DNA molecule. Computer artwork of a DNA (deoxyribonucleic acid) double helix surrounded by letters representing the genetic code. The helix molecule contains the code, which controls and transmits an organism's hereditary traits. The helix is composed of two twisting strands (orange/purple) of sugar-phosphate molecules, linked by complementary pairs of nucleotide bases. The molecular structure of these groups can be seen superimposed onto the helix. 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.