.

" DNA structure and sequencing ," by OpenStax College, BIology, CC BY 4.0 . Download the original article for free at http://cnx.org/contents/[email protected] .
" Historical basis of modern understanding ," by OpenStax College, Biology, CC BY 4.0 . Download the original article for free at http://cnx.org/contents/[email protected] .

Works cited:

Pray, L. A. (2008). Discovery of DNA structure and function: Watson and Crick. Nature Education , 1 (1), 100. Retrieved from http://www.nature.com/scitable/topicpage/discovery-of-dna-structure-and-function-watson-397 .
Aldridge, Susan. (2003). The DNA story. In Royal society of chemistry . Retrieved July 27, 2016 from http://www.rsc.org/chemistryworld/Issues/2003/April/story.asp .
Cobb, M. (2015, June 23). Sexism in science: Did Watson and Crick really steal Rosalind Franklin's data? The Guardian . Retrieved from http://www.theguardian.com/science/2015/jun/23/sexism-in-science-did-watson-and-crick-really-steal-rosalind-franklins-data .
The DNA riddle: King's College, London, 1951-1953. (n.d.) In The Rosalind Franklin papers . Retrieved from https://profiles.nlm.nih.gov/ps/retrieve/Narrative/KR/p-nid/187 .
Dugard, J. (2003, March 18). A grave injustice. Mail & Guardian . Retrieved from http://mg.co.za/article/2003-03-18-a-grave-injustice .
Tyson, P. (2003, April 22). Rosalind Franklin's legacy. In NOVA . Retrieved from http://www.pbs.org/wgbh/nova/tech/rosalind-franklin-legacy.html .
Rosalind Franklin. (2016, January 15). Retrieved January 15, 2016 from Wikipedia: https://en.wikipedia.org/wiki/Rosalind_Franklin .
Banquet speech: James Watson's speech at the Nobel banquet in Stockholm, December 10, 1962. (2016). In Nobelprize.org . Retrieved from http://www.nobelprize.org/nobel_prizes/medicine/laureates/1962/watson-speech.html .
Molecular structure and function: Evolution with a twist. (n.d.) In Biology-101: Brantley . Retrieved July 27, 2016 from http://www.science-projects.com/Helices.htm .
B-form, A-form, and Z-form of DNA. (2014, May 4). Retrieved July 27, 2016 from BioWiki: bio.libretexts.org/Core/Genetics/Unit_I%3A_Genes,_Nucleic_Acids,_Genomes_and_Chromosomes/2%3A_Structures_of_nucleic_acids/B-Form,_A-Form,_Z-Form_of_DNA.
Cambridge Physics. (n.d). A working model! In The structure of DNA: Crick and Watson, 1953 . Retrieved from http://www-outreach.phy.cam.ac.uk/camphy/dna/dna14_1.htm .
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 .

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Rosalind Franklin’s Overlooked Role in the Discovery of DNA’s Structure

By: Sarah Pruitt

Published: March 25, 2024

It’s one of the most famous moments in the history of science: On February 28, 1953, Cambridge University molecular biologists James Watson and Francis Crick determined that the structure of deoxyribonucleic acid, or DNA—the molecule carrying the genetic code unique to any individual—was a double helix polymer, a spiral consisting of two strands of DNA wound around one another.

Nearly 10 years later, Watson and Crick, along with biophysicist Maurice Wilkins, received the 1962 Nobel Prize in Physiology or Medicine for uncovering what they called the “secret of life.” Yet another person was missing from the award ceremony, whose work was vital to the discovery of DNA’s structure. Rosalind Franklin was a chemist and X-ray crystallographer who studied DNA at King’s College London from 1951 to 1953, and her unpublished data paved the way for Watson and Crick’s breakthrough.

An Unflattering Portrayal in Watson's Account

Franklin, who died of ovarian cancer in 1958 at the age of 37, was ineligible to receive the Nobel, which is not given posthumously. Yet debate over her role in the discovery of DNA’s structure and her failure to be recognized for it began simmering after the publication of Watson’s bestselling book The Double Helix: A Personal Account of the Discovery of the Structure of DNA in 1968 and its highly unflattering portrait of Franklin.

“Watson portrayed Franklin as this kind of evil figure—a schoolmarmish, shrewish person,” says Nathaniel Comfort, a historian of medicine at Johns Hopkins University who is working on a biography of the famed molecular biologist. Watson also related in his book that he and Crick had gained access to Franklin’s data without her knowledge, including the now-famous Photograph 51, an X-ray image of DNA that immediately convinced Watson that the molecule’s structure must be a helix.

Watson’s treatment of Franklin in The Double Helix provoked a robust backlash among those who viewed her as a victim of betrayal, sexism and misogyny, including Franklin’s friend Anne Sayre, who published a biography of Franklin in 1975 . Comfort argues that this view also obscures the more complicated truth of Franklin’s contributions. As he and Matthew Cobb argued in a 2023 article in Nature , a reconsideration of the available evidence suggests that Franklin should be recognized not as a martyr, but as an equal contributor to solving the double helix structure of DNA.

Rosalind Franklin: Expert Crystallographer

Rosalind Elsie Franklin. (Credit: Universal History Archive/Getty Images)

In 1951, Franklin joined a team of biophysicists led by John Randall at King’s College who were using X-ray crystallography to study DNA. The molecule had been discovered in 1869, but its structure and function weren’t yet understood. After learning X-ray crystallography at a government-run lab in France, she was already an expert in the scientific technique, which involves beaming X-rays at crystalline structures and taking photographs of the patterns created by atoms in the structures diffracting the X-rays. By measuring the sizes, angles and intensities of the patterns, researchers can create a 3-D picture of the crystalline structure.

From the beginning, Franklin famously clashed with Wilkins, who was Randall's deputy, and the two began working largely separately from one another. Wilkins had previously identified two forms of DNA appearing in the X-ray images; Franklin discovered that by adjusting the level of humidity in the specimen chamber, she could convert the crystalline, relatively dry “A” form of DNA into the wetter, paracrystalline “B” form. She shared these key insights into DNA at a seminar in November 1951, which Watson attended.

“Her notes for that lecture are very detailed,” Comfort says, adding that Franklin initially assumed both the A and B forms had a helical structure. “She describes DNA as a big helix, describes the two forms and lays out their differences…and [explains] how the structure switches from A to B depending on the relative humidity in the sample chamber.”

Franklin’s ‘Photograph 51’

Despite capturing clear evidence of the B form’s double helical structure—most notably in what became known as Photograph 51, taken in May 1952—Franklin chose to focus on the drier A form of DNA, which produced a much sharper, more detailed image than the B form. This focus pointed her away from the idea of a helix, because the A form did not appear to be helical.

“For a chemist and an X-ray crystallographer, she was doing the [form] that made the most sense,” Comfort says. “She wasn't a biologist, and so she didn't appreciate that in a living cell, the more hydrated B form was going to be much more present, because a cell is a very wet place.”

In February 1953, Wilkins showed Photograph 51 of the B form of DNA to his friend Watson at Cambridge, who along with Crick was attempting to determine the molecule’s structure mainly through building and analyzing physical models. Wilkins received the image from Raymond Gosling, who worked for both Wilkins and Franklin and had taken the photo with Franklin.

Watson later claimed that seeing Photograph 51 immediately convinced him that a DNA helix must exist. “The instant I saw the picture my mouth fell open and my pulse began to race,” he wrote in The Double Helix . Soon after that, Crick’s supervisor passed along a report on Franklin’s unpublished results, which he had received during a visit to the King’s College lab in December 1952. By late February 1953, Watson and Crick had constructed their model of the DNA double helix, which they formally announced in a landmark paper in Nature that April.

To Comfort, Watson’s version of events doesn’t ring entirely true when it comes to Photograph 51 and its importance. “Watson talks [in The Double Helix ] about realizing only then that there was an A and a B form…but Franklin talked about that at the end of 1951, and she and Wilkins talked about it openly,” Comfort says. “I think he was writing it as though the photograph was the magic key because it made a good discovery narrative that allowed him to boil down and communicate an enormously complex, highly technical kind of science.”

Franklin’s Understanding of DNA’s Structure

Comfort also discounts the idea that Franklin, an expert crystallographer, did not understand the significance of the X-ray diffraction image she and Gosling had taken of DNA’s B form 10 months earlier. “She was way too good for that,” he says.

In fact, Franklin was simply more focused on the A form of DNA at the time, and was also in the process of leaving King’s College behind for a new job at Birkbeck College, also in London. Before she left, however, Franklin started a new laboratory notebook, with notes on the B form of DNA.

By late February 1953, Franklin’s notes reveal that she had not only accepted that DNA had a helical structure, probably with two strands; she had also recognized that the component nucleotides, or bases, on each strand were related in a way that made the strands complementary, allowing the molecule to easily replicate. “Franklin’s colleague Aaron Klug analyzed her research notes and said that Franklin was ‘two steps away’ from the double helix,” Comfort says. “Given a couple more months, she surely would have had it.”

Both Wilkins and Franklin (with Gosling) published separate papers in the same April 1953 issue of Nature , largely supporting Watson and Crick’s model of DNA’s structure. The earliest presentation of the double helix that June was signed by authors of all three papers, suggesting—as Comfort and Cobb point out in their article—that the discovery of DNA was seen at the time as a joint effort, not just the triumph of Watson and Crick.

Taking Full Measure of Franklin’s Contributions

Over the next five years, Franklin led a team of researchers studying ribonucleic acid, or RNA, in viruses such as polio and the tobacco mosaic virus (TMV). Diagnosed with ovarian cancer in 1956, Franklin continued her work until days before her death in April 1958. Franklin also remained in regular contact with Watson and Crick after she left King’s College, even becoming good friends with Crick and his wife, Odile.

Franklin’s unjust exclusion from the Nobel Prize, combined with Watson’s decidedly sexist portrayal in The Double Helix led many to see her as a victim of chauvinism and betrayal. A more complicated view of events reveals a scientist who was an equal contributor to the discovery of DNA’s structure, as well as a trailblazer in the all-important field of virology.

“Franklin had an incredible series of insights into how the RNA is packed within the protein shell of TMV,” Comfort says. “She was widely recognized and seen as being at the top of her field.”

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Franklin's X-Ray Crystallography Experiments: Refractions & Reflections on the Nature of Science

Regular substances like crystals diffract X-rays in characteristic patterns according to their physical structure. The X-ray crystallograph at right (" P ho to 51 ") shows an exceptionally clear diffraction pattern of a crystallized DNA molecule. The X-pattern in the middle is characteristic of a helical molecule with regular repeats; the broad bands at top and bottom indicate the periodicity of the repeats. The photograph is of the highly hydrated B form of DNA , rather than the drier A form , which does not show a distinct helical structure. The photo does not, without mathematical analysis, indicate whether there are 2, 3, or 4 helices, which requires measurement of the intervals between elements of the X-pattern.

