Mitochondrial DNA

Mitochondrial DNA (mtDNA) is the small circular genome found inside mitochondria, separate from the DNA in the cell nucleus. Human mtDNA contains 16,569 base pairs and is inherited almost entirely from the mother.

Human mtDNA has 37 genes: 13 for proteins of the electron transport chain and ATP production, 22 for transfer RNAs and 2 for ribosomal RNAs. Most mitochondrial proteins are instead encoded in nuclear DNA and imported. The existence of this separate genome is a main piece of evidence that mitochondria descend from free-living bacteria that were taken into an ancestral cell.

A cell contains hundreds to thousands of mtDNA copies. Sperm mitochondria are usually destroyed or diluted after fertilization, so the egg supplies nearly all of them. Because mtDNA does not recombine and mutates relatively fast, branches of the maternal family tree, called haplogroups, are used to trace human migrations and ancestry. A cell can hold both normal and mutated copies, a state called heteroplasmy.

Mutations in mtDNA cause disorders such as Leber hereditary optic neuropathy and MELAS, which typically affect energy-hungry tissues like muscle and brain. Because it is present in so many copies, mtDNA can be recovered from old bones, hair and ancient remains, and it was used to identify remains of historical figures. Sequencing methods are covered in DNA sequencing, and the molecule is part of the wider genome.

• Mitochondria — the organelles that hold it
• Genome — the genetic material as a whole
• Cell — the unit that contains it