is the basic hereditary material in all cells and contains
all the information necessary to make proteins.
DNA is a linear polymer that is made up
of nucleotide units . The nucleotide unit consists of a base,
a deoxyribose sugar, and a phosphate. There are four types
of bases: adenine (A), thymine (T), guanine (G), and cytosine
(C). Each base is connected to a sugar via a ß glycosyl
linkage. The nucleotide units are connected via the O3' and
O5' atoms forming phosphodiester linkages.
In normal DNA, the bases form pairs: A
to T and G to C. This is called complementarity. A duplex
of DNA is formed by two complementary chains that are arranged
in an anti-parallel manner.
The results of fiber and single crystal
x-ray crystallographic studies have shown that DNA can have
several conformations. The most common one is called B-DNA.
B-DNA is a right-handed double helix with a wide and narrow
groove. The bases are perpendicular to the helix axis.
DNA can also be found in the A form in which the major groove
is very deep and the minor groove is quite shallow.
A very unusual form of DNA is the left-handed
Z-DNA. In this DNA, the basic building block consists
of two nucleotides, each with different conformations. Z-DNA
forms excellent crystals.
Several years ago it was discovered that
nucleic acids can form four stranded structures and a
few examples of these molecules now exist. Occasionally mutations
occur in which a base is changed. Base pairs still form, but
they are not in the usual Watson-Crick geometry. Examples
of these mismatches have been characterized.
DNA structures have appeared that
are very unusual in that the end pairs are flipped
out or there are bulges.