Under the supervision Rosalind Franklin (1920 - 1958), grad student Raymond Gosling (1926 - 2015), made Photo 51 in May 1952. Maurice Wilkins (1916 - 2004), working in the same lab group, with Gosling's assistance had previously made photos of the B form as early as 1951. Wilkins and Franklin had a severe personality crash, in part on their mutual misunderstanding that "the DNA problem" had been assigned to each of them, exclusively. Wilkins approached DNA as a biological problem, whereas Franklin approached DNA as a physical problem in crystalline structure of the A form.

Latter-day revisionism (e.g., BBC's TV documentary " The Secret of Photo 51 ") has suggested that Wilkins and (or) Watson gained improper access to Ph oto 51 , without her knowledge or permission, such that she was cheated out of proper recognition. Opinion and evidence vary as to how and when Franklin interpreted her evidence as bearing on the form of the DNA molecule. When she decided to leave Kings College, Franklin delivered her notebooks (including Photo 51) to Wilkins, with instructions to use them as he wished. Wilkins showed Photo 51 to Watson, who immediately realized that the X -structure implied two strands. In combination with his work on model building, the familiar " D ouble Helix " with paired bases on the inside quickly emerged. The DNA model and crystallographic evidence were published as twin papers in Nature .

Watson, Crick, & Wilkins subsequently received the Nobel Prize in 1962 for solving the structure of DNA . Wilkins had by then amassed a great deal of additional crystallographic evidence for the double-helical structure. Franklin had moved on to other crystallographic studies, notably the structure of Tobacco Mosaic and Polio viruses. In 1958, she died of cancer, possibly from exposure to X-rays . The Nobel is not awarded posthumously, nor to more than three persons. Watson's autobiographic account of the discovery of " The Double Helix " (1968) paints an unflattering personal portrait of Franklin, and has been widely criticized as inaccurate and sexist. Watson, Crick, and Wilkins repeatedly acknowledged that they could not have solved the structure without the crystallographic evidence.

HOMEWORK : Priority of discovery, acknowledgement of ideas, and ownership of data continue to be controversial topics in science. The story of the discovery of DNA structure is an exceptionally well-documented one. From the evidence and statements of participants, evaluate and comment on the following statements:

1) Watson and (or) Wilkins ripped off Franklin, who was badly treated because she was a woman. 2) If Gosling made Photo 51 while working for Franklin, who " owned it ", and who was entitled to see it? 3) " Grad students in those days were treated like serfs ." 4) " She was definitely anti-helical ." 5) " I showed them the base pairing, I wasn't properly acknowledged."

I. INTRODUCTION

Ii. three-dimensional helix versus a flat sinusoidal aperture, iii. diffraction from a helix: the x-shape pattern, a. experimental setup using a spring from a ballpoint pen, b. modelling the x-shape with a simple area integral, c. the missing fourth order and the double helix, iv. diffraction from sinusoidally arranged holes, a. experimental setup for diffraction from sinusoidally arranged holes, b. the height of the x-shape and the number of phosphates per pitch, v. conclusion, acknowledgments, rosalind franklin's x-ray photo of dna as an undergraduate optical diffraction experiment.

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J. Thompson , G. Braun , D. Tierney , L. Wessels , H. Schmitzer , B. Rossa , H. P. Wagner , W. Dultz; Rosalind Franklin's X-ray photo of DNA as an undergraduate optical diffraction experiment. Am. J. Phys. 1 February 2018; 86 (2): 95–104. https://doi.org/10.1119/1.5020051

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Rosalind Franklin's X-ray diffraction patterns of DNA molecules rendered the important clue that DNA has the structure of a double helix. The most famous X-ray photograph, Photo 51, is still printed in most Biology textbooks. We suggest two optical experiments for undergraduates that make this historic achievement comprehensible for students by using macromodels of DNA and visible light to recreate a diffraction pattern similar to Photo 51. In these macromodels, we replace the double helix both mathematically and experimentally with its two-dimensional (flat) projection and explain why this is permissible. Basic optical concepts are used to infer certain well-known characteristics of DNA from the diffraction pattern.

Rosalind Franklin used X-ray diffraction to determine the structure of DNA molecules. One of her best X-ray pictures is numbered Photo 51 and is shown in Fig. 1(a) . This photo was instrumental to J. D. Watson and F. Crick in deducing the double-helix model of DNA. Because of its historical significance, Photo 51 is still printed in most textbooks about X-ray diffraction or genetics. These diffraction patterns were a telltale indicator that DNA is a double helix. In addition, the radius, pitch, pitch angle, and the number of phosphate molecules per pitch of the DNA helix could be determined. Although this photo is used in most biology, genetics, biophysics, or modern physics textbooks, it remains unclear to students how or why these conclusions can be drawn. We suggest two experiments that can help students make these connections. The experiments can be incorporated in both introductory and upper-level courses; however, the modelling is geared toward upper-level courses.

$Fig. 1. (a) The well-known Photo 51, the diffraction pattern from DNA in its so-called B configuration. The dimensions of DNA are: pitch P = 3.4 nm, radius R = 1 nm, and a phase difference between the two helices (sine waves) of ΔP = 3P/8. Several important features include the characteristic X-shape or distorted rhombus, the ten diffracted orders per X, and the missing fourth order. (b) A two-dimensional projection of the phosphate molecules in the DNA backbone. The projection outlines two sine waves. We justify this flat model theoretically in Sec. II.$

(a) The well-known Photo 51, the diffraction pattern from DNA in its so-called B configuration. The dimensions of DNA are: pitch P = 3.4 nm, radius R = 1 nm, and a phase difference between the two helices (sine waves) of ΔP = 3P/8. Several important features include the characteristic X-shape or distorted rhombus, the ten diffracted orders per X, and the missing fourth order. (b) A two-dimensional projection of the phosphate molecules in the DNA backbone. The projection outlines two sine waves. We justify this flat model theoretically in Sec. II .

Franklin (and separately Stokes) worked on creating a mathematical model for diffraction from DNA. 1,2 Historically, this pattern was calculated using a lattice approach, where one would look for Bragg diffracting conditions. 3 The diffraction pattern was then described mathematically by Watson and Crick, as well as Stokes. 2,4 Kittel 5 deduced the diffraction pattern of a helical structure using Bragg reflection, the approach used by most crystallographers. The successful structure analysis of the DNA molecule resulting in the double helix model is an example of how an experiment and its analysis changed our view of the world. The achievement was rightfully honored with the Nobel prize in 1962. Unfortunately, Franklin died before the prize was awarded, but her contribution to the effort has been acknowledged by Watson 6 and Klug 7,8 (Klug later won the Nobel prize in 1982 for work he started with Rosalind Franklin).

It is desirable to discuss this crucial experiment and its outcome with undergraduate students in biophysics, modern physics, and optics courses. Most approaches to make the connection between the structure of DNA and its X-ray diffraction pattern more plausible use mathematics that one can find in solid state physics textbooks. 5,9,10 However, a good part of the problems that students have with these approaches comes from their unfamiliarity with complex experiments described by a scientific and mathematical language. Students often fail to associate theory and experimental outcomes when performing a difficult experiment. In this work, we repeat the scattering, but instead of X-rays, as in the original experiment, we use monochromatic light with a much longer wavelength, similar to Lucas et al . 11,12 The longer wavelength allows us to recreate the diffraction pattern of Photo 51 by using larger, homemade models of the DNA helix, such as the spring from a ballpoint pen or sinusoidally arranged holes in a piece of cardboard (which represent the heavy phosphate molecules in the DNA backbone). The advantage of this approach is that both the helix and its diffraction pattern can be seen with the naked eye. Students have the benefit of creating the diffracting structure first, and so, they know the structure that produces the diffraction pattern, whereas in real X-ray diffraction patterns, the goal is to determine a structure too small to be seen.

Regarding the mathematical approach, we show that in order to analyze the resulting scattering pattern, we only need Huygens' elementary wave model in the Fraunhofer approximation. Within this approximation, the helix does not need to be three-dimensional; instead, a flat projection of the helix perpendicular to its axis [see Fig. 1(b) ] can be used. The diffraction of light from a helical structure thus requires only the Fourier transform as a mathematical tool. Mathematically, we decompose the aperture into three substructures: the heavy phosphates that lay on the DNA backbone (represented by the holes in a piece of cardboard), the arrangement of these holes sinusoidally (representing the helical shape in the flat model), and a function used to offset the helix to two locations so that the single helix becomes a double helix. According to the convolution theorem, the resulting pattern is then readily described as a product of the Fourier transforms of each substructure. To conclude our investigations, we compare the diffraction patterns of our experiments with our theoretical results, as well as the historic pattern in Photo 51. These experiments can be performed in introductory or upper-level courses and as a modelling exercise in upper-level physics courses.

The scattering of light by small objects is calculated from the Huygens-Fresnel integral in a model where elementary spherical waves are stimulated by an incident light wave u ( r → ) at different points of the object and summed to give at point B: 13

Here, ρ ( r → ) is the object's scattering density, k 0 is the wave number of the incident monochromatic plane wave u ( r → ) = u 0 e i k → 0 ⋅ r → ⁠ , and R → B is the vector from the scattering point S , at the location r → = ( x , y , z ) of the object, to the point of interest B , at location r → B = ( x B , y B , z B ) ⁠ , is a volume element in real space, so that R → B = r → B − r → = ( x B − x , y B − y , z B − z ) (see Fig. 2 ). Integration is over all real space with d τ = d x d y d z as volume element.

Fig. 2. Light scattering by a small object. The vector R→B=r→B−r→ is directed from the scattering point S at location r→ of the object to the test point B at location r→B. Note that k→0 ⋅r→ is zero for normal incidence on flat objects.

Light scattering by a small object. The vector R → B = r → B − r → is directed from the scattering point S at location r → of the object to the test point B at location r → B ⁠ . Note that k → 0 ⋅ r → is zero for normal incidence on flat objects.

