Caption

DNA. Computer artwork of a DNA (deoxyribonucleic acid) double helix. The spiralling strands (orange) of the DNA helix are composed of complex chemical units called nucleotides. Nucleotides are made up of a constant sugar-phosphate group and a variable base group (coloured rods). Complementary hydrogen bonding (base pairing) 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 functioning of every living organism.