Mitochondrial DNA

Mitochondrial DNA (mtDNA) is DNA that is located in mitochondria. This is in contrast to most DNA of eukaryotic organisms, which is found in the nucleus. Nuclear and mtDNA are thought to be of separate evolutionary origin, with the mtDNA being derived from bacteria that were engulfed by early precursors of eukaryotic cells. Thus in cells in current organisms, the vast majority of proteins found in the mitochondria (~1500 in mammals) are encoded by nuclear DNA: some, if not most, are thought to have been originally of bacterial origin and have since been transferred to the nucleus during evolution. In mammals, 100% of the mtDNA contribution to a zygote is inherited from the mother and this is true for most, but not all, organisms. Currently, human mtDNA is present at 100-10,000 copies per cell, with each circular molecule consisting of 16,569 base pairs with 37 genes, 13 proteins (polypeptides), 22 transfer RNA (tRNAs) and two ribosomal RNAs (rRNAs).

Unlike nuclear DNA in which the genes are rearranged by ~50% each generation (due to the process called recombination), there is usually no change in mtDNA from parent to offspring by this mechanism. Because of this and the fact that its mutation rate is higher than nuclear DNA and easily measured, mtDNA is a powerful tool for tracking matrilineage, and has been used in this role for tracking many species back hundreds of generations. Human mtDNA can also be used to identify individuals, however is not a failsafe way to discriminate involvement of people at crime scenes and is no longer commonly used in court cases for this purpose.

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.

Mitochondrial inheritance

Female inheritance

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). Some in vitro fertilization techniques, particularly injecting a sperm into an oocyte, may interfere with this.

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. Vilà et al have published studies tracing the matrilineal descent of domestic dogs to wolves. 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.

Because mtDNA is not highly conserved, and has a rapid mutation rate, it can be used in phylogenetic study. Biologists sequence a few selected genes across different species, and can build an evolutionary tree depending on how conserved or divergent the sequences happen to be.

Male inheritance

It has also been reported that mitochondria can occasionally be inherited from the father^{[citation needed]}.

Mixed inheritance

Occasionally the single parent inheritance process goes wrong, for example in inter-species hybrids. It has been proven that about 1-2% of a organism's mitochondria can be inherited from the father ^[1].

Genetic influence

Genetic illness

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).

See also

• Human mitochondrial genetics
• Paternal mtDNA transmission
• Single origin theory

References

• Petros, J et al (2006) Emory University School of Medicine 'DNA Key To Predicting Prostate And Renal Cancer' Summary in Science Daily [1]
• San Mauro, Diego, David J. Gower, Rafael Zardoya and Mark Wilkinson (January 2006). "A hotspot of gene order rearrangement by tandem duplication and random loss in the vertebrate mitochondrial genome". Molecular Biology and Evolution 23: 227–234.
• Sutovsky, P., et. al (Nov. 25, 1999). "Ubiquitin tag for sperm mitochondria". Nature 402: 371-372. DOI:10.1038/46466. Discussed in [2].
• Vila C, Savolainen P, Maldonado JE, Amorim IR, Rice JE, Honeycutt RL, Crandall KA, Lundeberg J, Wayne RK (1997). "Multiple and ancient origins of the domestic dog". Science 276 (5319): 1687-9. DOI:10.1126/science.276.5319.1687 PMID 9180076.
• Behar, D.,et al (January 2005) 'Nearly Half Of Ashkenazi Jews Descended From Four 'Founding Mothers' American Journal of Human Genetics Discussed in Science Daily [3]
• Trejaut JA, Kivisild T, Loo JH, Lee CL, He CL, et al. (2005) Traces of archaic mitochondrial lineages persist in Austronesian-speaking Formosan populations. PLoS Biol 3(8): e247 Discussed in Science Daily [4]
• J.P. Jakupciak, W.Wang, M.E. Markowitz, D. Ally, M. Coble, S. Srivastava, A. Maitra, P.E. Barker, D. Sidransky, and C.D. O'Connell. Mitochondrial DNA as a Cancer Biomarker. Journal of Molecular Diagnostics 2005 7: 258-267 Discussed in Science Daily [5]
• Orlando et al.: "Correspondence: Revisiting Neandertal diversity with a 100,000 year old mtDNA sequence." Current Biology 16, R400-402, June 6, 2006 Discussed in Science Daily [6]

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Mitochondrial Databases

Mitosearch (FTDNA)

EMPOP - Mitochondrial DNA Control Region Database

External links

• Mitomap - a human mitochondrial genome database [7]
• A polymorphism in mitochondrial DNA associated with IQ?
• mtDNA sequencing information

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