DNA damage, conceptual image. Computer illustration representing a DNA (deoxyribonucleic acid) molecule being damaged. 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. Damage can occur to the DNA sequence through normal metabolic processes or external factors such as ionising radiation.