Friday night, after cutting out the cardboard bases, still deep in defeat Watson went home and then to the theatre. Saturday morning, February 28, he came in, cleared a place to work, got out his cardboard cutouts.
Though I initially went back to my like-with-like prejudices, I saw all too all too well that they led nowhere. When Jerry [Donohue] came in I looked up, saw that it was not Francis, and begin shifting the bases in and out of various other pairing possibilities. Suddenly 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. All the hydrogen bonds seem to form naturally; no fudging was required to make the two types of base pair identical in shape.1Watson stumbled into this part of the solution visually, from a shape, a representation, and that had happened several times before; that is the way his mind works. Note two of the four kinds of bases have the same contour. Watson found that the purine adenine, a fused double ring with other atoms fringing it at several points, could form two hydrogen bonds with the pyrimidine thymine, a single ring, when he placed the two cutouts side by side in the right way. The bond were the correct length, and were straight lines, N—H--O or N--H—N, as Pauling's model-building precepts required. Guanine and cytosine made hydrogen bonds the same way. The pairing could not be switched, however, for then the various atoms around the fringes got in each other's way. But when an A-T pair was laid on top of a G-C pair, the two compound shapes were exactly congruent. Such pairs could fit inside the backbones without bulges or pinches.
Donahue said these pairs agreed with what he knew. Crick, when he came in, immediately pointed out that the way the bases in these pairs would attach to their sugars meant that the two backbones ran in opposite directions, just as they had to do. Each chain could include both purines and pyrimidines, with pairs flipped over. That satisfied the dyadic symmetry. Chargaff's ratios were satisfied, too. The bases could appear in any order on one chain. Once that order was fixed, though, the base pairing, guanine always with cytosine and adenine with thymine, determined a complementary order on the opposite chain.
That morning, Watson and Crick knew, although still in mind only, the entire structure: it had emerged from the shadow of billions of years, absolute and simple, and was seen and understood for the first time. Twenty angstrom units in diameter, seventy-none billionths of an inch. Two chains twinning coaxially, clockwise, one up the other down, a complete turn the screw in 34 angstroms. The bases flat in their pairs in the middle, 3.4 angstroms and a tenth of a revolution separating a pair from the one above or below. The chains held by the pairing closer to each other around the circumference one way than the other, by an eighth of a turn, one groove up the outside narrow, the other wide. A melody for the eye of the intellect, not a note wasted. In itself, physically, structure carried the means of replication—positive to negative, complementary. As the strands unwound, at double template was there in the base pairing, so that only complementary nucleotides could form bonds and drop into place as the daughter strands grew. ... one doubts, of course, that Crick and Watson altogether realized, that morning, what they had seen. "We have discovered the secret of life," Crick told everyone within earshot over drinks that noon at the Eagle. It was not the entire secret of life, yet truly for the first time at the ultimate biological level structure had become one with function, the antimony dialectically resolved. The structure of DNA is flawlessly beautiful.
Horace Freeland Judson
The Eighth Day of Creation Expanded edition 1996, pp. 148-150
Cold Spring Harbor Laboratory Press
1. James D. Watson, The Double Helix, p. 194