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Mitochondrial DNA ('''mtDNA''') is the DNA located in organelles called Mitochondria . Most other DNA present in Eukaryotic organisms is found in the Cell Nucleus . Nuclear and mitochondrial DNA are thought to be of separate Evolution ary origin, with the mtDNA being derived from the circular genomes of the Bacteria that were engulfed by the early ancestors of today's eukaryotic cells. In the cells of current organisms, the vast majority of the proteins present in the mitochondria (numbering approximately 1500 different types in Mammal s) are coded for by nuclear DNA, but the genes for some of them, if not most, are thought to have originally been of bacterial origin, having since been transferred to the eukaryotic nucleus during Evolution . In mammals, all mitochonrial DNA in a fertilized egg ( Zygote ) is inherited only from the mother, as the Sperm does not contribute any mitochondria; this holds true for most other organisms as well. Currently, human mtDNA is present at 100-10,000 separate copies per cell, with each circular Molecule consisting of 16,569 Base Pair s with 37 genes, 13 proteins (polypeptides), 22 Transfer RNA (tRNAs) and two Ribosomal RNA s (rRNAs). One mitochondrion can contain 2-10 copies of its DNA.1 USE IN IDENTIFICATION Unlike nuclear DNA, which is sourced from both parents and whose genes are rearranged in the process of Recombination , there is usually no change in mtDNA from parent to offspring; although mtDNA does recombine, it recombines with copies of itself within the same mitochondrion. Because of this, and the fact that the mutation rate of mtDNA is higher than that of nuclear DNA and is easily measured, mtDNA is a powerful tool for tracking ancestry through females ( Matrilineage ), and has been used in this role for tracking the ancestry of many species back hundreds of generations. Human mtDNA can also be used to identify individuals. ORIGIN OF MITOCHONDRIAL DNA Endosymbiotic Theory suggests that Eukaryotic Cell s first appeared when a Prokaryotic Cell (a bacterium) 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 . This organelle would eventually become today's mitochondrion, and the genome of that first absorbed bacterium would have given rise to today's mitochondrial DNA as evolution progressed. MITOCHONDRIAL INHERITANCE Female inheritance In Sexually Reproducing Organisms , mitochondria are normally inherited exclusively from the mother. The mitochondria in mammalian sperm are usually destroyed by the egg cell after fertilization. Also, most mitochondria are present at the base of the sperms tail, used for propelling the sperm cells, and this tail is lost during 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 .2}} Discussed in {Link without Title} . 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 Maternal 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 Region s (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 Human ity 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 they can build an evolutionary tree depending on how conserved or divergent the sequences happen to be. Male inheritance It has been reported that mitochondria can occasionally be s. Paternally inherited mitochondria have also been reported in some insects such as the fruit flyKondo R, Matsuura ET, Chigusa SI (1992). Further observation of paternal transmission of Drosophila mitochondrial DNA by PCR selective amplification method. Genet Res 59: 81-84. and the honeybee.Meusel MS, Moritz RF (1993). Transfer of paternal mitochondrial DNA during fertilization of honeybee (Apis mellifera L.) eggs. Curr Genet 24: 539-543. Evidence supports rare instances of male mitochondrial inheritance in some mammals as well. Specifically, documented occurrences exist for mice,Gyllensten U, Wharton D, Josefsson A (1991). Paternal inheritance of mitochondrial DNA in mice. Nature 352: 255-257. Shitara H, Hayashi JI, Takahama S, Kaneda H, Yonekawa H (1998). Maternal inheritance of mouse mtDNA in interspecific hybrids: segregation of the leaked paternal mtDNA followed by the prevention of subsequent paternal leakage. Genetics 148: 851-857. where it was subsequently rejected. It has also been found in sheep,Zhao X, et al. (2004). Further evidence for paternal inheritance of mitochondrial DNA in the sheep (Ovis aries). Heredity 93:399-403.and in cloned cattle. Steinborn R, Zakhartchenko V, Jelyazkov J, Klein D, Wolf E, Muller M et al (1998). Composition of parental mitochondrial DNA in cloned bovine embryos. FEBS Lett 426: 352-356. It has been found in a single case in a human male and was linked to infertilitySchwartz M, Vissing J (2002). Paternal inheritance of mitochondrial DNA. N Engl J Med 22: 576-580. While many of these cases involve cloned embryos or subsequent rejection of the paternal mitochondria, others document ''in vivo'' inheritance and persistence under lab conditions. 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
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