(8 of 9)
DNA must be a helix after all, and on a cold train ride back to Cambridge, Watson decided that two helical sugar-phosphate backbones made more sense than three. "Thus by the time I had cycled back to college and climbed over the back gate, I had decided to build two-chain models. Francis would have to agree. Even though he was a physicist, he knew that important biological objects came in pairs."
It wasn't just the clarity of Franklin's picture that excited Watson. It was also the fact that the pattern repeated itself every 34 angstroms (an angstrom is one ten-billionth of a meter). That gave Crick and Watson crucial information about the angles between bonded molecules. Even better, the image suggested that the bases attached to the backbone were neatly stacked one on top of the other.
But were the two backbones on the inside of DNA or on the outside? Inside was a lot more straightforward; with the attached bases pointing outward, whatever code they might carry would be easily accessible. There seemed no chemically viable way to parse it, however, although Watson spent several days trying. Finally, he writes, "as I took apart a particularly repulsive backbone-centered molecule, I decided that no harm could come from spending a few days building backbone-out models." This would raise the tricky question of how to pack strings of bases against one another. But Watson put aside that worry for the moment.
On Feb. 8, 1953, the Cricks had Wilkins and Watson to lunch, and the Cavendish scientists learned several things. First, it was O.K. with Wilkins if they proceeded with their model building (a good thing, since they had already started and had no intention of stopping now). More important, they evidently also learned that the King's group had prepared a report on its DNA studies for the Medical Research Council, which funded the work. It wasn't a confidential document, so Watson and Crick got hold of a copy. In it were some more crucial clues, including the fact that DNA had a particular type of structural symmetry that implied that the molecule was made of two chains running in opposite directions.
But there remained the problem of how to fit the bases together. Watson kept trying to do it by pairing like with like--an A attached to one backbone linked to an A on the other. Chemically, it would work. The bases were different enough in size and shape, though, that this scheme led to either a gap between bases or misshapen backbones. Worse yet, when Watson happened to show his idea to Jerry Donohue, an American crystallographer doing a stint at the Cavendish, Donohue informed him that the bases came in more than one chemical form. Watson was using the form prescribed in standard textbooks. But the textbooks, Donohue insisted, were wrong.
It took about a week for Watson and Crick to see that Donohue was right. The Cavendish machine shop would have to build new pieces for their models. Watson couldn't wait. He spent the afternoon of Feb. 27 cutting his own pieces out of cardboard. Then he went out to the theater.