DNA molecule. Computer artwork of the molecular structure 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 (white vertical bars). There are four nucleotide bases: adenine, guanine, cytosine and thymine. It is the sequence of base pairs, arranged in discrete segments known as genes, that determines the genetic code.