Genetic code. Computer artwork of DNA (deoxyribonucleic acid) double helix molecules surrounded by letters representing the genetic code. The genetic code, carried by DNA molecules, contains the instructions which control and transmit an organism's hereditary traits. Each double helix is composed of two twisting strands (pale white) of sugar-phosphate molecules, linked by complementary pairs of nucleotide bases (coloured spheres). There are four types of nucleotide base; adenine (A), guanine (G), cytosine (C) and thymine (T). The sequence of base pairs, arranged in discrete segments known as genes, determines the genetic code.