DNA 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.