The scattering density ρ ( r → ) describes the number of scattering centers and their strengths per unit volume; in Secs. II and III , we will only consider opaque-transparent apertures, where ρ ( r → ) = 0 for the opaque regions and ρ ( r → ) = 1 for transparent regions. The result one gets from Eq. (1) depends on the approximation used to express the distance vector R → B ⁠ . If quadratic terms in r → are included, complex interference effects like the Gouy phase can be calculated. To calculate the far field interference of a monochromatic plane wave scattered by a small object, we only need the linear approximation; for x , y , z ≪ x B , y B , z B and x B , y B ≪ z B ⁠ , we find that

Substituting Eq. (2) into Eq. (1) and writing the scattering density ρ ( r → ) as a Fourier-integral, we obtain

where k → B = k 0 r ̂ B is the wave vector of the outgoing wave and k → 0 the wave vector of the incoming wave, with | k → B | = | k → 0 | = k 0 as required by conservation of energy. Integration is over all k-space and d τ k = d k x d k y d k z is a volume element in reciprocal space. Thus, in the far field, the amplitude u B of the scattered wave is proportional to the Fourier transform ρ ̃ ( k → B − k → 0 ) of the scattering density, with k → = k → B − k → 0 as required by momentum conservation.

The important question to ask is how the depth of the object (in the z -direction) contributes to the amplitude u B in the far field, where r B is very large compared to both the size of the scatterer and the size of the diffraction pattern. If, as is the case of Fraunhofer diffraction, the incident wave propagates in the z -direction along the optical axis, its wave vector has only a z -component: k → 0 = ( 0 , 0 , k 0 ) ⁠ . Since | r → B | = r B ≈ z B ⁠ , the z -component k 0 r ̂ B of the outgoing wave vector is compensated in ρ ̃ ( k → B − k → 0 ) by the wave vector of the incoming wave (see Fig. 2 ) and has only components with k z = 0

with ρ ′ ( x , y ) = ∫ − ∞ + ∞ ρ ( x , y , z ) d z ⁠ . Thus, we see that the incoming and outgoing (in the far field) wave vectors have nearly the same z -components, which means that the depth of the scattering object is not important in this approximation, and it can therefore be projected parallel to the z -axis onto the xy -plane. In our experiment, this means that the spring from a ballpoint pen, when used as a diffracting object, can be approximated as a sinusoidal curve (of finite thickness) in the xy -plane. We will use this result in our experiments and in our mathematical modelling when we diffract from flat apertures instead of from three-dimensional structures. Of course, disregarding the depth of the scattering object in Fraunhofer diffraction implies that no noticeable information about the chirality (the sense of the winding) of the spring can be obtained from the far-field spectrum.

The X-shape (or distorted rhombic shape) in Photo 51 was an important hint that DNA is helical with a radius R of 1 nm and a pitch P of 3.4 nm. Also noticeable was a missing fourth layer line [see Fig. 1(a) ], which helped in identifying that there is a second helix that is offset from the first by 3/8 of the pitch.

By diffracting light from the spring of a ballpoint pen, students can rework the thought processes that led scientists to draw these conclusions. In Fraunhofer diffraction, the spring creates the characteristic X-shape pattern similar to that of DNA 14 [see Figs. 1(a) and 3(b) ]. In our approach, the wavelength λ of the light is much smaller than the grating constant d and the depth D ( ≈d ) of the object (the Klein–Cook parameter Q ≪ 1 ⁠ ), and so, the diffraction from a helix is in the far field like the diffraction from a flat sinusoidal curve. Thus, we can make deductions about the structure and dimensions of DNA from Photo 51 at a very basic level. In particular, the X-shape results from the parallel sections of the projected wire [the two flanks of the sine curve indicated by the dashed lines in Fig. 3(a) ], which diffract like two sets of multiple slits or gratings oriented at an angle 2 α to each other. Using the diffraction pattern, the spacing d between parallel sections of the projected wire can be obtained using the equation mλ = d sin θ , where θ is the angle of the m- th maximum in the intensity pattern on one of the legs of X, with the zeroth order being the center. The pitch angle α can be obtained by simply measuring the half acute angle of the X-shaped pattern in Fig. 3(b) .

$Fig. 3. (a) A ballpoint pen spring (length ∼30 mm, used as our helical diffracting object), where about four pitches are being illuminated by coherent laser light (λ = 633 nm, 10 mW) expanded to a diameter of ∼5 mm by a microscope lens and a secondary lens of focal length f = 10 cm. (b) The diffraction pattern as observed on a screen 6 m away, showing the characteristic X-shape (Ref. 14). The intensity maxima on the legs of the X are caused by the grating of the pitch, and the missing fifth, tenth, etc., orders are a result of the thickness of the wire. The horizontal maxima stem from the two-slit interference of the vertical parts of the spring. A polarizer was placed at the zeroth order and adjusted to near total extinction to prevent oversaturation of the image.$

(a) A ballpoint pen spring (length ∼30 mm, used as our helical diffracting object), where about four pitches are being illuminated by coherent laser light (λ = 633 nm, 10 mW) expanded to a diameter of ∼5 mm by a microscope lens and a secondary lens of focal length f = 10 cm. (b) The diffraction pattern as observed on a screen 6 m away, showing the characteristic X-shape (Ref. 14 ). The intensity maxima on the legs of the X are caused by the grating of the pitch, and the missing fifth, tenth, etc., orders are a result of the thickness of the wire. The horizontal maxima stem from the two-slit interference of the vertical parts of the spring. A polarizer was placed at the zeroth order and adjusted to near total extinction to prevent oversaturation of the image.

Franklin and Stokes created a mathematical model for diffraction from DNA, 1,2 but it is not derived in their 1953 Nature publication. A simple approach for students to model the diffraction pattern is to replace the opaque helix mathematically with its two-dimensional projection and to make it transparent. We end up with a flat sinusoidal “slit” aperture and use an area integral to calculate the diffraction pattern. Replacing an opaque object with its inverse (transparent) counterpart is possible because open areas of an aperture contribute mainly with their edges. 15 The result is that an (opaque) obstacle has an identical diffraction pattern in the Fraunhofer region as a transparent aperture of the same shape (Babinet's principle), 16 except for an additional bright spot in the center (Poisson's spot). Thus, the helical wire is represented by the area between two sinusoidal curves that extend in the x -direction and that are offset by ± a with respect to the y -axis. This creates a small “aperture” that simulates the projected figure of a helical wire of thickness ∼2 a [see Fig. 4(a) ]. It should be noted that the spacing between these two sine curves varies and has only the thickness 2 a at the minima and maxima of the sinusoidal wave. Students can now calculate the diffraction pattern with a simple area integral, which they typically learn in calculus classes, and then compare it with the observed pattern from the three-dimensional spring.

$Fig. 4. Apertures and diffraction patterns of single and double helix apertures. (a) A single helix projection and the absolute value of its Fourier transform calculated from the electric field amplitude given by Eq. (6). The single solid sinusoidal aperture creates a characteristic X that is associated with helical diffraction patterns. The parameters used reflect the geometry of the spring from Fig. 3(a): P = 1.8 mm, R = 2 mm, and a = 0.2 mm. (b) A double helix projection and the absolute value of the Fourier transform calculated from the electric field amplitude given by Eq. (6) and multiplied by Eq. (14). Notice that adding a second helix that is 3P/8 out of phase with the first creates destructive interference at the fourth diffracted order. The aperture functions were calculated using Maple as horizontal structures, and their diffraction patterns were rotated to reflect the geometry of our setup.$

Apertures and diffraction patterns of single and double helix apertures. (a) A single helix projection and the absolute value of its Fourier transform calculated from the electric field amplitude given by Eq. (6) . The single solid sinusoidal aperture creates a characteristic X that is associated with helical diffraction patterns. The parameters used reflect the geometry of the spring from Fig. 3(a) : P = 1.8 mm, R = 2 mm, and a = 0.2 mm. (b) A double helix projection and the absolute value of the Fourier transform calculated from the electric field amplitude given by Eq. (6) and multiplied by Eq. (14) . Notice that adding a second helix that is 3P /8 out of phase with the first creates destructive interference at the fourth diffracted order. The aperture functions were calculated using Maple as horizontal structures, and their diffraction patterns were rotated to reflect the geometry of our setup.

If n windings of the spring are illuminated by a uniform electric field u 0 then Huygens waves emerge from a sinusoidal aperture given by

Here, R and P are the radius and pitch of the helix as defined earlier but now have the meaning of amplitude and wavelength for the flat projection. Huygens waves create a diffraction pattern in the far field that is given by the Fourier transform in Eq. (4) . Thus ρ ̃ ( k x , k y , 0 ) ⁠ , becomes

where F { ρ ( x , y ) } denotes the Fourier transform and k = 2π/ λ. Because the scattering density ρ is uniform within the aperture and zero outside, the limits of the integral are defined by the aperture itself. The limits ± x 0 of the x integral should be chosen to reflect the extent of the illuminated spring. The diffraction amplitude can be calculated using software such as Maple or Mathematica . In order to compare the diffraction pattern of the model with the experiment, one needs to plot the intensity I ∝ ρ ̃ ( k x , k y ) ρ ̃ ( k x , k y ) * ⁠ . However, squaring ρ ̃ would lead to very small intensities in the higher diffracted orders, and so, we instead plotted the absolute value of Eq. (6) [see Fig. 4(a) ].

It is valuable to reflect on the two features of this diffraction amplitude. First, the integral with respect to x represents the periodic maxima in the diagonal ± k x , ± k y directions caused by the interference from multiple illuminated windings. Second, the factor sin ( k y a ) / k y a stems from the interference of the wire itself (i.e., single-slit interference) and causes a wavelike modulation of the X-pattern in the k y -direction, which leads to missing orders for specific values of a . In our experiment, the thickness-to-pitch ratio is such that the fifth order vanished [see Fig. 3(b) ]. However, because in our simple model, the thickness varies across the pitch, the missing orders due to the wire thickness cannot be reproduced correctly.

How can the missing fourth order indicate a second helix? It is instructive and relatively simple to see that the fourth order could be missing as a consequence of the presence of a second helix if this second helix is shifted along the x -axis with respect to the first helix by a specific fraction f of the spacing d . (This fraction can also be defined in terms of the pitch P .) An easy way to understand this is as follows. The multiple slits of the single sinusoidal aperture create maxima at m λ = d sin θ ⁠ , with m being an integer. The m- th maximum is suppressed if it coincides with a minimum of the double slits with slit distance f d between the two sinusoidal aperture [see Fig. 4(b) ], i.e., with an odd integer of λ / 2 ⁠ : ( 2 l + 1 ) λ / 2 = f d sin θ = f m λ ⁠ , with integer l < m ⁠ . The fourth maximum ( m = 4) vanishes for f = 1 / 8 , 3 / 8 , 5 / 8 , 7 / 8 ⁠ , which are the fractions of d by which the second helix can be offset with respect to the first. The values f = 3 / 8 , 5 / 8 result in the same relative spacing of the two helices. Similarly, f = 1 / 8 , 7 / 8 results in the same relative offset, but this offset is probably too small to be feasible for the molecular structure of DNA. If students use these considerations, they can figure out which separation would cause other orders to disappear. On the other hand, it is a nice exercise to see that if f = 3 / 8 ⁠ , the 12th, 20th, etc., orders would vanish as well as the fourth.

