DNA molecule. Computer artwork of the molecular structure (left) of a DNA (deoxyribonucleic acid) double helix. DNA carries the genetic code, unique for every individual, that controls the growth and functioning of every living organism. The helix is composed of two twisting sugar-phosphate strands, (blue) linked on the inside of the helix by complementary pairs of nucleotide bases (coloured horizontal bars). The four colours match the four nucleotide bases: adenine, guanine, cytosine and thymine. The sequence of base pairs, arranged in discrete segments known as genes, determines the genetic code. This code can be displayed as bands (blue) in an autoradiogram (top right).