Deoxyribonucleic acid. One of two types of molecules that encode genetic information. (The other is RNA. In humans DNA is the genetic material; RNA is transcribed from it. In some other organisms, RNA is the genetic material and, in reverse fashion, the DNA is transcribed from it.)
DNA is a double-stranded molecule held together by weak hydrogen bonds between base pairs of nucleotides. The molecule forms a double helix in which two strands of DNA spiral about one other. The double helix looks something like an immensely long ladder twisted into a helix, or coil. The sides of the ladder are formed by a backbone of sugar and phosphate molecules, and the rungs consist of nucleotide bases joined weakly in the middle by the hydrogen bonds.
There are four nucleotides in DNA. Each nucleotide contains a base: adenine (A), guanine (G), cytosine (C), or thymine (T). Base pairs form naturally only between A and T and between G and C so the base sequence of each single strand of DNA can be simply deduced from that of its partner strand.
The genetic code in DNA is in triplets such as ATG. The base sequence of that triplet in the partner strand is therefore TAC.