In order to model the entire diffraction pattern of a double helix as seen in Fig. 4(b) , we used the convolution theorem as described in Sec. IV B Eq. (10) .

From the diffraction pattern of DNA, Rosalind Franklin also deduced that there were ten phosphates per pitch along the DNA backbone. Precisely how this conclusion can be drawn is now discussed.

That the diffracting object—the helix—can be replaced by its two dimensional projection allows us to create a more realistic and detailed aperture than our previous DNA model. Specifically, we can mimic the ten phosphate groups per pitch. In contrast to our simplified model, DNA is not a wire wound into a helical shape. From the viewpoint of X-rays, there are large areas filled with nothing between the phosphates on the DNA backbone. We could of course use beads to represent these phosphates, but they would have to be arranged on an almost transparent wire or background and affixed at specific locations. Instead, we use the two dimensional projection of the helix and again use Babinet's principle. This approach has the benefit of avoiding Poisson's spot, which generally makes viewing more difficult. Thus, we create an aperture where the phosphate molecules, which contribute mostly to the diffraction of X-rays, are represented by circular holes in black cardboard, rather than beads, arranged in a sine wave pattern. A similar aperture has been suggested by Lucas et al . 11

We describe here how students can make their own model and build the mathematical bridge to its diffraction pattern. In the case of DNA, which has ten phosphate groups per turn (pitch P ), we generate ten holes per wavelength (see Fig. 5 ). This pattern was created by first plotting a sine wave using Maple . The ratio of amplitude to wavelength of the sine wave was the same as the ratio of radius to pitch of the DNA. Our helix had a radius of 3.56 mm and a pitch of 11.97 mm. Then, circles of radius a were plotted at previously calculated positions along the sine wave, ten circles per wavelength. (Note that in this two-dimensional projection, the distance between the circles is not equally spaced like the distance between the phosphate molecules on the three dimensional helical DNA strand.) We placed this printed plot over the black cardboard and punched holes using a needle of radius a = 0.3 mm. A smaller needle would have resulted in not enough light getting through, and a larger needle would have led to overlapping holes in the extrema of the sinusoidal arrangement. Although the hole size does affect the diffraction pattern, as long as the holes remained less than 40% of the size of the sinusoidal amplitude (i.e., the DNA's radius), the finer diffraction detail like the X-shape could still be observed.

$Fig. 5. The 2D projection of a helix is a sinusoidal wave. Our apertures were made by punching holes of radius a into cardboard, with the holes representing the phosphate molecules. The laser light (λ = 633 nm, 10 mW) is expanded using a microscope objective and a lens (f = 0.25 m) to cover the entire aperture. The position of the second lens was then adjusted slightly to create a slightly converging beam until the diffraction pattern could be clearly seen on a distant screen (L ∼ 4 m away). A focusing lens placed after the aperture was used to record the diffraction pattern with a CCD camera.$

The 2D projection of a helix is a sinusoidal wave. Our apertures were made by punching holes of radius a into cardboard, with the holes representing the phosphate molecules. The laser light (λ = 633 nm, 10 mW) is expanded using a microscope objective and a lens ( f = 0.25 m) to cover the entire aperture. The position of the second lens was then adjusted slightly to create a slightly converging beam until the diffraction pattern could be clearly seen on a distant screen ( L ∼ 4 m away). A focusing lens placed after the aperture was used to record the diffraction pattern with a CCD camera.

The double helix in DNA is represented by adding a second sinusoidal arrangement of holes which is shifted by 3/8 of the sinusoid's wavelength [see Fig. 1(b) ]. We note that in actuality, we adjusted the offset between the helices for ease of aperture creation purposes. With the offset of 3 P/ 8, there exists an overlap of certain holes, which would have been difficult to recreate when punching them. Thus, for simplification, we adjusted the offset until the overlapping holes completely coincided with each other such that the two holes from the two different helices could now be represented by a single hole, as depicted in Fig. 1(b) . After performing some calculations, we found that this adjustment had a slight effect on the diffraction pattern. However, these differences were not important since the main purpose of the offset was to observe a missing fourth layer line in our diffraction pattern, and this result remained unaltered with our simplification.

The apertures were then placed into an expanded laser beam (see Fig. 5 ). The second lens of the telescopic beam expander was shifted slightly along the optic axis toward the aperture until the diffraction pattern appeared focused at a reasonable distance ( L ∼ 4 m). In contrast to the experiment with a spring, 14 at first glance, the diffraction pattern does not look like the typical X-shape pattern. Instead, there is a huge Airy disk that is created by the circular apertures. However, a closer look reveals that the zeroth order and the surrounding rings have repeating rhombic structures within them [see Fig. 6(a) ], not just one X-shape as in the experiment with the solid helix. It is the height of these rhombuses or X-shapes which contains the information about the number of holes (phosphates) per pitch.

$Fig. 6. Diffraction patterns from sinusoidally arranged holes (representing phosphate molecules). The ring structure is caused by diffraction from the holes; the finer, diagonal structures inside the bright center and inside the rings are due to the arrangement of the holes in a sinusoidal pattern. (a) Photo of the experimentally generated pattern for a single helix; a round polarizer was placed at the zeroth order and adjusted to near total extinction to prevent oversaturation of the camera with the 633 nm laser light. (b) Calculated Fourier transform for a single helix (absolute value of Eq. (15) with f = 0) with parameter values P = 3.4, R = 1, f = 0, a = 0.1 R, and 5 illuminated pitches (these values are the same for all figures except as noted otherwise). (c) A zoomed-in photograph of the experimental diffraction pattern using a black-and-white CCD camera. As a result of the ten phosphates per period, the rhombuses are ten diffracted orders high. (d) Calculated transform of the inner center. (e) The double sinusoidal experimental diffraction pattern recorded with the CCD camera. The weakening of the fourth diffracted order (m = 4) is due to the offset of the double helix by 3P/8. (f) Calculated transform of a double helix made of holes (with f = 3/8).$

Diffraction patterns from sinusoidally arranged holes (representing phosphate molecules). The ring structure is caused by diffraction from the holes; the finer, diagonal structures inside the bright center and inside the rings are due to the arrangement of the holes in a sinusoidal pattern. (a) Photo of the experimentally generated pattern for a single helix; a round polarizer was placed at the zeroth order and adjusted to near total extinction to prevent oversaturation of the camera with the 633 nm laser light. (b) Calculated Fourier transform for a single helix (absolute value of Eq. (15) with f = 0) with parameter values P = 3.4, R = 1, f = 0, a = 0.1 R , and 5 illuminated pitches (these values are the same for all figures except as noted otherwise). (c) A zoomed-in photograph of the experimental diffraction pattern using a black-and-white CCD camera. As a result of the ten phosphates per period, the rhombuses are ten diffracted orders high. (d) Calculated transform of the inner center. (e) The double sinusoidal experimental diffraction pattern recorded with the CCD camera. The weakening of the fourth diffracted order ( m = 4) is due to the offset of the double helix by 3 P /8. (f) Calculated transform of a double helix made of holes (with f = 3/8).

In order to zoom in on the bright center, we replaced the observation screen with a CCD camera that had its front lens removed, making sure that the exposed sensor was placed exactly on the center of our diffraction pattern. Using a neutral density filter (ND = 2.5), we decreased the intensity of the laser beam so that it did not oversaturate the camera. A second converging lens placed after the aperture brought the diffraction pattern closer and made it smaller so that it was easier to capture its image with the camera [see Figs. 6(c) and 6(e) ].

Our apertures (representing a helix and a double helix) are now made of two or three substructures (see Fig. 7 ): a circular hole ρ O simulating the phosphate molecule, the sinusoidal arrangement of the holes H S representing the helix, and the two positions of this sinusoidal arrangement H P creating the double helix. One can describe the aperture as a convolution of these aperture functions 16,17

Fig. 7. The convolution theorem states that the Fourier transform of an aperture function constructed by convoluting substructures can be calculated by taking the product after the Fourier transforms of the substructures. The top images show the field distributions across the aperture substructures. From left to right are: (a) the hole ρ0; (b) the sinusoidal position of the holes HS; and (c) the position of the two sine functions for the double helix HP. Panel (d) shows the final aperture function—a double sine function outlined by holes—which is the convolution of the three substructures. The bottom images show the Fourier transforms of substructures in the top row. From left to right: (e) a Bessel function; (f) a rhombus composed of a reciprocal grating with grating constant q/P in the kx direction and a reciprocal grating in the ky direction; and (g) a cosine wave leading to missing orders. Panel (h) shows the final transform, which corresponds to the product of the three substructure transforms.

The convolution theorem states that the Fourier transform of an aperture function constructed by convoluting substructures can be calculated by taking the product after the Fourier transforms of the substructures. The top images show the field distributions across the aperture substructures. From left to right are: (a) the hole ρ 0 ; (b) the sinusoidal position of the holes H S ; and (c) the position of the two sine functions for the double helix H P . Panel (d) shows the final aperture function—a double sine function outlined by holes—which is the convolution of the three substructures. The bottom images show the Fourier transforms of substructures in the top row. From left to right: (e) a Bessel function; (f) a rhombus composed of a reciprocal grating with grating constant q/P in the k x direction and a reciprocal grating in the k y direction; and (g) a cosine wave leading to missing orders. Panel (h) shows the final transform, which corresponds to the product of the three substructure transforms.

The circular aperture ρ 0 with radius a exposed to a planar electromagnetic wave is a piece-wise step function (i.e., a cylinder with radius a and height 1; see Fig. 7 , upper left corner)

The other apertures are arrays of Dirac delta functions δ ( x ) arranged in a desired pattern (also called Dirac combs). A convolution of ρ O with a Dirac comb creates mathematically an aperture consisting of holes as repeated identical substructures. Convolving the aperture ρ 0 with the array function

arranges the holes in a sinusoidal pattern. Here, P is the pitch of the helix, R is the radius of the helix, n is an index for the n- th hole, N is the total number of holes illuminated by the incident light wave, and q is the number of holes (phosphate molecules) per pitch. The axial spacing in the x -direction between the phosphate molecules is equidistant such that there are q of them per period P , resulting in the locations x n = nP/q with respect to the x -axis and a grating constant P/q. However, the projections of the phosphate molecules are sinusoidally—not equally—spaced along the y -axis. For DNA, the dimensions are P = 3.4 nm, q = 10 phosphates/period, and R = 1 nm.

