282 JAMES WATSON AND FRANCIS CRICK
In assembling the cardboard
pieces for their 3-D jigsaw, Watson
and Crick were juggling a vast
amount of data, working from
mathematics, X-ray images, their
own knowledge of chemical bonds
and their angles, and other data—
all approximate and subject to
ranges of errors. Their final
breakthrough came when they
realized that making slight
adjustments to the configurations
of thymine and guanine allowed
the pieces to fit together, producing
an elegant double helix in which
the pairs of bases linked along the
middle. Unlike the protein alpha-
helix, which had 3.6 subunits in
one complete turn, DNA had about
10.4 subunits per turn.
The model that Watson and
Crick described consisted of two
helical or corkscrew phosphate-
sugar backbones curling around
each other, like the uprights of a
“twisted ladder,” connected by
pairs of bases serving as rungs.
The sequence of bases worked like
letters in a sentence, carrying small
units of information that combined
to make an overall instruction,
or gene—which in turn told thecell how to make the particular
protein or other molecule that was
the physical manifestation of the
genetic data and had a particular
role in the cell’s fabric and function.Zip and unzip
Each pair of bases is connected
by what chemists call hydrogen
bonds. These are made and broken
relatively easily, so the sections of
the double-helix can be “unzipped”
by undoing the bonds, which then
exposes the code of bases as a
template for making a copy.
This zip-unzip allowed two
processes to occur. First, a mirror
complementary copy of nucleic acid
could be made from one unzippedThese are human male chromosomes.
Before Crick and Watson’s discovery,
it had been known that chromosomes
carry genes that pass from a dividing
cell to a daughter cell.