DNA. Computer artwork of a DNA (deoxyribonucleic acid) double helix. The spiralling strands of the DNA helix are composed of complex chemical units called nucleotides. Complementary pairing (rods) between the base groups of nucleotides on opposite strands holds the helix together. The sequence of the four base groups (adenine, cytosine, guanine, thymine) along the DNA helix is unique for every individual. Known as the genetic code, it provides the instructions necessary for the development and functioning of every living organism.