An additional convolution of the aperture ρ O * H S with the array

shifts the sinusoidal array of holes to the positions ( 1 / 2 ) ( 3 P / 8 , 0 ) and − ( 1 / 2 ) ( 3 P / 8 , 0 ) ⁠ , where P is the pitch of the helix, i.e., the wavelength of the sinusoidal aperture. Hence, two sinusoidal arrangements are created that are separated by an axial distance of 3 P/ 8. This is the axial separation between the two helices in DNA.

The convolution theorem states that the Fourier transform of convoluted functions is equal to the product of the Fourier transform of each function, that is,

The Fourier transform of a circular hole is 16

where a is the circle's radius and k a 2 = k x 2 + k y 2 ⁠ . Fortunately, the Fourier transform of H S is rather simple; we find that

Finally, the Fourier transform of H P is

which is simply a cosine function in the ± k x direction of reciprocal space. Putting everything together, the resulting Fourier transform for a double helix is just the product of Eqs. (12)–(14) , given by

where f = 0 for the single helix and f = 3 / 8 for the double helix mimicking the DNA. The Hermitian product ρ ̃ ( k x , k y ) ρ ̃ ( k x , k y ) ∗ then describes the diffraction pattern. The absolute square of Eq. (15) can be plotted [see Figs. 6(b) , 6(d) , and 6(f) ] and compared to the experimental images [Figs. 6(a) , 6(c) , and 6(e) ].

Notice how each term in Eqs. (11) and (15) leads to a specific aspect of the diffraction pattern, each playing a crucial role as graphically presented in Fig. 7 :

F { ρ O } leads to the Airy disk and rings.

F { H S } is a summation over “waves” in reciprocal space. It represents the periodic maxima and minima in the diagonal ± k x , ± k y directions on the observing screen. Note that the vertical periodicity of the diffraction pattern has the reciprocal grating constant q / P ⁠ , and so, the rhombus (or the X) has a height of q diffracted orders. Thus, counting the diffracted orders reveals the number of phosphates per pitch. In Fig. 6 , this can be seen by counting the diffracted orders in the height of a rhombus from 0 to 10. In Fig. 1(b) , it can be better seen by counting the diffracted orders in one leg of the X from −5 to +5. In the limit of a solid sinusoidal line, q → ∞ ⁠ , and so, the rhombus does not repeat and is instead infinitely high (an X-shape) as in the experiment with the helical wire. In the case q = 1 ⁠ , there is no reciprocal grating in the ± k y direction and therefore no X-shape [see Fig. 8(a) ].

Finally, the cosine term in F { H P } modulates the pattern in the ± k x direction, which leads to missing orders in the pattern when the argument of the cosine term is f P k x / 2 = ( 2 l + 1 ) π / 2 with l = integer or f = ( 2 l + 1 ) / 2 m because k x = 2 π m / P for the m- th diffracted order. Figure 4(b) shows a result of multiplying F { ρ } of Eq. (6) with F { H P } ⁠ .

$Fig. 8. First quadrant of the diffraction pattern for (a) one hole (q = 1), (b) two holes (q = 2), and (c) four holes (q = 4) per pitch. The dotted lines and arrows show how the pattern is related to the diffracting structure. (Note the first minimum of the Airy disk that suppresses the pattern in an area outlining a quarter of a circle.) Panels (d)–(g) were all made using P = 3.4, R = 1, and 5 illuminated pitches. The dashed quarter circle outlines the first minimum of the Airy disk. Panel (g) shows that when the holes overlap, the rhombus grows into an X (the scale in this panel was changed to show the full pattern, which makes comparison with Fig. 3(b) easier).$

First quadrant of the diffraction pattern for (a) one hole ( q = 1), (b) two holes ( q = 2), and (c) four holes ( q = 4) per pitch. The dotted lines and arrows show how the pattern is related to the diffracting structure. (Note the first minimum of the Airy disk that suppresses the pattern in an area outlining a quarter of a circle.) Panels (d)–(g) were all made using P = 3.4, R = 1, and 5 illuminated pitches. The dashed quarter circle outlines the first minimum of the Airy disk. Panel (g) shows that when the holes overlap, the rhombus grows into an X (the scale in this panel was changed to show the full pattern, which makes comparison with Fig. 3(b) easier).

$Fig. 9. As the hole size increases, the Airy disk it creates in reciprocal space decreases: (a) a = 0.1 R, corresponding to our experimental setup; (b) a = 0.2 R, the diffraction pattern of the holes begins to “suffocate” the rhombic pattern and makes it less visible in the rings; (c) a = 0.3 R, the tip of the rhombus is now located in the first Airy ring, which is the condition for DNA (where the radii of the phosphate molecules are about one third of the helix's radius); (d) a = 0.4 R, the rhombic pattern still exists outside the Airy disk but is too weak to be seen. All figures were made for P = 3.4, R = 1, and 5 illuminated pitches.$

As the hole size increases, the Airy disk it creates in reciprocal space decreases: (a) a = 0.1 R , corresponding to our experimental setup; (b) a = 0.2 R , the diffraction pattern of the holes begins to “suffocate” the rhombic pattern and makes it less visible in the rings; (c) a = 0.3 R , the tip of the rhombus is now located in the first Airy ring, which is the condition for DNA (where the radii of the phosphate molecules are about one third of the helix's radius); (d) a = 0.4 R , the rhombic pattern still exists outside the Airy disk but is too weak to be seen. All figures were made for P = 3.4, R = 1, and 5 illuminated pitches.

$Fig. 10. As the number of illuminated pitches (N) increases, the diffraction pattern becomes sharper (similar to two-beam versus multiple-beam interference).$

As the number of illuminated pitches ( N ) increases, the diffraction pattern becomes sharper (similar to two-beam versus multiple-beam interference).

$Fig. 11. As the shift ΔP between the two sinusoidal functions changes, the resulting interference pattern also changes, leading to different missing diffracted orders: (a) ΔP = P/2, all odd orders vanish; (b) ΔP = P/4, the second, sixth, and tenth orders vanish; the 4th and 12th orders are seen to vanish for both (c) ΔP = P/8 and (d) ΔP = 3P/8.$

As the shift Δ P between the two sinusoidal functions changes, the resulting interference pattern also changes, leading to different missing diffracted orders: (a) Δ P = P /2, all odd orders vanish; (b) Δ P = P /4, the second, sixth, and tenth orders vanish; the 4th and 12th orders are seen to vanish for both (c) Δ P = P /8 and (d) Δ P = 3 P /8.

It is very instructive for students to adjust (experimentally or by modeling) certain parameters like the hole size a (Fig. 9 ), the number of illuminated pitches N (by expanding the beam) (Fig. 10 ), the shift Δ P between the helices (Fig. 11 ), and the number of holes per pitch q and to observe the effect on the diffraction pattern. Varying the number of holes per pitch will clarify how Rosalind Franklin could conclude that there are ten phosphates per pitch and not two or seven (Fig. 8 ). Fig. 1(b) can be used as a starter template for the experiments.

One didactical aspect of this work is that for a volume scatterer the far field (Fraunhofer region) is equivalent to the far field of a plane scatterer. And the plane scatterer is constructed from the volume scatterer by projecting all its scattering elements onto a plane perpendicular to the incident beam. This means it makes no difference if we scatter from a helix or a plane, sinusoidal slit. Based on this insight, our two dimensional representations of DNA generated mathematically as well as experimentally enable another didactical aspect of this work: they replicate the famous X-structure in Rosalind Franklin's Photo 51 and its missing fourth order and provide an explanation for the overall structure of the pattern as a product of the Fourier transforms of three substructures: the scattering molecules, their arrangement along one sinusoidal line, and the arrangement of the two sinusoidal lines themselves. Using mathematical modeling or optical experiments, students can experience the interplay between these substructures, which will provide them with new insights into X-ray diffraction and Fourier transformation.

Our structure analysis is of course complex, and it should encourage the readers to do their own investigations and improvements by changing the theoretical and experimental parameters as suggested. It is perhaps the main outcome of this work that only playful experience with the experimental and theoretical background of physical phenomena can lead to a profound understanding of major discoveries and why they are worthy to be honored with a Nobel prize.

This work was funded by the Research Corporation for Science Advancement (Grant No. CC6339) and the Fredrick A. Hauck Research Grant and the Women of Excellence Giving Circle at Xavier University. L. Wessels was supported by the Jonathan F. Reichert Foundation. The authors thank Jessica Murphy for the expert preparation of the figures.

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Franklin's X-ray diffraction, explanation of X-ray pattern.

Description

How an X-ray diffraction pattern is created and how the DNA X-ray diffraction pattern can be interpreted to give the dimensions. (DNAi location: Code > Finding the Structure > piece of the puzzle > Franklin's X-ray)

This is the X-ray crystallograph pattern of DNA obtained by Rosalind Franklin and Raymond Gosling in 1952. It is know as the B-form. It was clearer than the other X-ray patterns because water was included in the DNA sample. Both James Watson and Francis Crick were struck by the simplicity and symmetry of this pattern. The distinctive "X" in this X-ray photo is the telltale pattern of a helix. Because the X-ray pattern is so regular, the dimensions of the helix must also be consistent. For example, the diameter of the helix stays the same..........

x ray diffraction,x ray crystallography,rosalind franklin dna,diffraction pattern,ray pattern,s college

Source: DNALC.DNAi

15875. Wilkins' X-ray

Maurice Wilkins obtained some of the first X-ray diffraction patterns of DNA from which dimensions could be calculated.

15337. Use of X-ray crstallography to prove that DNA is crystalline, Maurice Wilkins

Maurice Wilkins talks about obtaining an X-ray diffraction pattern.

16422. Animation 19: The DNA molecule is shaped like a twisted ladder.

James Watson and Francis Crick explain how they solved the structure of DNA. Erwin Chargaff explain how he measured the levels of each of the four nitrogenous bases.

Source: DNALC.DNAFTB

16439. Biography 19: Rosalind Elsie Franklin (1920-1958)

James Watson and Francis Crick solved the structure of DNA. Other scientists, like Rosalind Franklin and Maurice Wilkins, also contributed to this discovery.

Source: DNAFTB

15692. Rosalind Franklin

Rosalind Franklin in the 1950s.

15493. The double helical structure of DNA, 3D animation with no audio

Animation of 2D DNA model becoming three dimensional.

15676. DNA helix

Image depicting DNA helix model and table.

