Rosalind Franklin studied X-ray diffraction images of DNA. The famous Photo 51 and related analysis provided important evidence for understanding the molecule's structure.
Building a molecular model involved several researchers and questions about the use of data. Franklin also studied viruses, so her scientific work should not be reduced to a single photograph.
Discovery of DNA structure
In November 1951, James Watson and Francis Crick of the Cavendish Laboratory in Cambridge University had started to build a molecular model of the B-DNA using data similar to that available to both teams at King's. Based on Franklin's lecture in November 1951 that DNA was helical with either two or three strands, they constructed a triple-helix model, which was immediately proven to be flawed. In particular, the model had the phosphate backbone of the molecules forming a central core. However, Franklin pointed out that the progressive solubility of DNA crystals in water meant that the strongly hydrophilic phosphate groups were likely to be on the outside of the structure; while the experimental failure to titrate the CO- and NH2 groups of the bases meant that these were more likely to be inaccessible in the interior of the structure.
This initial setback led Watson and Crick to focus on other topics for most of the next year.
Model building had been applied successfully in the elucidation of the structure of the alpha helix by Linus Pauling in 1951, but Franklin was opposed to prematurely building theoretical models, until sufficient data were obtained to properly guide the model building. She took the view that building a model was to be undertaken only after enough of the structure was known. Franklin's conviction was only reinforced when Pauling and Corey also came up with an erroneous triple-helix model in their late-1952 paper (published in February 1953). Ever cautious, Franklin wanted to eliminate misleading possibilities. Photographs of her Birkbeck work table show that Franklin routinely used small molecular models of DNA, although certainly not ones on the grand scale successfully used at Cambridge.
The arrival in Cambridge of Linus Pauling's flawed paper in January 1953 prompted the head of the Cavendish Laboratory, Lawrence Bragg, to encourage Watson and Crick to resume their own model building. Six weeks of intense efforts followed, as they tried to guess how the nucleotide bases pack into the core of the DNA structure, within the broad parameters set by the experimental data from the team at King's, that the structure should contain one or more helices with a repeat distance of 34 Angstroms, with probably ten elements in each repeat; and that the hydrophilic phosphate groups should be on the outside (though as Watson and Crick struggled to come up with a structure, they at times departed from each of these assumptions during the process).
Crick and Watson received a further impetus in the middle of February 1953 when Crick's thesis advisor, Max Perutz, gave Crick a copy of a report written for a Medical Research Council biophysics committee visit to King's in December 1952, containing many of Franklin's crystallographic calculations. This decisively confirmed the 34 Angstrom repeat distance; and established that the structure had C2 symmetry, immediately confirming to Crick that it must contain an equal number of parallel and anti-parallel strands running in opposite directions.
Since Franklin had decided to transfer to Birkbeck College and Randall had insisted that all DNA work must stay at King's, Wilkins was given copies of Franklin's diffraction photographs by Gosling. By 28 February 1953 Watson and Crick felt they had solved the problem enough for Crick to proclaim (in the local pub) that they had "found the secret of life". However, they knew they must complete their model before they could be certain. The closeness of fit to the experimental data from King's was an essential corroboration of the structure.