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. The image was tagged "photo 51" because it was the 51st diffraction photograph that Franklin had taken. It was critical evidence in identifying the structure of DNA.
According to a later account by Raymond Gosling, although Photo 51 was an exceptionally clear diffraction pattern of the "B" form of DNA, Franklin was more interested in solving the diffraction pattern of the "A" form of DNA, so she put Gosling's Photo 51 to the side. When it had been decided that Franklin would leave King's College, Gosling showed the photograph to Maurice Wilkins (who would become Gosling's advisor after Franklin left).
A few days later, Wilkins showed the photo to James Watson after Gosling had returned to working under Wilkins' supervision. Franklin did not know this at the time because she was leaving King's College London. Randall, the head of the group, had asked Gosling to share all his data with Wilkins. Watson recognized the pattern as a helix because his co-worker Francis Crick had previously published a paper of what the diffraction pattern of a helix would be. Watson and Crick used characteristics and features of Photo 51, together with evidence from multiple other sources, to develop the chemical model of the DNA molecule. Their model, along with papers by Wilkins and colleagues, and by Gosling and Franklin, were first published, together, in 1953, in the same issue of Nature.
In 1962, the Nobel Prize in Physiology or Medicine was awarded to Watson, Crick and Wilkins. The prize was not awarded to Franklin; she had died four years earlier, and although there was not yet a rule against posthumous awards, the Nobel Committee generally does not make posthumous nominations. Gosling's work also was not cited by the prize committee.
The photograph provided key information that was essential for developing a model of DNA. The diffraction pattern determined the helical nature of the double helix strands (antiparallel). The outside of the DNA chain has a backbone of alternating deoxyribose and phosphate moieties, and the base pairs, the order of which provides codes for protein building and thereby inheritance, are inside the helix. Watson and Crick's calculations from Gosling and Franklin's photography gave crucial parameters for the size and structure of the helix.
Photo 51 became a crucial data source that led to the development of the DNA model and confirmed the prior postulated double helical structure of DNA, which were presented in the series of three articles in the journal Nature in 1953.
As historians of science have re-examined the period during which this image was obtained, considerable controversy has arisen over both the significance of the contribution of this image to the work of Watson and Crick, as well as the methods by which they obtained the image. Franklin had been hired independently of Maurice Wilkins, who, taking over as Gosling's new supervisor, showed Photo 51 to Watson and Crick without Franklin's knowledge. Whether Franklin would have deduced the structure of DNA on her own, from her own data, had Watson and Crick not obtained Gosling's image, is a hotly debated topic, made more controversial by the negative caricature of Franklin presented in the early chapters of Watson's history of the research on DNA structure, The Double Helix. Watson admitted his distortion of Franklin in his book, noting in the epilogue: