linguistic:paraphrasing, structural:summarization, stylistic:style_transfer, information:highlighting
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 stands, 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. But 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 NH 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 in the late 1952 (published in February 1953) with an erroneous triple helix model. Ever cautious, Franklin wanted to eliminate misleading possibilities. Photographs of her Birkbeck work table show that Franklin routinely used small molecular models, although certainly not ones on the grand scale successfully used at Cambridge for DNA.

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.

Watson and Crick finished building their model on 7 March 1953, a day before they received a letter from Wilkins stating that Franklin was finally leaving and they could put "all hands to the pump". This was also one day after Franklin's two A-DNA papers had reached Acta Crystallographica. Wilkins came to see the model the following week, according to Franklin's biographer Brenda Maddox, on 12 March, and allegedly informed Gosling on his return to King's.

One of the most critical and overlooked moments in DNA research was how and when Franklin realised and conceded that B-DNA was a double helical molecule. When Klug first examined Franklin's documents after her death, he initially came to an impression that Franklin was not convinced of the double helical nature until the knowledge of the Cambridge model. But Klug later discovered the original draft of the manuscript (dated 17 March 1953) from which it became clear that Franklin had already resolved the correct structure. The news of Watson–Crick model reached King's the next day, 18 March, suggesting that Franklin would have learned of it much later since she had moved to Birkbeck. Further scrutiny of her notebook revealed that Franklin had already thought of the helical structure for B-DNA in February 1953 but was not sure of the number of strands, as she wrote: "Evidence for 2-chain (or 1-chain helix)." Her conclusion on the helical nature was evident, though she failed to understand the complete organisation of the DNA strands, as the possibility of two strands running in opposite directions did not occur to her.

Towards the end of February Franklin began to work out the indications of double strands, as she noted: "Structure B does not fit single helical theory, even for low layer-lines." It soon dawned to her that the B-DNA and A-DNA were structurally similar, and perceived A-DNA as an "unwound version" of B-DNA. Franklin and Gosling wrote a five-paged manuscript on 17 March titled "A Note on Molecular Configuration of Sodium Thymonucleate." After the Watson–Crick model was known, there appeared to be only one (hand-written) modification after the typeset at the end of the text which states that their data was consistent with the model, and appeared as such in the trio of 25 April 1953 Nature articles; the other modification being a deletion of "A Note on" from the title.

As Franklin considered the double helix, she also realised that the structure would not depend on the detailed order of the bases, and noted that "an infinite variety of nucleotide sequences would be possible to explain the biological specificity of DNA". However she did not yet see the complementarity of the base-pairing – Crick and Watson's breakthrough of 28 February, with all its biological significance; nor indeed at this point did she yet have the correct structures of the bases, so even if she had tried, she would not have been able to make a satisfactory structure. Science historians Nathaniel C. Comfort, of Johns Hopkins University, and Matthew Cobb, of the University of Manchester, explained that "She did not have time to make these final leaps, because Watson and Crick beat her to the answer."

Weeks later, on 10 April, Franklin wrote to Crick for permission to see their model. Franklin retained her scepticism for premature model building even after seeing the Watson–Crick model, and remained unimpressed. She is reported to have commented, "It's very pretty, but how are they going to prove it?" As an experimental scientist, Franklin seems to have been interested in producing far greater evidence before publishing-as-proven a proposed model. Accordingly, her response to the Watson–Crick model was in keeping with her cautious approach to science.

Crick and Watson published their model in Nature on 25 April 1953, in an article describing the double-helical structure of DNA with only a footnote acknowledging "having been stimulated by a general knowledge of Franklin and Wilkins' 'unpublished' contribution." Actually, although it was the bare minimum, they had just enough specific knowledge of Franklin and Gosling's data upon which to base their model. As a result of a deal struck by the two laboratory directors, articles by Wilkins and Franklin, which included their X-ray diffraction data, were modified and then published second and third in the same issue of Nature, seemingly only in support of the Crick and Watson theoretical paper which proposed a model for the B-DNA. Most of the scientific community hesitated several years before accepting the double helix proposal. At first mainly geneticists embraced the model because of its obvious genetic implications.