Fatefully, such amity did not prevail at a laboratory over at King's College, London, where a woman named Rosalind Franklin was creating the world's best X-ray diffraction pictures of DNA. Maurice Wilkins, a colleague who was also working on DNA, disliked the precociously feminist Franklin, and the feeling was mutual. By Watson's account, this estrangement led Wilkins to show Watson one of Franklin's best pictures yet, which hadn't been published. "The instant I saw the picture my mouth fell open," Watson recalled. The sneak preview "gave several of the vital helical parameters."

Franklin died of cancer in 1958, at 37. In 1962 the Nobel Prize, which isn't given posthumously, went to Watson, Crick and Wilkins. In Crick's view, if Franklin had lived, "it would have been impossible to give the prize to Maurice and not to her" because "she did the key experimental work." And her role didn't end there. Her critique of an early Watson and Crick theory had sent them back to the drawing board, and her notebooks show her working toward the solution until they found it; she had narrowed the structure down to some sort of double helix. But she never employed a key tool — the big 3-D molecular models that Watson and Crick were fiddling with at Cambridge.

It was Watson who fit the final piece into place. He was in the lab, pondering cardboard replicas of the four bases that, we now know, constitute DNA's alphabet: adenine, thymine, guanine and cytosine, or A, T, G and C. He realized that "an adenine-thymine pair held together by two hydrogen bonds was identical in shape to a guanine-cytosine pair." These pairs of bases could thus serve as the rungs on the twisting ladder of DNA.

Here — in the "complementarity" between A and T, between C and G — lay the key to replication. In the double helix, a single strand of genetic alphabet — say, CAT--is paired, rung by rung, with its complementary strand, GTA. When the helix unzips, the complementary strand becomes a template; its G, T and A bases naturally attract bases that amount to a carbon copy of the original strand, CAT. A new double helix has been built.

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