15262. Rosalind Franklin's reasoning on the DNA structure, Raymond Gosling

Raymond Gosling - Rosalind Franklin's graduate student - talks about Franklin's view on model building.

16440. Biography 19: Maurice Hugh Frederick Wilkins (1916-2004)

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Photo-51-credit-Kings-College-London-Archives

The structure of DNA: How Dr Rosalind Franklin contributed to the story of life

11 October 2016

Technology & Science

The discovery of the structure of DNA in 1953 was made possible by Dr Rosalind Franklin’s X-ray diffraction work at King’s. Her creation of the famous Photo 51 demonstrated the double-helix structure of deoxyribonucleic acid: the molecule containing the genetic instructions for the development of all living organisms.

Dr Franklin joined the laboratory of John Randall at King’s in 1950 with a PhD from Cambridge and X-ray diffraction experience in Paris. At King’s, by controlling the water content of the DNA specimens, she showed that the molecule could exist in two forms (A and B). In May 1952 she and PhD student Ray Gosling captured the image of the B form that Jim Watson of Cambridge saw early in 1953, giving him and Francis Crick vital information for the building of their DNA model in March.

A paper by Franklin and Gosling, together with one by Dr Maurice Wilkins and colleagues from King’s, accompanied the announcement of Watson and Crick’s momentous discovery in Nature in May 1953. Franklin moved to Birkbeck College, London, and she died of cancer in 1958. She had helped to discover the story of life, and to lay the foundations of structural molecular biology.

Photographs courtesy of King’s College London Archives.

In this story

Rosalind Franklin

Biophysicist

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Replicating X-ray of DNA

Print Experiment

Goal: In this experiment/demo students would reproduce the experiment done by Rosalind Franklin that showed that the structure of the DNA molecule was a double helix. However, the students will be using light rather than x-rays and a helical tungsten filament rather than DNA.

Background:

One of the most useful ways to understand how something works is to look at its structure. This is why the discovery of the helical structure of DNA started a DNA revolution that would last the next 50 years. Knowing the structure of DNA allowed us to understand how genes work and how they are replicated from one cell to another for generations. It allowed us to better understand inherited diseases and how one single change or mutation could lead to those diseases. Knowing the structure of DNA lead to DNA fingerprinting, which has revolutionized forensic sciences by matching DNA samples to crime suspects, and can also be used to determine the paternity or ancestry of humans or pets. Recently, new developments in DNA technology (including cutting edge CRISPR technology) could lead to gene editing and gene therapies. But, this all started with determining the structure of DNA. Back in the 1800’s, scientists knew about genetic traits, but it wasn't until 1943 that they learned that DNA was the "genetic factor" and not until 1953 that the helical structure of DNA that we know today was described.

While the discovery of the structure of DNA involved four scientists, many scientific breakthroughs had to occur for the structure of DNA to be found. Some of these include: 1) the progress made by X-ray crystallographers in studying organic macromolecules; 2) the growing evidence supplied by geneticists that it was DNA, not protein, in chromosomes that was responsible for heredity; 3) Erwin Chargaff’s experimental finding that there are equal numbers of A and T bases and of G and C bases in DNA; and 4) Linus Pauling’s discovery that the molecules of some proteins have helical shapes—arrived at through the use of atomic models and a keen knowledge of the possible disposition of various atoms. However, the most important scientific breakthrough was a photograph, photograph 51, which was taken by Rosalind Franklin and her student.

In 1951, Franklin was offered a 3-year research scholarship at King's College in London to set up and improve the x-ray crystallography unit there. X-ray crystallography is a biophysical technique where scientists make crystals of molecules and shine x-rays on them. The x-rays diffract and the diffracted pattern can be used to reconstruct the molecule’s structure. DNA is too small to be seen with the naked eye, but using x-ray crystallography, we can determine the positions of all the atoms in the DNA, and thereby generate a 3D model based on those observations.

When Franklin arrived at King’s College, Maurice Wilkins was already using X-ray crystallography to try to solve the structure of DNA. It turned out that Franklin arrived while Wilkins was away, and on his return, Wilkins assumed that she was hired to be his assistant. It was a bad start to a relationship that never got any better. However, pushing forward, Franklin and her student, Raymond Gosling, were able to use X-ray crystallography to get high-resolution photographs of crystallized DNA fibers, including photograph 51. From photograph 51, she deduced the basic dimensions of the DNA strand and that it was a helix.

She presented her data at a lecture in King's College at which James Watson was in attendance. Watson admitted to not paying attention at Franklin's talk and not being able to fully describe the lecture and the results to Francis Crick. Watson and Crick had been working on solving the DNA structure. Franklin did not know Watson and Crick as well as Wilkins did and never truly collaborated with them. It was Wilkins who showed Watson and Crick the X-ray data that Franklin had obtained. The data confirmed the 3-D structure that Watson and Crick had theorized for DNA. In 1953, both Wilkins and Franklin published papers on their X-ray data in the same Nature issue with Watson and Crick's paper on the structure of DNA.

In this experiment, you will use some household items to reproduce the photograph that launched a DNA revolution, photograph 51. However, instead of a DNA molecule, you will use a filament from an incandescent light bulb, which has a helical shape. Instead of x-rays, you will shine visible light from a laser pointer on the filament. Looking at the diffraction pattern of the filament on a wall, you will be able to see the characteristic “cross pattern” that is also shown in photograph 51

A small helical structure (I got mine from an old incandescent light bulb)

Laser pointer (make sure it is not and LED one, I got mine from a pet store)

White paper

Tungsten filament from the light bulb is helical just like DNA

The laser pointer will act as our X-ray beam

The white paper will be our x-ray film

Safety Precautions:

Even though it is a laser pointer and not an x-ray beam, avoid direct exposure to the beam

Do not stare into the beam

Do not view the beam through optical instruments (this includes your camera phone)

Make sure that your laser is aligned with the filament.

Use a leveled surface for support

Have a partner carefully draw the pattern on the paper. It does not have to be perfect.

Scientists measure the space in between the spots to calculate the size of the helix. This is why a good look at photograph 51 gave away the structure! They had already since helical structures before (they knew helical proteins). The spacing in between the spots could be measured and it gave away that the there would be no steric constraints if the phosphates were on the outside.

Extras - other things you could do

You can find other patterns to explore and learn. Try other shapes and see what you can learn from the different diffraction patterns. You can see more diffraction pattern calculations here .

Scientific Biographies

Francis Crick, Rosalind Franklin, James Watson, and Maurice Wilkins

These four scientists—Crick, Franklin, Watson, and Wilkins—codiscovered the double-helix structure of DNA, which formed the basis for modern biotechnology.

At King’s College London, Rosalind Franklin obtained images of DNA using X-ray crystallography, an idea first broached by Maurice Wilkins. Franklin’s images allowed James Watson and Francis Crick to create their famous two-strand, or double-helix, model.

In 1962 Watson (b. 1928), Crick (1916–2004), and Wilkins (1916–2004) jointly received the Nobel Prize in Physiology or Medicine for their 1953 determination of the structure of deoxyribonucleic acid (DNA). Wilkins’s colleague Franklin (1920–1958), who died from cancer at the age of 37, was not so honored. The reasons for her exclusion have been debated and are still unclear. There is a Nobel Prize stipulation that states “in no case may a prize amount be divided between more than three persons.” The fact she died before the prize was awarded may also have been a factor, although the stipulation against posthumous awards was not instated until 1974.

Discovering the Structure of DNA

The molecule that is the basis for heredity, DNA, contains the patterns for constructing proteins in the body, including the various enzymes. A new understanding of heredity and hereditary disease was possible once it was determined that DNA consists of two chains twisted around each other, or double helixes, of alternating phosphate and sugar groups, and that the two chains are held together by hydrogen bonds between pairs of organic bases—adenine (A) with thymine (T), and guanine (G) with cytosine (C). Modern biotechnology also has its basis in the structural knowledge of DNA—in this case the scientist’s ability to modify the DNA of host cells that will then produce a desired product, for example, insulin.

The background for the work of the four scientists was formed by several scientific breakthroughs: the progress made by X-ray crystallographers in studying organic macromolecules; the growing evidence supplied by geneticists that it was DNA, not protein, in chromosomes that was responsible for heredity; Erwin Chargaff’s experimental finding that there are equal numbers of A and T bases and of G and C bases in DNA; and Linus Pauling ’s discovery that the molecules of some proteins have helical shapes—arrived at through the use of atomic models and a keen knowledge of the possible disposition of various atoms.

Rosalind Franklin

Of the four DNA researchers, only Rosalind Franklin had any degrees in chemistry. She was born into a prominent London banking family, where all the children—girls and boys—were encouraged to develop their individual aptitudes. She attended Newnham College, one of the women’s colleges at Cambridge University. She completed her degree in 1941 in the middle of World War II and undertook graduate work at Cambridge with Ronald Norrish, a future Nobel laureate.

She resigned her research scholarship in just one year to contribute to the war effort at the British Coal Utilization Research Association. There she performed fundamental investigations on the properties of coal and graphite. She returned briefly to Cambridge, where she presented a dissertation based on this work and was granted a PhD in physical chemistry.

After the war, through a French friend, she gained an appointment at the Laboratoire Centrale des Services Chimiques de l’Etat in Paris, where she was introduced to the technique of X-ray crystallography and rapidly became a respected authority in this field. In 1951 she returned to England to King’s College London, where her charge was to upgrade the X-ray crystallographic laboratory there for work with DNA.

Maurice Wilkins

Already at work at King’s College was Maurice Wilkins, a New Zealand–born but Cambridge-educated physicist. As a new PhD he worked during World War II on the improvement of cathode-ray tube screens for use in radar and then was shipped out to the United States to work on the Manhattan Project.

Like many other nuclear physicists, he became disillusioned with his subject when it was applied to the creation of the atomic bomb; he turned instead to biophysics, working with his Cambridge mentor, John T. Randall—who had undergone a similar conversion—first at the University of St. Andrews in Scotland and then at King’s College London.

It was Wilkins’s idea to study DNA by X-ray crystallographic techniques, which he had already begun to implement when Franklin was appointed by Randall. The relationship between Wilkins and Franklin was unfortunately a poor one and probably slowed their progress.

James Watson and Francis Crick

Meanwhile, in 1951, 23-year-old James Watson, a Chicago-born American, arrived at the Cavendish Laboratory in Cambridge. Watson had two degrees in zoology: a bachelor’s degree from the University of Chicago and a doctorate from Indiana University, where he became interested in genetics. He had worked under Salvador E. Luria at Indiana on bacteriophages, the viruses that invade bacteria in order to reproduce—a topic for which Luria received a Nobel Prize in Physiology or Medicine in 1969.

