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Photograph 51 is the nickname given to an X-ray diffraction image of DNA taken by Raymond Gosling in May 1952, working as a PhD student under the supervision of Rosalind Franklin, at King's College London in Sir John Randall's group. It was critical evidence in identifying the structure of DNA.
James Watson was shown the photo by Maurice Wilkins without Rosalind Franklin's approval or knowledge (although by this time Gosling had returned to the supervision of Wilkins). Along with Francis Crick, Watson used characteristics and features of Photo 51 to develop the chemical model of the DNA molecule. 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 the Nobel Prize's rules require that it be awarded only to living persons.
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 molecules, 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 articles in the Nature journal by Raymond Gosling.
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 was hired independently of Maurice Wilkins, who, nonetheless, showed Photo 51 to Watson and Crick, without her 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: "Since my initial impressions about [Franklin], both scientific and personal (as recorded in the early pages of this book) were often wrong, I want to say something here about her achievements."