Photo 51 is a 1952X-ray basedfiber diffraction image of aparacrystalline gel composed ofDNA fiber^[1] taken byRaymond Gosling,^[2][3] a postgraduate student working under the supervision ofMaurice Wilkins andRosalind Franklin atKing's College London, while working inSir John Randall's group.^{[4][5][6][7][8]} The image was tagged "photo 51" because it was the 51st diffraction photograph that Gosling had taken.^[9] It was critical evidence^[10] in identifying the structure of DNA.^[11]

Photo 51 was taken byRaymond Gosling, working underRosalind Franklin, on 2 May 1952.^[12] According to a later account by Gosling, althoughPhoto 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'sPhoto 51 to the side. When it had been decided that Franklin would leave King's College, and with Gosling now reporting to Wilkins,^[13] Gosling showed the photograph toMaurice Wilkins^[14][15] (who would become Gosling's advisor after Franklin left).

A few days later, Wilkins showed the photo toJames Watson after Gosling had returned to working under Wilkins' supervision. Franklin did not know this at the time because she was leavingKing's College London. Randall, the head of the group, had asked Gosling to share all his data with Wilkins.^[5] Watson recognized the pattern as a helix because his co-workerFrancis Crick had previously published a paper of what the diffraction pattern of a helix would be.^[14] Watson and Crick used characteristics and features ofPhoto 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 ofNature.

In 1962, theNobel 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,^[16] the Nobel Committee generally does not make posthumous nominations.^[17] 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.^[11][18] 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.^[18]

Photo 51 became a crucial data source^[19] that led to the development of the DNA model and confirmed the prior postulateddouble helical structure of DNA, which were presented in the series of three articles in the journalNature 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, showedPhoto 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,^{[11][18][20][21]} 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.^[18][22][23] 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.^[24]

• A 56-minute documentary,DNA – Secret of Photo 51, was broadcast in 2003 onPBSNOVA.^[7] Narrated bySigourney Weaver, the program features interviews with Wilkins, Gosling,Aaron Klug,Brenda Maddox,^[25] including Franklin's friends Vittorio Luzzati,Donald Caspar, Anne Piper, and Sue Richley.^[26] The UK version produced by theBBC is titledRosalind Franklin: DNA's Dark Lady.^[27]
• The first episode of aPBS documentary serial,DNA, which aired on 4 January 2004^[28] as "The Secret of Life", centres on and features the contributions of Franklin. Narrated byJeff Goldblum, it features Watson, Wilkins, Gosling and Peter Pauling (son ofLinus Pauling).^[29]
• A play entitledPhotograph 51 byAnna Ziegler focuses on the role ofX-ray crystallographer Rosalind Franklin in the discovery of the structure of DNA.^[30][31] This play won the thirdSTAGE International Script Competition in 2008.^[32] In 2015, the play was put on at London West End, withNicole Kidman playing Franklin.^[33]
• A 107-minute documentary,Life Story,BBCHorizon science series 1987, starringJuliet Stevenson asRosalind Franklin, Nicholas Fry asRaymond Gosling^[34]

• List of photographs considered the most important

• X-ray crystallography
• DNA
• Black-and-white photographs
• Genetics in the United Kingdom
• History of genetics
• Works originally published in Nature (journal)
• 1952 works
• 1952 in art
• 1950s photographs

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