DNA molecules. Computer artwork of the molecular structure of DNA (deoxyribonucleic acid) double helices. The atoms which make up the molecules are shown as coloured spheres. DNA molecules carry the genetic code which controls and transmits an organism's hereditary traits. They are composed of two twisting sugar-phosphate strands (gold/red spheres), linked on the inside of the helix by complementary pairs of nucleotide bases (blue, gold and red spheres). There are four types of nucleotide base; adenine, guanine, cytosine and thymine. The sequence of base pairs, arranged in discrete segments known as genes, determines the genetic code.