What was Rosalind Franklin's famous 'Photo 51'?

Answer

The 1952 diffraction image showed the helical pattern and helped Watson and Crick build their 1953 model.

Context

Photo 51 was an X-ray diffraction image of the B form of DNA, taken in 1952 in the laboratory of King's College London, where Rosalind Franklin led the DNA diffraction work with her graduate student Raymond Gosling, who took the exposure. When X-rays pass through a crystalline fiber, the pattern on the film reflects the arrangement of atoms. Photo 51 showed a distinct X-shaped pattern, which is characteristic of a helix, and the spacing of the spots allowed calculation of the helix's dimensions.

The image reached James Watson and Francis Crick in early 1953, when Maurice Wilkins showed it to Watson, apparently without Franklin's knowledge, and Crick and Watson also saw a King's College report with her data. These details helped them to settle on a double helix with the bases inside and the sugar-phosphate backbones outside, and they published their model in Nature in April 1953, alongside papers by Wilkins, Franklin and colleagues.

Historians continue to debate how much credit Franklin received. She died in 1958 at 37, and the 1962 Nobel Prize went to Watson, Crick and Wilkins; Nobel Prizes are not awarded posthumously. Recent archival studies, including work from King's College, argue that Franklin was a co-contributor who understood the structure's implications and not merely an unwitting source of data.