Deoxyribonucleic acid, a chemical found primarily in the nucleus of cells. DNA carries the instructions for making all the structures and materials the body needs to function. DNA is composed of a chain of nucleotides linked with ribose sugar molecules; specific patterns of nucleotides represent particular genes.

DNA is a large, double-stranded, helical molecule. The rungs of this double helix are formed by weak bonds between base pairs of nucleotides. The four nucleotides in DNA contain the bases: adenine (A), guanine (G), cytosine (C), and thymine (T). In nature, base pairs form only between A and T and between G and C; thus the base sequence of each single strand can be deduced from that of its partner.