Mitochondrial DNA



Mitochondrial DNA (mtDNA) is the located in  called. Most other DNA present in organisms is found in the. Nuclear and mitochondrial DNA are thought to be of separate ary origin, with the mtDNA being derived from the circular genomes of the that were engulfed by the early ancestors of today's eukaryotic cells. Each mitochondrion is estimated to contain 2-10 mtDNA copies. In the cells of extant organisms, the vast majority of the proteins present in the mitochondria (numbering approximately 1500 different types in 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. Among multicellular animals, nearly all of the mtDNA in a fertilized egg is inherited from only one parent - the female. One mechanism for this is simple dilution: an egg contains 100,000 to 1,000,000 mitochondria, whereas a contains only 10 to 100. Another mechanism, documented for a few organisms, is that the sperm mitochondria do not enter the egg. Whatever the mechanism, this single parent (uniparental) pattern of mtDNA inheritance is found in most animals, most plants and in fungi as well.

In humans (and probably in metazoans in general), 100-10,000 separate copies of mtDNA are usually present per cell (egg and sperm cells are exceptions). In mammals, each circular mtDNA consists of 15,000-17,000 s, which encode the same 37 genes: 13 for proteins (polypeptides), 22 for  (tRNA) and one each for the small and large subunits of  (rRNA). This pattern is also seen among most metazoans, although in some cases one or more of the 37 genes is absent and the mtDNA size range is greater. Even greater variation in mtDNA gene content and size exists among fungi and plants, although there appears to be a core subset of genes that are present in all eukaryotes (except for the few that have no mitochondria at all). Some plant species have enormous mtDNAs (as many as 2,500,000 base pairs per mtDNA molecule!) but, surprisingly, even those huge mtDNAs contain the same number and kinds of genes as related plants with much smaller mtDNAs.

Use in identification
Unlike nuclear DNA, which is inherited from both parents and in which genes are rearranged in the process of, there is usually no change in mtDNA from parent to offspring. Although mtDNA also recombines, it does so with copies of itself within the same mitochondrion. Because of this and because the mutation rate of animal mtDNA is higher than that of nuclear DNA, mtDNA is a powerful tool for tracking ancestry through females and has been used in this role to track the ancestry of many species back hundreds of generations. Human mtDNA can be used to identify individuals.

Because the base sequence of animal mtDNA changes rapidly, it is useful for assessing genetic relationships of individuals or groups within a species and also for identifying and quantifying the phylogeny (evolutionary relationships; see ) among different species, provided they are not too distantly related. To do this, biologists determine and then compare the mtDNA sequences from different individuals or species. Data from the comparisons is used to construct a network of relationships among the sequences, which provides an estimate of the relationships among the individuals or species from which the mtDNAs were taken. This approach has limits that are imposed by the rate of mtDNA sequence change. In animals, the rapid rate of change makes mtDNA most useful for comparisons of individuals within species and for comparisons of species that are closely or moderately-closely related, among which the number of sequence differences can be easily counted. As the species become more distantly related, the number of sequence differences becomes very large; changes begin to accumulate on changes until an accurate count becomes impossible.

Female inheritance
In, 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 sperm's tail, which is used for propelling the sperm cells. Sometimes the tail is lost during fertilization. In it was reported that paternal sperm mitochondria (containing mtDNA) are marked with  to select them for later destruction inside the. 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 far back in time. (, 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 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 is based on the same type of analysis, attempting to discover the origin of ity by tracking the lineage back in time.

Because mtDNA is not highly conserved and has a rapid mutation rate, it is useful for studying the evolutionary relationships of organisms. Biologists can determine and then compare mtDNA sequences among different species and use the comparisons to build an evolutionary tree for the species examined.

Male inheritance
It has been reported that mitochondria can occasionally be  in some species such as s. Paternally inherited mitochondria have also been reported in some insects such as the fruit fly and the honeybee.

Evidence supports rare instances of male mitochondrial inheritance in some mammals as well. Specifically, documented occurrences exist for mice, where it was subsequently rejected. It has also been found in sheep, and in cloned cattle. It has been found in a single case in a human male and was linked to infertility.

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 illness
Mutations of mitochondrial DNA can lead to a number of illnesses including and  (KSS), which causes a person to lose full function of their heart, eye, and muscle movements. (See also ).