5.
The Basics of DNA
 We
now know that the DNA double helix looks like a twisted
ladder but before 1953 chemists could only see fragments
of the 'rung' of this ladder. They knew that these individual
parts had a 'sugar-phosphate' chemical group connected
at right angles to a 'base'.
The
sugar-phosphate groups of these 'rungs' can bond to
each other to form a very long chain, commonly known
as the sugar-phosphate backbone of DNA. But each rung
can have a different base, from a selection of four.
The bases are adenine, cytosine, guanine and thymine.
The order in which they occur along the DNA chain builds
a distinct genetic code from the four letters.
Chemists
knew that these chains could form, but didn't know how
they were arranged inside the molecule. How many chains
were there? Two, three, or more? How did the chains
bond to each other? Were the bases at the centre of
the molecule or sticking out on the outside?
It
was hoped that X-ray analysis of the DNA crystals would
answer some of these questions. By studying the X-ray
photographs possible structures could be rejected or
accepted.
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