Watson went to Denmark for postdoctoral work, to continue studying viruses and to remedy his relative ignorance of chemistry. At a conference in the spring of 1951 at the Zoological Station at Naples, Watson heard Wilkins talk on the molecular structure of DNA and saw his recent X-ray crystallographic photographs of DNA. He was hooked.

Watson soon moved to the Cavendish Laboratory, where several important X-ray crystallographic projects were in progress. Under the leadership of William Lawrence Bragg, Max Perutz was investigating hemoglobin and John Kendrew was studying myoglobin, a protein in muscle tissue that stores oxygen. (Perutz and Kendrew received the Nobel Prize in Chemistry for their work in the same year that the prize was awarded to the DNA researchers—1962.)

Working under Perutz was Francis Crick, who had earned a bachelor’s degree in physics from University College London and had helped develop radar and magnetic mines during World War II. Crick, another physicist in biology, was supposed to be writing a dissertation on the X-ray crystallography of hemoglobin when Watson arrived, eager to recruit a colleague for work on DNA.

Inspired by Pauling’s success in working with molecular models, Watson and Crick rapidly put together several models of DNA and attempted to incorporate all the evidence they could gather. Franklin’s excellent X-ray photographs, to which they had gained access without her permission, were critical to the correct solution. The four scientists announced the structure of DNA in articles that appeared together in the same issue of Nature .

Separate Career Paths

Then they moved off in different directions. Franklin went to Birkbeck College, London, to work in J. D. Bernal’s laboratory, a much more congenial setting for her than King’s College. Before her untimely death from cancer she made important contributions to the X-ray crystallographic analysis of the structure of the tobacco mosaic virus, a landmark in the field.

By the end of her life she had become friends with Francis Crick and his wife and had moved her laboratory to Cambridge, where she undertook dangerous work on the poliovirus. Wilkins applied X-ray techniques to the structural determination of nerve cell membranes and of ribonucleic acid (RNA)—a molecule that is associated with chemical synthesis in the living cell—while rising in rank and responsibility at King’s College.

Watson’s subsequent career eventually took him to the Cold Spring Harbor Laboratory (CSHL) of Quantitative Biology on Long Island, New York, where as director from 1968 onward he led it to new heights as a center of research in molecular biology. From 1988 to 1992 he headed the National Center for Human Genome Research at the National Institutes of Health. Afterward he returned to CSHL as chancellor. Watson’s racist remarks about the intelligence of Africans in 2007 led the CSHL to force him into retirement, though the Lab named him an emeritus professor and honorary trustee.

When Watson doubled down on his racist views in a 2018 documentary, the lab revoked these honors and severed ties with Watson. Watson’s fame as a discoverer of the structure of DNA also made his continued public expression of sexist views on women in science and his previous eugenicist comments on homosexuality particularly harmful during the first decades of the 21st century.

The information contained in this biography was last updated on July 28, 2022.

Featured image: Rosalind Franklin in Paris, ca. 1949. Vittorio Luzzati.

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Concept 19 The DNA molecule is shaped like a twisted ladder.

James Watson (1928-)
Francis Crick (1916-2004)
Maurice Wilkins (1916-2004)

Rosalind Franklin (1920-1958)

Rosalind Elsie Franklin was born in London, England. Her family was well-to-do and both sides were very involved in social and public works. Franklin's father wanted to be a scientist, but World War I cut short his education and he became a college teacher instead. Rosalind Franklin was extremely intelligent and she knew by the age of 15 that she wanted to be a scientist. Her father actively discouraged her interest since it was very difficult for women to have such a career. However, with her excellent education from St. Paul's Girls' School ? one of the few institutions at the time that taught physics and chemistry to girls ? Franklin entered Cambridge University in 1938 to study chemistry.

When she graduated, Franklin was awarded a research scholarship to do graduate work. She spent a year in R.G.W. Norrish 's lab without great success. Norrish recognized Franklin's potential but he was not very encouraging or supportive toward his female student. When offered the position as an assistant research officer at the British Coal Utilization Research Association (CURA), Franklin gave up her fellowship and took the job.

CURA was a young organization and there was less formality on the way research had to be done. Franklin worked fairly independently, a situation that suited her. Franklin worked for CURA until 1947 and published a number of papers on the physical structure of coal.

Franklin's next career move took her to Paris. An old friend introduced her to Marcel Mathieu who directed most of the research in France. He was impressed with Franklin's work and offered her a job as a "chercheur" in the Laboratoire Central des Services Chimiques de l'Etat. Here she learned X-ray diffraction techniques from Jacques Mering.

In 1951, Franklin was offered a 3-year research scholarship at King's College in London. With her knowledge, Franklin was to set up and improve the X-ray crystallography unit at King's College. Maurice Wilkins was already using X-ray crystallography to try to solve the DNA problem at King's College. Franklin arrived while Wilkins was away and on his return, Wilkins assumed that she was hired to be his assistant. It was a bad start to a relationship that never got any better.

Working with a student, Raymond Gosling, Franklin was able to get two sets of high-resolution photos of crystallized DNA fibers. She used two different fibers of DNA, one more highly hydrated than the other. From this she deduced the basic dimensions of DNA strands, and that the phosphates were on the outside of what was probably a helical structure.

She presented her data at a lecture in King's College at which James Watson was in attendance. In his book The Double Helix , Watson admitted to not paying attention at Franklin's talk and not being able to fully describe the lecture and the results to Francis Crick. Watson and Crick were at the Cavendish Laboratory and had been working on solving the DNA structure. Franklin did not know Watson and Crick as well as Wilkins did and never truly collaborated with them. It was Wilkins who showed Watson and Crick the X-ray data Franklin obtained. The data confirmed the 3-D structure that Watson and Crick had theorized for DNA. In 1953, both Wilkins and Franklin published papers on their X-ray data in the same Nature issue with Watson and Crick's paper on the structure of DNA.

Franklin left Cambridge in 1953 and went to the Birkbeck lab to work on the structure of tobacco mosaic virus. She published a number of papers on the subject and she actually did a lot of the work while suffering from cancer. She died from cancer in 1958.

In 1962, the Nobel Prize in Physiology or Medicine was awarded to James Watson, Francis Crick, and Maurice Wilkins for solving the structure of DNA. The Nobel committee does not give posthumous prizes.

DNA was first crystallized in the late 70's — remember, the 1953 X-ray data were from DNA fibers. So, the real "proof" for the Watson-Crick model of DNA came in 1982 after the B-form of DNA was crystallized and the X-ray pattern was solved.

If the DNA of one human cell is stretched out, it would be almost 6 feet long and contain over three billion base pairs. How does all this fit into the nucleus of one cell?

CLASSICAL GENETICS

GENETIC ORGANIZATION AND CONTROL

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Maurice Wilkins: Behind the Scenes of DNA

The "third man." Although Maurice Wilkins shared the 1962 Nobel Prize in physiology or medicine with James Watson and Francis Crick, his name is not as commonly known as one of the discoverers of the structure of DNA. His autobiography is called The Third Man of the Double Helix because much of the glory went to Watson and Crick, but much of the work was done by him. In fact, if things had worked out slightly differently, the famous pair of Watson and Crick could have been the famous pair of Wilkins and Franklin.

His education. Maurice Wilkins was born in Pongaroa, New Zealand, on December 15, 1916, to Irish parents. His family moved to England when Wilkins was six years-old and he began a British education, complete with a degree in physics from the University of Cambridge. At Cambridge he received his first training in X-ray crystallography, a technique he would later use to study DNA fibers.

The bomb. After earning his Ph.D., he contributed to the war effort by improving cathode-ray screens for radar and working on the Manhattan Project. He was unhappy about his role in developing the atomic bomb, determining to make a positive impact using science. As a result, he joined the biophysics unit at St. Andrews that later moved to King's College.

The misunderstanding. What Wilkins did not know was that when Franklin was recruited, she was told that she would be in charge of the X-ray studies of DNA. Wilkins thought that Franklin would be his assistant. This caused tension between the pair, and their personalities only served to deepen the divide. Wilkins was relatively quiet, reserved, and non-confrontational; meanwhile, Franklin was brusque, outspoken, and well-known as a person that did not suffer fools. Unfortunately, Franklin believed Wilkins fell into the latter category, and they generally avoided each other.

The lost opportunity. If Wilkins and Franklin had cooperated better, they might have been the first to discover DNA's structure. Indeed, much of Watson and Crick's model was based on photographs taken by Wilkins and Franklin. Wilkins work was published as supporting data to the Watson-Crick model, and he went on to do much of the experimental work to prove the model correct. However, Watson and Crick exclusively became household names.

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Rosalind Franklin

(1920-1958)

Who Was Rosalind Franklin?

Rosalind Franklin earned a Ph.D. in physical chemistry from Cambridge University. She learned crystallography and X-ray diffraction, techniques that she applied to DNA fibers. One of her photographs provided key insights into DNA structure. Other scientists used it as evidence to support their DNA model and took credit for the discovery. Franklin died of ovarian cancer in 1958, at age 37.

Early Years

Rosalind Elsie Franklin was born into an affluent and influential Jewish family on July 25, 1920, in Notting Hill, London, England. She displayed exceptional intelligence from early childhood, knowing from the age of 15 that she wanted to be a scientist. She received her education at several schools, including North London Collegiate School, where she excelled in science, among other things.

Franklin enrolled at Newnham College, Cambridge, in 1938 and studied chemistry. In 1941, she was awarded Second Class Honors in her finals, which, at that time, was accepted as a bachelor's degree in the qualifications for employment. She went on to work as an assistant research officer at the British Coal Utilisation Research Association, where she studied the porosity of coal—work that was the basis of her 1945 Ph.D. thesis "The physical chemistry of solid organic colloids with special reference to coal."

In the fall of 1946, Franklin was appointed at the Laboratoire Central des Services Chimiques de l'Etat in Paris, where she worked with crystallographer Jacques Mering. He taught her X-ray diffraction, which would play an important role in her research that led to the discovery of "the secret of life"—the structure of DNA. In addition, Franklin pioneered the use of X-rays to create images of crystallized solids in analyzing complex, unorganized matter, not just single crystals.

