Mitochondrial DNA (mtDNA, or less popularly, mDNA) is DNA which is not located in the nucleus of the cell but in the mitochondria. Mitochondria are parts of the cell that generate fuel in the form of adenosine triphosphate (ATP), which drives the varied machinery of the cell.
Unlike most of the cell, the function of which is defined by the nuclear DNA, the mitochondria have their own DNA and are assumed to have evolved separately. Human mitochondrial DNA consists of 5-10 rings of DNA and appears to carry 16,569 base pairs with 37 genes (13 proteins, 22 tRNAs and two rRNAs) which are concerned with the production of proteins involved in cellular respiration. However they all need sub-units created by nuclear DNA in order to work.
mtDNA is typically passed on only from the mother during sexual reproduction (mitochondrial genetics), meaning that the mitochondria are clones. This means that there is little change in the mtDNA from generation to generation, unlike nuclear DNA which changes by 50% each generation. Since the mutation rate is easily measured, mtDNA is a powerful tool for tracking matrilineage, and has been used in this role for tracking many species back thousands of generations.
Origin of mitochondrial DNA
The existence of mitochondrial DNA also supports the endosymbiotic theory, which suggests that eukaryotic cells first appeared when a prokaryotic cell was absorbed into another cell without being digested. These two cells are thought to have then entered into a symbiotic relationship, forming the first organelle. The existence of separate mitochondrial DNA suggests that, at one point, mitochondria were separate entities from their current host cells.
Mutations of mitochondrial DNA can lead to a number of illnesses including exercise intolerance and Kearns-Sayre syndrome (KSS), which causes a person to lose full function of their heart, eye, and muscle movements. (See also Mitochondrial disease).
Mitochondria in mammalian sperm are usually destroyed by the egg cell after fertilization. In 1999 it was reported that paternal sperm mitochondria (containing mtDNA) are marked with ubiquitin to select them for later destruction inside the embryo (Sutovsky et. al. 1999). Occasionally this process goes wrong, for example in inter-species hybrids. It has also been reported that mitochondria can occasionally be inherited from the father, e.g. in bananas (Schwartz and Vissing, 2002).
The fact that mitochondrial DNA is maternally inherited enables researchers to trace uterine lineage far back in time. (Y chromosomal DNA, paternally inherited, is used in an analogous way to trace the agnate lineage.) This is accomplished in humans by sequencing one or more of the hypervariable control regions (HVR1 or HVR2) of the mitochondrial DNA. HVR1 consists of about 440 base pairs. These 440 base pairs are then compared to the control regions of other individuals (either specific people or subjects in a database) to determine maternal lineage. Most often, the comparison is made to the revised Cambridge Reference Sequence. Vilà et al have published studies tracing the matrilineal descent of domestic dogs to 4 individuals. The concept of the Mitochondrial Eve is based on the same type of analysis, attempting to discover the origin of humanity by tracking the lineage back in time.
See also: single origin theory.
- Marianne Schwartz and John Vissing, "Paternal Inheritance of Mitochondrial DNA", New England Journal of Medicine, Aug 22, 2002; 347:576-580. 
- "Mitochondria can be inherited from both parents", New Scientist article on Schwartz and Vissing's report; 
- Sutovsky, P., et. al. 1999. "Ubiquitin tag for sperm mitochondria." Nature 402(Nov. 25):371-372. Abstract available at  and discussed in .
- Wikipedia article on Mitochondrial DNA
- Mitomap - a human mitochondrial genome database 
- A polymorphism in mitochondrial DNA associated with IQ?
- mtDNA sequencing information