DNA, or deoxyribonucleic acid, is the fundamental hereditary material found in almost all living organisms. In human genetics, DNA is of central importance because it stores, transmits and expresses the genetic information that determines many biological characteristics of an individual. It is mainly located in the nucleus of human cells, where it is associated with proteins to form chromosomes. A small amount of DNA is also present in mitochondria. DNA is a long molecule made up of smaller units called nucleotides. Each nucleotide consists of three components: a deoxyribose sugar, a phosphate group and a nitrogenous base. There are four types of nitrogenous bases in DNA: adenine (A), thymine (T), guanine (G) and cytosine (C). The nucleotides are joined together to form long chains. DNA normally consists of two complementary strands arranged in the form of a double helix. The two strands are held together by hydrogen bonds between their nitrogenous bases. Adenine pairs specifically with thymine, while guanine pairs