DNA. Computer artwork of a DNA (deoxyribonucleic acid) double helix seen against autoradiograms of genetic sequences (upper right). The spiralling strands of the DNA helix (blue) are composed of complex chemical groups called nucleotides, which consist of a sugar phosphate and a base group. Complementary pairing between the base groups of nucleotides on opposite strands holds the helix together. The sequence of the 4 base groups (adenine, cytosine, guanine and thymine; coloured rods) along the DNA helix is unique for every individual and is known as the genetic code. The autoradiograms depict DNA base sequences, each band (dark blue) representing an individual base.