The Gene: An Intimate History - Siddhartha Mukherjee Page 0,71

revealed the most precious piece of her data to Watson.V (“Perhaps I should have asked Rosalind’s permission and I didn’t,” a contrite Wilkins would later write. “Things were very difficult. . . . If there had been anything like a normal situation here, I’d have asked her permission naturally, though if there had been anything like a normal situation, the whole question of permission wouldn’t have come up. . . . I had this photograph, and there was a helix right on the picture, you couldn’t miss it.”)

Watson was immediately transfixed. “The instant I saw the picture my mouth fell open and my pulse started to race. The pattern was unbelievably simpler than those obtained previously. . . . The black cross could arise only from a helical structure. . . . After only a few minutes’ calculations, the number of chains in the molecule could be fixed.”

In the icy compartment of the train that sliced across the fens back to Cambridge that evening, Watson sketched what he remembered of the picture on the edge of a newspaper. He had come back the first time from London without notes. He wasn’t going to repeat the same error. By the time he had returned to Cambridge and jumped over the back gate of the college, he was convinced that DNA had to be made of two intertwined, helical chains: “important biological objects come in pairs.”

The next morning, Watson and Crick raced down to the lab and started model building in earnest. Geneticists count; biochemists clean. Watson and Crick played. They worked methodically, diligently, and carefully—but left enough room for their key strength: lightness. If they were to win this race, it would be through whimsy and intuition; they would laugh their way to DNA. At first, they tried to salvage the essence of their first model, placing the phosphate backbone in the middle, and the bases projecting out to the sides. The model wobbled uneasily, with molecules jammed too close together for comfort. After coffee, Watson capitulated: perhaps the backbone was on the outside, and the bases—A, T, G, and C—faced in, apposed against each other. But solving one problem just created a bigger problem. With the bases facing outside, there had been no trouble fitting them: they had simply circled around the central backbone, like a spiral rosette. But with the bases turned inside, they had to be jammed and tucked against each other. The zipper’s teeth had to intercalate. For A, T, G, and C to sit in the interior of the DNA double helix, they had to have some interaction, some relationship. But what did one base—A, say—have to do with another base?

One lone chemist had suggested, insistently, that the bases of DNA must have something to do with each other. In 1950, the Austrian-born biochemist Erwin Chargaff, working at Columbia University in New York, had found a peculiar pattern. Whenever Chargaff digested DNA and analyzed the base composition, he always found that the A and the T were present in nearly identical proportion, as were the G and the C. Something, mysteriously, had paired A to T and G to C, as if these chemicals were congenitally linked. But although Watson and Crick knew this rule, they had no idea how it might apply to the final structure of DNA.

A second problem arose with fitting the bases inside the helix: the precise measurement of the outer backbone became crucial. It was a packing problem, obviously constrained by the dimensions of the space. Once again, unbeknownst to Franklin, her data came to the rescue. In the winter of 1952, a visiting committee had been appointed to review the work being performed at King’s College. Wilkins and Franklin had prepared a report on their most recent work on DNA and included many of their preliminary measurements. Max Perutz had been a member of the committee; he had obtained a copy of the report and handed it to Watson and Crick. The report was not explicitly marked “Confidential,” but nor was it evident anywhere that it was to be made freely available to others, to Franklin’s competitors, in particular.

Perutz’s intentions, and his feigned naïveté about scientific competition, have remained mysterious (he would later write defensively, “I was inexperienced and casual in administrative matters and, since the report was not ‘Confidential,’ I saw no reason for withholding it.”). The deed, nonetheless, was done: Franklin’s report found its way to Watson’s and Crick’s hands. And with the sugar-phosphate backbone placed on the