Photo 51

Photo 51 is an X-ray based fiber diffraction image of a paracrystalline gel composed of DNA fiber[1] taken by Raymond Gosling,[2][3] 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.

[12] 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.

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.

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][16][18][19] 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.