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Rosalind Franklin, and Raymond Gosling were investigating DNA using their expertise in producing photographic representations of three-dimensional molecular structures. X-ray diffraction, a standard technique for establishing three-dimensional models of complex molecules, was brought to London by Wilkins from the Manhattan Project, which created the first atomic bombs. The principle is similar to the kind of silhouette portraits that were fashionable in the eighteenth and nineteenth centuries: shine a beam at your subject and capture the light and dark that is projected past it. The human subject in portraiture is a solid to the visible light that this technique uses, but in the molecular version the X-rays penetrate the molecule under scrutiny and create signature shadows behind it, regular but cryptic swirls cast onto the photographic plate. Mathematical deduction is required to figure out the arrangement of atoms that could produce such a pattern, but the effect is the same: a unique portrait of a molecule otherwise too small to see. Franklin was particularly skilled at this technique, and of the many photographs that Franklin, along with Gosling, developed while performing this arduous method, Photo 51 became the key to one of the great achievements in human history.
    The Cambridge scientists Francis Crick and James Watson acquired the photograph. Science always builds on the work of others, but it was with their insight and genius that they deduced from Franklin and Gosling’s photo that DNA took the form of a twisted ladder: the iconic double helix. On April 25, 1953, in a brief paper published in the scientific journal
Nature,
Crick and Watson showed that the rungs of this twisted ladder contained paired chemical letters—
A
for adenine,
T
for thymine,
C
for cytosine, and
G
for guanine. Each letter is bound to one vertical section of the ladder and pairs up with a corresponding letter on the other upright to form a rung. It is this pairing that makes the helix doubled, and the pairing is very precise:
A
always pairs with
T;
C
always pairs with
G
. Crick and Watson concluded the paper with one of science’s great understatements: “It has not escaped our notice that the specific pairing we have postulated immediately suggests a copying mechanism for the genetic material.”
    This is the first marvelous thing about DNA. If you split the double helix into its two component strands, you immediately have the information to replace the missing strand: where there is an
A,
the other strand should have a
T,
and where there is a
C,
the other strand needs a
G.
Therefore, DNA possesses an ability, inherent in its structure, to provide the instructions for its own replication. Thanks to Crick and Watson, building on the work of Wilkins, Gosling, and particularly Rosalind Franklin, we were given a molecule that could be copied and passed from generation to generation. 3
    The letter molecules, known as nucleobases or simply bases, bind to one another, holding the two vertical parts of the ladder together. There are millions of such pairings on each strand of DNA, something like six and a half feet of it in every cell that has a nucleus, though it is tightly wound up and then winds up on itself again around small lumps of proteins, like beads on a string. And this winds up on itself yet again, which forms a chromosome, like a thick tug-of-war rope.
    The number and length of chromosomes varies hugely between all species, and that variation doesn’t appear to relate to either the size or complexity of the host. We have twenty-three pairs, and bacteria tend to have just one, in a neat loop. But some species of carp have over a hundred, and this doesn’t come anywhere near the number in some plant chromosomes, which can be in the thousands. The full collection of an organism’s DNA, packaged into chromosomes, is called a genome. In humans, those twenty-three chromosomes, our genome, contain around three