What did Photo 51 reveal about the shape of DNA, and whose picture was it?

Photo 51 is a 1952 X-ray picture of DNA fibers, taken by Raymond Gosling, a PhD student of Rosalind Franklin at King's College London. It earned its name simply because it was the 51st diffraction photograph Gosling had taken. Watson recognized its pattern as a helix, because Francis Crick had previously published a paper on what the diffraction pattern of a helix would be.

The photo, together with evidence from other sources, let Watson and Crick build their model of DNA. It has a backbone of alternating deoxyribose and phosphate on the outside, and the base pairs, whose order codes for proteins and thereby inheritance, inside. Their model was published in 1953 in the same issue of Nature as papers by Wilkins and colleagues and by Gosling and Franklin.

Then came the controversy. By Gosling's later account, the picture was an exceptionally clear pattern of the B form of DNA, but Franklin was more interested in the A form and put it aside. The group's head had asked Gosling to share all his data with Maurice Wilkins, and Wilkins showed the photo to Watson and Crick without Franklin's knowledge. Whether that was wrong is disputed: one biography alleges that Wilkins acted without Franklin's permission, while others say Gosling gave it to Wilkins because she was leaving King's. Whether Franklin would have solved the structure alone is hotly debated.

Franklin died of ovarian cancer in 1958, at 37. The 1962 Nobel Prize in Physiology or Medicine went to Watson, Crick and Wilkins, and the rules at the time did not allow posthumous awards unless the nomination had been made while the candidate was alive. Franklin was never nominated.