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
DNA to be stretched out in a perfect, linear B-DNA
conformation. In nearly all cells, from simple bacteria
through complex eukaryotes, the DNA must be compacted
by more than a thousandfold in order even to fit inside the
cell or nucleus. Furthermore, refined resolution of the
structure of DNA, based on X-ray crystallography of short
synthetic pieces of DNA, has shown that there is
considerable variance of the helical structure of DNA,