The rest of the story of the discovery of the structure of DNA has been told and retold numerous times, but the recently discovered correspondence of Francis Crick does shed some new light on the frantic activity that preceded the publication of the Watson and Crick model.

Pauling’s blunder served as the catalyst that convinced Bragg to allow Watson and Crick to go back to DNA modeling. Within a couple of weeks, Watson went to London, where Wilkins, also pleased with Pauling’s glitch, took the liberty of showing him Franklin’s famous photograph 51 of the B form of DNA (figure 14), without Franklin’s knowledge. Much ink has been devoted to the question of the ethical nature of this particular act. In my humble opinion, three main parts to this story deserve attention. First, there was apparently no problem with Wilkins himself having a copy of the photo (given to him by Gosling), since Franklin was about to leave King’s to work at Birkbeck College, and she had been informed by the lab director, Sir John Randall, that the results of all the DNA work belonged exclusively to King’s. Second, there is little doubt (in my mind at least) that Franklin should have been consulted before her unpublished results were shared with members of another laboratory. Finally, there is disagreement on whether or not Watson and Crick acknowledged Franklin’s contribution adequately in their paper. You can be the judge of that. They wrote, “We have also been stimulated by a knowledge of the general nature of the unpublished experimental results and ideas of Dr. M. H. F. Wilkins, Dr. R. E. Franklin and their co-workers at King’s College, London.” Be that as it may, the effect the photo had on Watson was dramatic: The dark cross was the unmistakable sign of a helical structure. No wonder that, as he later described, his “mouth fell open,” and his “pulse began to race.”

Watson and Crick spent the following weeks trying frantically to build models in which the bases would form the rungs of the helical ladder they had in mind. The first attempts were unsuccessful. Ignoring the clue from Chargaff’s ratios, Watson mistakenly thought that he should pair every base with its twin, forming rungs composed of adenine-adenine (A-A), cytosine-cytosine (C-C), guanine-guanine (G-G), and thymine-thymine (T-T). However, since the bases C and T were different in length from G and A, this created rungs of unequal lengths, which was inconsistent with the symmetric pattern exhibited in photograph 51. There was also the question of the bond between the two bases in each rung and between the rung and the “legs” of the ladder (which were supposed to be composed of sugars and phosphates). Here again Watson and Crick were heading the wrong way, but their office mate Jerry Donohue came to the rescue. As a former student of Pauling’s, Donohue knew everything there was to know about hydrogen bonds. Donohue pointed out to Watson and Crick that even many textbooks had the hydrogen atoms in the wrong positions in thymine and guanine. Placing these atoms in their correct locations opened new possibilities for bonding the bases to each other. By shifting the bases in and out of other pairing possibilities (than the like-with-like), Watson suddenly realized that an A-T pair held together by two hydrogen bonds was identical to a G-C pair held similarly. The rungs became of equal length. Moreover, this pairing provided a natural explanation to Chargaff’s rules. Clearly, if A always paired with T, and G with C, then the numbers of A and T molecules in any section of DNA would be equal, and similarly for G and C. Another source of valuable information became available around that time via Max Perutz: a copy of Franklin’s report, written for a visit of the Medical Research Council biophysics committee to King’s. From the symmetry of the crystalline DNA described in that report, Crick concluded that the two strands of DNA were antiparallel—they ran in opposite directions.

Figure 15