Pictures of the double helix of deoxyribonucleic acid
(DNA) have become so common that everyone is familiar
with its overall shape and structure (Figure 1a).This
structure is known as B-DNA, and represents an average
conformation of DNA, based on fibre diffraction studies.
However, this average shape of DNA is very unlikely to
exist within the cells of living organisms, for several
reasons. First, there is simply not enough room for the
based on the sequence. For example, a 200-bp piece of
DNA can run as if it were more than 1000 bp on an
acrylamide gel, if it has the right sequence. The double helix
is not the same uniform structure. The structure (and
function) of DNA depends on the sequence of the DNA.
There are three families of DNA helices: A-DNA, which
can readily form within certain stretches of purines (e.g.
GAGGGA); B-DNA, which is favoured by mixed
sequences (although the exact conformation depends on
the particular nucleotide sequence, as described below);
and Z-DNA, which is favoured by alternating pyrimidine–
purine steps (e.g. CGCGCG). The A-and B-DNA families
are right-handed helices, while the Z-DNA family has a