FUNDAMENTAL BUILDING BLOCKS 281
themselves. Instead they collected
data from others, including the
results of chemical experiments
that gave information about the
angles of the links, or bonds,
between the various ingredient
atoms and subgroups of DNA.
They also pooled their joint
knowledge of X-ray crystallography
and approached those researchers
who had made the highest-quality
images of DNA and other similar
molecules. One such image was
“photo 51,” which became key to
their achieving their breakthrough.
Photo 51 was an X-ray
diffraction image of DNA that
resembled an “X” seen through the
slats of a Venetian blind—fuzzy to
our eyes, but at that time, among
the sharpest and most informative
of DNA’s X-ray pictures. Some
debate surrounds the identity of
the photographer who took this
historic picture. It came from the
laboratory of a British biophysicist
named Rosalind Franklin, an
expert in X-ray crystallography,
and her graduate student Raymond
Gosling, at King’s College, London.
Each has been credited with the
image at various times.
Cardboard models
Also working at King’s was
Maurice Wilkins, a physicist who
was interested in molecular biology.
In early 1953, in what was perhaps
a break with scientific protocol,
Wilkins showed the images taken by
Franklin and Gosling, without their
permission or knowledge, to James
Watson. The American immediately
recognized their significance, and
took the implications straight back
to Crick. Suddenly their work was on
the right path.
From this point, the exact
sequence of events becomes
unclear, and later accounts of the
discovery are conflicting. Franklin
had described in unpublished draft
reports her thoughts about the
structure and shape of DNA. These
were also incorporated by Watson
and Crick as they struggled with
their various proposals. The main
idea, derived from Pauling’s alpha-
helix model and supported by
Wilkins, centered on some form of
repeating helical pattern for the
giant molecule.One of Franklin’s considerations
was whether the structural
“backbone,” a chain of phosphate
and deoxyribose sugar subunits,
was in the center with the bases
projecting outward, or the other
way around. Another colleague
who provided help was Austrian-
born British biologist Max Perutz,
who would win the Nobel prize in
Chemistry in 1962 for his work on
the structure of hemoglobin and
other proteins. Perutz also had
access to Franklin’s unpublished
reports and passed them on to the
ever-networking Watson and Crick.
They pursued the idea that DNA’s
backbones were on the outside,
with the bases pointing inward and
perhaps connecting to each other
in pairs. They cut out and shuffled
around cardboard shapes that
represented these molecular
subunits: phosphates and sugars in
the backbone, and the four types of
base—adenine, thymine, guanine,
and cytosine.
In 1952, Watson and Crick had
met Erwin Chargaff, an Austrian-
born biochemist, who had devised
what became known as Chargaff’s
first rule. This stated that in DNA,
the amounts of guanine and
cytosine are equal, as are the
amounts of adenine and thymine.
Experiments had sometimes shown
that all four amounts were roughly
equal, and sometimes not. The
latter findings came to be seen as
errors in methodology, and equal
amounts of all four bases came to
be accepted as the rule of thumb.Making the pieces fit
By splitting the base quantities
into two sets of pairs, Chargaff had
shed light on the structure of DNA.
Watson and Crick now began to
think of adenine as only and always
linking to thymine, and guanine
to cytosine. ❯❯Rosalind Franklin’s draft reports
on her theoretical models for DNA’s
structure were key to Watson and
Crick’s discovery of the double helix,
but she received little recognition in
her lifetime.We have discovered the
secret of life.
Francis Crick