DNA, Scientific Discoveries and Credit Controversy

In January 1951, Franklin began working as a research associate at the King's College London in the biophysics unit, where director John Randall used her expertise and X-ray diffraction techniques (mostly of proteins and lipids in solution) on DNA fibers. Studying DNA structure with X-ray diffraction, Franklin and her student Raymond Gosling made an amazing discovery: They took pictures of DNA and discovered that there were two forms of it, a dry "A" form and a wet "B" form. One of their X-ray diffraction pictures of the "B" form of DNA, known as Photograph 51, became famous as critical evidence in identifying the structure of DNA. The photo was acquired through 100 hours of X-ray exposure from a machine Franklin herself had refined.

John Desmond Bernal, one of the United Kingdom’s most well-known and controversial scientists and a pioneer in X-ray crystallography, spoke highly of Franklin around the time of her death in 1958. "As a scientist Miss Franklin was distinguished by extreme clarity and perfection in everything she undertook," he said. "Her photographs were among the most beautiful X-ray photographs of any substance ever taken. Their excellence was the fruit of extreme care in preparation and mounting of the specimens as well as in the taking of the photographs."

Despite her cautious and diligent work ethic, Franklin had a personality conflict with colleague Maurice Wilkins, one that would end up costing her greatly. In January 1953, Wilkins changed the course of DNA history by disclosing without Franklin's permission or knowledge her Photo 51 to competing scientist James Watson, who was working on his own DNA model with Francis Crick at Cambridge.

Upon seeing the photograph, Watson said, "My jaw fell open and my pulse began to race," according to author Brenda Maddox, who in 2002 wrote a book about Franklin titled Rosalind Franklin: The Dark Lady of DNA.

The two scientists did, in fact, use what they saw in Photo 51 as the basis for their famous model of DNA, which they published on March 7, 1953, and for which they received a Nobel Prize in 1962. Crick and Watson were also able to take most of the credit for the finding: When publishing their model in Nature magazine in April 1953, they included a footnote acknowledging that they were "stimulated by a general knowledge" of Franklin's and Wilkins' unpublished contribution, when in fact, much of their work was rooted in Franklin's photo and findings. Randall and the Cambridge laboratory director came to an agreement, and both Wilkins' and Franklin's articles were published second and third in the same issue of Nature . Still, it appeared that their articles were merely supporting Crick and Watson's.

According to Maddox, Franklin didn't know that these men based their Nature article on her research, and she didn't complain either, likely as a result of her upbringing. Franklin "didn't do anything that would invite criticism … [that was] bred into her," Maddox was quoted as saying in an October 2002 NPR interview.

Franklin left King's College in March 1953 and relocated to Birkbeck College, where she studied the structure of the tobacco mosaic virus and the structure of RNA. Because Randall let Franklin leave on the condition that she would not work on DNA, she turned her attention back to studies of coal. In five years, Franklin published 17 papers on viruses, and her group laid the foundations for structural virology.

Illness and Death

In the fall of 1956, Franklin discovered that she had ovarian cancer. She continued working throughout the following two years, despite having three operations and experimental chemotherapy. She experienced a 10-month remission and worked up until several weeks before her death on April 16, 1958, at the age of 37.

QUICK FACTS

Name: Rosalind Elsie
Birth Year: 1920
Birth date: July 25, 1920
Birth City: Notting Hill, London, England
Birth Country: United Kingdom
Gender: Female
Best Known For: British chemist Rosalind Franklin is best known for her role in the discovery of the structure of DNA, and for her pioneering use of X-ray diffraction.
World War II
Education and Academia
Science and Medicine
Astrological Sign: Leo
Newnham College
Cambridge University
Death Year: 1958
Death date: April 16, 1958
Death City: London, England
Death Country: United Kingdom

CITATION INFORMATION

Article Title: Rosalind Franklin Biography
Author: Biography.com Editors
Website Name: The Biography.com website
Url: https://www.biography.com/scientists/rosalind-franklin
Access Date:
Publisher: A&E; Television Networks
Last Updated: June 15, 2020
Original Published Date: April 2, 2014

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COMMENTS

Rosalind Franklin
One of these scientists was Rosalind Franklin (25 July 1920 - 16 April 1958). She was an expert in a technique called X-ray crystallography. Her work would hold the key to discovering the structure of DNA, the blueprint of life. A Scientist from a Young Age. At the age of 15, Rosalind Franklin decided she wanted to become a scientist.
Photograph 51, by Rosalind Franklin (1952)
On 6 May 1952, at King's College London in London, England, Rosalind Franklin photographed her fifty-first X-ray diffraction pattern of deoxyribosenucleic acid, or DNA. Photograph 51, or Photo 51, revealed information about DNA's three-dimensional structure by displaying the way a beam of X-rays scattered off a pure fiber of DNA. Franklin took Photo 51 after scientists confirmed that DNA ...
Photo 51
Photo 51 is an X-ray based fiber diffraction image of a paracrystalline gel composed of DNA fiber taken by Raymond Gosling, a postgraduate student working under the supervision of Maurice Wilkins and Rosalind Franklin at King's College London, while working in Sir John Randall's group.
Discovery of the structure of DNA (article)
Franklin's crystallography gave Watson and Crick important clues to the structure of DNA. Some of these came from the famous "image 51," a remarkably clear and striking X-ray diffraction image of DNA produced by Franklin and her graduate student. (A modern example of the diffraction pattern produced by DNA is shown above.)
What Rosalind Franklin truly contributed to the discovery of DNA's
(X-ray diffraction experiments in the 1930s had inadvertently used a mixture of the A and B forms of DNA, yielding muddy patterns that were impossible to fully resolve.) ... Rosalind Franklin was ...
Rosalind Franklin's Overlooked Role in the Discovery of DNA ...
English chemist and X-ray crystallographer Rosalind Elsie Franklin poses for a portrait, circa 1955. In 1951, Franklin joined a team of biophysicists led by John Randall at King's College who ...
What was Rosalind Franklin's true role in the discovery of DNA's double
This X-ray diffraction image, taken by a graduate student of Rosalind Franklin, shows the B form of DNA. The image, dubbed Photograph 51, is said to have inspired James Watson to realize that DNA ...
Rosalind Franklin
Rosalind Elsie Franklin (25 July 1920 - 16 April 1958) was a British chemist and X-ray crystallographer whose work was central to the understanding of the molecular structures of DNA (deoxyribonucleic acid), RNA (ribonucleic acid), viruses, coal, and graphite. Although her works on coal and viruses were appreciated in her lifetime, Franklin's contributions to the discovery of the structure ...
Rosalind Franklin: A Crucial Contribution
Rosalind Franklin: A Crucial Contribution ... In 1946, Franklin moved to Paris where she perfected her skills in X-ray crystallography, which would become her life's work. Although she loved the ...
Franklin's X-Ray Crystallography Experiments:
Franklin's X-Ray Crystallography Experiments: Refractions & Reflections on the Nature of Science. Regular substances like crystals diffract X-rays in characteristic patterns according to their physical structure. The X-ray crystallograph at right ("Photo 51") shows an exceptionally clear diffraction pattern of a crystallized DNA molecule.
Rosalind Franklin's X-ray photo of DNA as an undergraduate optical
Rosalind Franklin used X-ray diffraction to determine the structure of DNA molecules. One of her best X-ray pictures is numbered Photo 51 and is shown in Fig. 1(a). This photo was instrumental to J. D. Watson and F. Crick in deducing the double-helix model of DNA.
Franklin's X-ray diffraction, explanation of X-ray pattern
This is the X-ray crystallograph pattern of DNA obtained by Rosalind Franklin and Raymond Gosling in 1952. It is know as the B-form. It was clearer than the other X-ray patterns because water was included in the DNA sample. Both James Watson and Francis Crick were struck by the simplicity and symmetry of this pattern.
The structure of DNA: How Dr Rosalind Franklin contributed to the story
The discovery of the structure of DNA in 1953 was made possible by Dr Rosalind Franklin's X-ray diffraction work at King's. Her creation of the famous Photo 51 demonstrated the double-helix structure of deoxyribonucleic acid: the molecule containing the genetic instructions for the development of all living organisms.
Replicating X-ray of DNA
In 1951, Franklin was offered a 3-year research scholarship at King's College in London to set up and improve the x-ray crystallography unit there. X-ray crystallography is a biophysical technique where scientists make crystals of molecules and shine x-rays on them. The x-rays diffract and the diffracted pattern can be used to reconstruct the ...
Evolution: Library: The Discovery of DNA's Structure
Taken in 1952, this image is the first X-ray picture of DNA, which led to the discovery of its molecular structure by Watson and Crick.Created by Rosalind Franklin using a technique called X-ray ...
Francis Crick, Rosalind Franklin, James Watson, and Maurice Wilkins
These four scientists—Crick, Franklin, Watson, and Wilkins—codiscovered the double-helix structure of DNA, which formed the basis for modern biotechnology. At King's College London, Rosalind Franklin obtained images of DNA using X-ray crystallography, an idea first broached by Maurice Wilkins. Franklin's images allowed James Watson and ...
Rosalind Franklin
Rosalind Franklin contributed in double helical model of DNA by performing X-Ray diffraction experiments. The photographs taken by her and her student, Raymo...
Rosalind Franklin was so much more than the 'wronged heroine ...
In essence, it is because of Franklin, her collaborators and successors, that today's researchers are able to use tools such as DNA sequencing and X-ray crystallography to investigate viruses ...
(PDF) Rosalind Franklin's X-ray photo of DNA as an undergraduate
Rosalind Franklin used X-ray diffraction to determine the. structure of DNA molecules. One of her best X-ray pictures. is numbered Photo 51 and is shown in Fig. 1 (a). This photo. was instrumental ...
Rosalind Franklin :: DNA from the Beginning
In 1951, Franklin was offered a 3-year research scholarship at King's College in London. With her knowledge, Franklin was to set up and improve the X-ray crystallography unit at King's College. Maurice Wilkins was already using X-ray crystallography to try to solve the DNA problem at King's College.
How Rosalind Franklin Discovered the Helical Structure of DNA
Abstract. Rosalind Franklin, a chemical physicist (1920-1958), used X-Ray diffraction to determine the structure of DNA. In 1953 she described the DNA has a helical structure with a period of 34 A ...
Maurice Wilkins: Behind the Scenes of DNA
He was very successful in isolating single fibers of DNA and had already gathered some data about nucleic acid structure when Rosalind Franklin, an expert in X-ray crystallography, joined the unit ...
Rosalind Franklin
(1920-1958) Who Was Rosalind Franklin? Rosalind Franklin earned a Ph.D. in physical chemistry from Cambridge University. She learned crystallography and X-ray diffraction, techniques that she ...

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Francis Crick, Rosalind Franklin, James Watson, and Maurice Wilkins

Discovering the Structure of DNA

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Concept 19 The DNA molecule is shaped like a twisted ladder.

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