Mitochondria are small organelles that lie in the cytoplasm of eukaryotic cells, such as those of humans. Their primary purpose is to provide energy to the cell. Mitochondria are thought to be the vestigial remains of symbiotic bacteria that were once free living. One indication that mitochondria were once free living is that they contain a relatively small circular segment of DNA, called mitochondrial DNA (mtDNA). The overwhelming majority of a human's DNA is contained in chromosomes in the nucleus of the cell, but mtDNA is an exception. Individuals inherit their cytoplasm and the organelles it contains exclusively from their mothers, as these are derived from the ovum (egg cell) only, not from the sperm. When a mutation arises in the mtDNA molecule, the mutation is therefore passed in a direct female line of descent. These rare mutations are derived from copying mistakes - when the DNA is copied it is possible that a single mistake occurs in the DNA sequence, an outcome which is called a single-nucleotide polymorphism (SNP).
- How energy shapes life: our mitochondrial partners by Claire L Asher, UCL Genetics, Evolution and Environment blog, 21 June 2013
- What is mitochondrial DNA? from the Genetics Home Reference Handbook
- What is mitochondrial DNA? from the Genetics Home Reference Guide to Understanding Genetic Conditions
- Mitochondria An explanation from Nancy Custer
- Ian Logan's mtDNA website
- Mitomap A compendium of polymorphisms and mutations of the human mitochondrial DNA