Computer illustration showing ultraviolet (UV) damage to the structure of a molecule of DNA (deoxyribonucleic acid). The UV light (left) causes bonds to form between two adjacent pyrimidine bases (red, at right). This distorts the shape of the molecule and breaks the links with the complementary bases. Deoxyribonucleic acid (DNA) consists of long strands forming a double helix of phosphates and sugars. These two helices are held together by pairs of nucleotide bases (sticks). There are four different nucleotide bases. It is the varying sequence of these four bases along a DNA helix that forms the genetic code for that individual. A typical DNA molecule can contain a sequence that is many millions of bases long. The genetic code in DNA is the basis of all life on Earth.