The
next morning Watson began trying different arrangements
of the bases. He was intrigued to find that an adenine-thymine
pair hydrogen bonded together in exactly the same shape
as a guanine-cytosine pair. When he showed the structure
to Donohue the chemist agreed that the bonds formed
naturally.
These
two structures could fit regularly between two sugar-phosphate
backbones without any variation in diameter. They would
also explain Chargaff's rule, as a structure built out
of these pairs would always have as many adenines as
thymines, and as many cytosines as guanines.
Even
more importantly, this structure would explain how a
gene could copy itself. Each strand on its own could
build its counterpart from free bases, since the interactions
meant the same two bases always pair off together. You
only needed one of the two chains to know the exact
structure of the other, so the double helix could split
and form two new but identical DNA molecules.
