Photo 51 is an X-ray diffraction image of a paracrystalline gel composed of DNA fiber  taken by Raymond Gosling, a graduate student working under the supervision of Rosalind Franklin in May 1952 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 and Gosling had taken. It was critical evidence in identifying the structure of DNA.
Use in discovering structure of DNA
James Watson was shown the photo by his collaborator, Maurice Wilkins, as Raymond Gosling, the author of the picture, had returned under his supervision. Rosalind 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. Along with Francis Crick, Watson 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, and manuscripts by Wilkins and colleagues, and 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. Likewise, Gosling's work 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: "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."
- A 56-minute documentary DNA - Secret of Photo 51, was broadcast in 2003 on PBS NOVA. Narrated by Barbara Flynn, the program features interviews with Wilkins, Gosling, Aaron Klug, Brenda Maddox, including Franklin's friends Vittorio Luzzati, Caspar, Anne Piper, and Sue Richley. The UK version produced by BBC is titled Rosalind Franklin: DNA's Dark Lady.
- The first episode of a PBS documentary serial, DNA, which aired on 4 January 2004 as "The Secret of Life", centres on and features the contributions of Franklin. Narrated by Jeff Goldblum, it features Watson, Wilkins, Gosling and Peter Pauling (son of Linus Pauling).
- A play entitled Photograph 51 by Anna Ziegler focuses on the role of x-ray crystallographer Rosalind Franklin in the discovery of the structure of DNA. This play won the 3rd STAGE International Script Competition in 2008. In 2015, the play was put on at London West End, with Nicole Kidman playing Franklin.
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Raymond Gosling: 'And the best structure B pattern we ever got is photo 51, which I took and was called 51 because that was the 51st photograph that we’d taken, Rosalind and I, in our efforts to sort out this A and B difference'.
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