THE STRUCTURE OF DNAKIN
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Astbury’s PhD student, Florence
Bell, took the first of hundreds of X-ray
diffraction pictures of DNA that year.
The fact that it produced a pattern at
all suggested that DNA had a ‘solvable’
structure. Astbury and Bell’s pictures
look like smea rs compa red to t he
clear images that Rosalind Franklin
was able to produce in the early 1950s,
but their pictures did reveal one
crucial fact: the distance between the
bases in the DNA molecule.
In 1938, Astbury used the images he
and Bell produced to propose a
structure for DNA in which the bases
are stacked on top of each other, butconditions in which only DNA – not
protein – could be transferred. In this
way, t hey deter mined t hat only DNA
could pass on traits. Though many
would refuse to believe it, DNA had
been st rongly implicated as t he ca r rier
of inheritance and science had the
tools to find out what it looked like.
The stage was set for the race to find
the structure of DNA in the 1950s –
only not everyone knew it was a race.
DNA research was to benefit from
the post-World War Two mood in
science, as many physicists who had
been employed in wa r work tu r ned
their attention to the more benign 5the pictures weren’t detailed enough
for him to get further.Clues in bacteria
Meanwhile, back in the US, a medical
researcher named Oswald Avery was
busy refining a 1928 experiment by a
British microbiologist called Fred
Griffith. He had shown that it was
possible to make harmless bacteria
and their progeny dangerous by
mixing t hem wit h vir ulent bacteria,
suggesting that something was being
transferred from the virulent to
harmless bacteria. Avery and his
colleagues deliberately createdIt was a photo taken by biophysicist and cr ystallographer Rosalind Franklin that
held the key to determining the make-up and structure of the DNA moleculeTHE KEY EXPERIMENT
Rosalind Franklin’s key experiment – the results
of which James Watson glimpsed – was a series
of X-ray crystallography experiments with DNA
samples containing different amounts of water.
The most famous outcome of this is 1952’s
‘Photo 51’, which revealed key details about
the structure of DNA.
The more a feature is repeated within
a structure, the more the film will be
bombarded with X-rays diffracted in
the same way, and the darker the
corresponding patch in the image. The
large dark patches at the top and bottom
of the picture represent DNA’s bases,
while the X-shaped blobs indicate a
helix. The arms of the cross represent
the planes of symmetry in a helix viewed
from the side; the ‘zig’ and the ‘zag’ of its
turns. There are 10 spots on each arm of
the cross before you reach the large black
patch at the top, which corresponds with
10 bases stacked one on top of the other in
each turn of the helix. The fourth blob from
the centre is missing, which indicates that one
strand of DNA is slightly offset against the other.
Rosalind Franklin turned her attention to
Photo 51 in early 1953. Her notebooks suggest that
she had gleaned all its key information and may,
in time, have reached the same conclusions as
Watson and Crick.‘Photo 51’, taken via X-ray
crystallography by Rosalind
Franklin, that reveals DNA’s
double-helix structure“They created conditions in which only DNA
(not protein) could be transferred, hence
determining that only DNA could pass on traits”