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On Feb. 28, armed with his new cardboard bases, Watson began trying to match like with like again--and then he had an insight. "Suddenly," he writes, "I became aware that an adenine-thymine pair held together by two hydrogen bonds was identical in shape to a guanine-cytosine pair held together by at least two hydrogen bonds." If the bases were joined up this way, the backbones wouldn't be bumpy. Moreover, such an arrangement neatly explained what Chargaff had discovered in 1950. If A and T were always paired, there naturally had to be equal amounts of these two bases; same thing for G and C.
"Even more exciting," writes Watson, "this type of double helix suggested a replication scheme ... always pairing adenine with thymine and guanine with cytosine meant that the base sequences of the two intertwined chains were complementary to each other. Given the base sequence of one chain, that of its partner was automatically determined. Conceptually, it was thus very easy to visualize how a single chain could be the template for the synthesis of a chain with the complementary sequence."
He consulted Donohue. It made sense. Crick showed up about 40 minutes later; it made sense to him too. There were still details to work out, and Watson feared a repeat of their botch job in late 1951. "Thus," he writes, "I felt slightly queasy when at lunch Francis winged into the Eagle to tell everyone within hearing distance that we had found the secret of life."
But of course they had. Wilkins and Franklin would be informed within a few days--although they never told Franklin of the crucial role her photograph had played. The rest of the world would learn about the double helix in a one-page letter to Nature, which appeared on April 25, 1953. It began with the now famous understatement: "We wish to suggest a structure for the salt of deoxyribose nucleic acid (D.N.A.). This structure has novel features which are of considerable biological interest."
In retrospect, what they found is utterly straightforward and so elegant that Pauling or Wilkins or Franklin or someone would have come up with it, possibly within weeks. The reason we remember Watson and Crick instead is summed up nicely by Crick himself. "The major credit I think Jim and I deserve," he writes, "is for selecting the right problem and sticking to it. It's true that by blundering about we stumbled on gold, but the fact remains that we were looking for gold."