Rosalind Franklin and Photo 51: How X-rays Revealed the Shape of DNA

In May 1952, a physical chemist named Rosalind Franklin, working with her student Raymond Gosling at King's College London, recorded an X-ray photograph of DNA that is now known as Photo 51. It showed a clear X-shaped pattern of spots that told experts the molecule was a helix. Her careful measurements gave the key numbers for the width of the helix, the distance between turns and the spacing of the bases, and her data helped James Watson and Francis Crick build their famous double-helix model in 1953. Franklin's work also changed our understanding of coal, graphite and viruses. In this post we look at what she did, how X-ray diffraction reveals the helix, and what exam students should remember.

The Scientific Problem Before the Discovery

By the early 1950s scientists accepted that DNA carried genetic information, helped by Avery's 1944 experiments and the Hershey-Chase experiment in 1952. But nobody knew how a molecule made only of sugar, phosphate and four bases could store and copy information. Erwin Chargaff had shown that in any DNA the amount of adenine equals thymine and guanine equals cytosine (A = T and G = C), but the meaning of this rule was unclear. Linus Pauling had proposed a triple-helix model, which turned out to be wrong because it had the phosphates in the wrong place. The structure needed to be seen, and the only tool was X-ray diffraction.

The Discovery: Step-by-Step

1. Preparing fibres. Using DNA of high quality supplied by Rudolf Signer, Franklin and Gosling drew thin fibres in which molecules were aligned along the fibre axis.
2. Controlling humidity. Franklin showed that DNA exists in two forms depending on water content. At lower humidity, about 75%, the A form appears with a more crystalline pattern. At higher humidity, near 95%, the longer, thinner B form appears. Separating them was essential, because earlier workers had mixed the two patterns.
3. Long exposures. She used a finely focused X-ray beam and a special camera, with exposure times of many hours. Photo 51 of the B form needed an exposure of many hours.
4. Reading the pattern. The X-shaped cross indicates a helix. The spacing of layer lines gives the pitch, and a strong reflection far from the centre gives the spacing of stacked bases.
5. Placing the phosphates outside. From the A-form data and chemical reasoning, she concluded that the sugar-phosphate backbone lies on the outside and the bases point inward.

In April 1953, three papers appeared together in Nature: Watson and Crick's model, a paper by Wilkins, Stokes and Wilson, and a paper by Franklin and Gosling. The Franklin-Gosling paper provided experimental support for the helical structure.

The Chemistry Behind It

The main dimensions of B-DNA are:

Diameter of helix = about 20 Å (2 nm)
Pitch (one full turn) = 34 Å (3.4 nm)
Base-pair spacing = 3.4 Å (0.34 nm)
Base pairs per turn = about 10

The central cross pattern is a signature of a helix, because a helix can be thought of as a set of diffracting planes arranged along a spiral. Missing reflections in the pattern suggest two strands offset along the axis. The 3.4 Å reflection arises from stacked bases, and the 34 Å spacing of the layer lines gives the pitch.

Chemical forces then explain why the structure is stable. Adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three hydrogen bonds:

A=T (2 H-bonds)    G≡C (3 H-bonds)

The pairing of a purine with a pyrimidine keeps the width constant, and the base stacking (π–π and hydrophobic interactions) adds stability. The phosphate groups on the outside carry negative charges and make DNA soluble in water.

Franklin's earlier work on carbon is also important chemistry. She studied the porosity of coal and showed that carbons divide into two types: graphitising carbons, which turn into graphite on heating to high temperatures, and non-graphitising carbons, which keep a disordered structure. This distinction helped industry to use carbon in filters and gas masks. Later at Birkbeck College she worked on the structure of tobacco mosaic virus (TMV) and the polio virus, showing that the TMV rod is hollow.

Applications Today

• Molecular biology: The double helix underlies PCR, DNA sequencing, gene editing and cloning.
• Medicine: Many drugs, such as the anticancer drug cisplatin, bind to DNA, and their action is understood from structure.
• Forensics and ancestry: DNA fingerprinting depends on understanding DNA as a stable, copyable molecule.
• Materials: Carbon science based on her work supports activated carbon filters and carbon fibres.
• Virology: Structural virology, used in vaccine design, follows the approach she began.

Exam Corner

• B-DNA is a right-handed double helix with pitch 34 Å, diameter about 20 Å and 10 base pairs per turn.
• Chargaff's rule: A = T, G = C, so purines equal pyrimidines.
• A-form DNA is more compact and found at lower humidity; Z-DNA is left-handed.
• A–T has two hydrogen bonds; G–C has three.
• The Nobel Prize in Physiology or Medicine 1962 was awarded to Watson, Crick and Wilkins; Franklin had died in 1958 and Nobel Prizes are not given posthumously.

Common Misconceptions

• "Franklin discovered DNA." DNA was isolated by Miescher in 1869. Franklin helped determine its structure.
• "Franklin built the double-helix model." The model was built by Watson and Crick using her data and other evidence.
• "She was simply a technician." She was an independent expert in X-ray diffraction, who had already published important work on carbon.

Quick Revision Box

FAQ

What does the X shape in Photo 51 mean?

It is the characteristic diffraction pattern of a helix.

Why is humidity so important for DNA structure?

Water content changes how the molecule is packed. At high humidity the B form appears, which is the form present in living cells.

Who is Raymond Gosling?

He was Franklin's PhD student, who helped to take the X-ray photographs of DNA.

Did Franklin receive a Nobel Prize?

No. She died in 1958 and the prize for DNA structure was awarded in 1962 to Watson, Crick and Wilkins.

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