Mitochondrial DNA

The ring-shaped, double-stranded mtDNA molecule has the same basic structure in all mammals. It is approximately 16,500 bp in length and contains coding sequences for 13 genes, 2 ribosomal RNA molecules (12S and 16S), and 22 transfer RNA molecules, together with a non-coding control region (D-loop). In contrast to nuclear genes, there are no introns in mtDNA. Furthermore, mtDNA differs from nDNA in another crucial respect that simplifies analysis of its evolution. In mammals, mtDNA is exclusively or almost exclusively inherited maternally (i.e., from the mother), and there is no recombination of genes when the mitochondrion

Images Placental Mammals
The giant anteater (Myrmecophaga tridactyla) belongs to the Xenarthra cluster of placental mammals. (Photo by © Tom Brakefield/Corbis. Reproduced by permission.)

divides. Phylogenetic reconstructions may be based on part of mtDNA (e.g. using an individual gene, such as cytochrome b) or on the entire molecule, and many complete mtDNA sequences are now available for analysis. Overall, mtDNA tends to accumulate changes more rapidly than nDNA (about five times faster overall), and for this reason it is more suitable for analyses of relatively recent changes in the evolutionary tree of mammals. Because rapidly evolving DNA sequences become saturated with changes at an earlier stage, they are unsuitable for probing early parts of the tree. However, there are differences in rate of evolution between individual parts of the mtDNA molecule, so it is possible to select regions that are suitable for particular stages of mammalian evolution. Mitochondrial DNA sequences can be crudely divided into those that evolve relatively rapidly, hence being useful for comparisons of quite closely related species (e.g. control region, ATPase gene) and those that evolve relatively slowly, thus being useful for comparisons of more distantly related species (e.g. riboso-mal genes, tRNA genes, cytochrome b gene). For example, golden moles are of presumed African origin. This implies that there was an extensive African radiation from a single common ancestor that gave rise to ecologically divergent adaptive types. DNA studies suggest that the base of this radiation occurred during Africa's isolation in the Cretaceous period before land connections were developed with Europe in the early Cenozoic era. In another study, scientists examined the mtDNA of 654 domestic dogs, looking for variations. They were trying to determine whether dogs were domesticated in one or several places, and then attempting to identify the place and time that such domestication occurred. Their results show that our common domestic dog population originated from at least five female wolf lines. They went on to speculate that while the archaeological record cannot define the number of geographical origins or their locations, their own data indicate a single origin of domestic dogs in East Asia some 15,000 to 40,000 years ago.

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