In Pursuit of the Gene

EPILOGUE © 281
In November 1951, Watson was invited to attend a colloquium at King’s
College where Rosalind Franklin presented her latest data. The following
week Watson and Crick began to construct a model, making use of what
they had gleaned from Wilkins over the previous months, the few facts that
Watson recalled, it turned out wrongly, from Franklin’s seminar, and some
theoretical ideas about helix diffraction recently derived by Crick. Within
the week, they had a model consisting of three sugar-phosphate chains
spiraling up a central core with the nucleotide bases pointing outward,
and Wilkins and Franklin were called up to Cambridge to view the final
product. In a matter of minutes, Franklin found a fatal flaw. In order to
account for the data, the sugar-phosphate chains had to be on the outside
of the structure, not buried within it. 9
More than a year after the fiasco of their first model, Watson and Crick
received word via Linus Pauling’s son, Peter, that Pauling had solved the
DNA structure. The preprint of the paper, however, revealed that Pauling’s
model, like Watson and Crick’s first model, was a three-chain structure
with the bases pointing out. Frantic to find the correct structure before
Pauling realized his mistake, Watson and Crick immediately resumed their
model building. This time, however, they did not have to rely on Watson’s
memory for the crystallographic facts, but instead were in possession of
Franklin’s actual data thanks to an internal Medical Research Council report,
which she had not intended to be made available to them. 10 After
briefly reconsidering models with the chains on the inside and bases facing
out, Watson began building a two-chain model with the helix spiraling
around a central core containing some later-to-be-determined configuration
of bases. At first Watson considered models in which the two chains
were held together by interactions between like bases. This model was
pleasing from a genetic point of view because it suggested that the one
chain had served as a template for the other, the scheme favored by Muller.
But the fact that the bases came in two distinct size classes—adenine and
guanine being two-ring structures while cytosine and thymine had only
one ring—presented a problem. In particular, the fact that a pair of tworinged
adenines or guanines would occupy more room within the central
core of the helix than a pair of single-ringed thymines or cytosines was difficult
to reconcile with the perfect regularity of the crystal structure.