The Gene: An Intimate History - Siddhartha Mukherjee Page 0,69

spoke “in a quick nervous style [with] . . . not a trace of warmth or frivolity in her words,” Watson would later write. “Momentarily I wondered how she would look if she took off her glasses and did something novel with her hair.” There was something purposefully severe and offhanded in Franklin’s manner of speaking; she delivered her lecture as if reading the Soviet evening news. Had anyone paid real attention to her subject—and not the styling of her hair—he might have noticed that she was circling a monumental conceptual advance, albeit with deliberate caginess. “Big helix with several chains,”IV she had written in her notes, “phosphates on the outside.” She had begun to glimpse the skeleton of an exquisite structure. But she gave only some cursory measurements, pointedly declined to specify any details about the structure, and then brought a witheringly dull academic seminar to its close.

The next morning, Watson excitedly brought news of Franklin’s talk to Crick. They were boarding a train for Oxford to meet Dorothy Hodgkin, the grande dame of crystallography. Rosalind Franklin had said little in her talk except to provide a few preliminary measurements. But when Crick quizzed Watson about the precise numbers, Watson could provide only vague answers. He had not even bothered to scribble numbers on the back of a napkin. He had attended one of the most important seminars in his scientific life—and failed to take notes.

Still, Crick got enough of a sense of Franklin’s preliminary thoughts to hurry back to Cambridge and begin building a model. They started the next morning, with lunch at the nearby Eagle Pub and some gooseberry pie. “Superficially, the X-ray data was compatible with two, three or four strands,” they realized. The question was, how to put the strands together and make a model of an enigmatic molecule?

A single strand of DNA consists of a backbone of sugars and phosphates, and four bases—A, T, G, and C—attached to the backbone, like teeth jutting out from a zipper strand. To solve the structure of DNA, Watson and Crick had to first figure out how many zippers were in each DNA molecule, what part was at the center, and which part at the periphery. It looked like a relatively simple problem—but it was fiendishly difficult to build a simple model. “Even though only about fifteen atoms were involved, they kept falling out of the awkward pincers set up to hold them.”

By teatime, still tinkering with an awkward model set, Watson and Crick had come up with a seemingly satisfactory answer: three chains, twisted around each other, in a helical formation, with the sugar phosphate backbone compressed in the center. A triple helix. Phosphates on the inside. “A few of the atomic contacts were still too close for comfort,” they admitted—but perhaps these would be fixed by additional fiddling. It wasn’t a particularly elegant structure—but maybe that was asking too much. The next step, they realized, was to “check it with Rosy’s quantitative measurements.” And then, on a whim—in a misstep that they would later come to regret—they called Wilkins and Franklin to come and have a look.

Wilkins, Franklin, and her student, Ray Gosling, took the train down from King’s the next morning to inspect the Watson and Crick model. The journey to Cambridge was loaded with expectations. Franklin was lost in her thoughts.

When the model was unveiled at last, it was an epic letdown. Wilkins found the model “disappointing”—but held his tongue. Franklin was not as diplomatic. One look at the model was enough to convince her that it was nonsense. It was worse than wrong; it was unbeautiful—an ugly, bulging, falling-apart catastrophe, a skyscraper after an earthquake. As Gosling recalled, “Rosalind let rip in her best pedagogical style: ‘you’re wrong for the following reasons’ . . . which she proceeded to enumerate as she demolished their proposal.” She may as well have kicked the model with her feet.

Crick had tried to stabilize the “wobbly unstable chains” by putting the phosphate backbone in the center. But phosphates are negatively charged. If they faced inside the chain, they would repel each other, forcing the molecule to fly apart in a nanosecond. To solve the problem of repulsion, Crick had inserted a positively charged magnesium ion at the center of the helix—like a last-minute dab of molecular glue to hold the structure together. But Franklin’s measurements suggested that magnesium could not be at the center. Worse, the structure modeled by Watson and Crick was so tightly packed that