The production of ATP by the process of oxidative phosphorylation is the principal function of the mitochondrion, which is an organelle of eukaryo-tes (fungi, plants and animals). Endosymbiosis may explain how mitochondria came to be incorporated within eukaryotic cells (Embley and Martin, 2006). While not the only endosymbiotic theory describing the evolution of mitochondria, the hydrogen hypothesis (Gray et al., 1999) postulates that small ancient bacteria capable of producing hydrogen (H2) were integrated into and survived within larger H2-consuming bacteria. Through this symbiotic relationship, primordial mitochondria evolved from the small H2-producing bacteria and acquired the ability to conduct oxidative phosphorylation. Eventually, the eukaryotic (enveloped) nucleus was derived from genes of the larger bacteria. Over evolutionary time, most of the initial mitochondrial genes (genes from the small bacteria) were transferred into the eukaryotic nucleus. Genes escaping transfer eventually developed into the present-day mtDNA genome.
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