Mitochondrial Genome

Given the bacterial-like features of mitochondria and chloroplasts (small size, circular genome, and ability to divide on their own), it is believed that each organelle traces its evolutionary history to a free-living bacterial ancestor that was engulfed by a larger cell and then entered into a symbiotic relationship with the host cell. This "serial endosymbiotic theory" proposes that the evolution of the modern eukaryotic cell was a step-wise association, with the acquisition of the mitochondria preceding that of the chloroplast. The most compelling evidence for the endosymbiosis theory has come from the analysis of complete genome sequences. Comparison of DNA sequence data has identified two specific groups of bacteria, α-Proteobacteria and Cyanobacteria, as the closest living relatives of mitochondria and chloroplasts, respectively. The mtDNA sequence information from numerous organisms has revealed remarkable similarity, reinforcing the idea of a single primary ancestor for the organelle originating very early in the evolution of the eukaryotic cell.

During the course of evolution, a large portion of mitochondrial genes were either lost or transferred to the nuclear genome. Elimination of genes from mtDNA is an ongoing evolutionary process made possible because their functions either become dispensable or can be replaced by nuclear functions. A comparison of the complete mtDNA sequence and the working draft of the human nuclear genome project reveals numerous areas of similarity. These regions represent mtDNA sequences that have been transferred from the cytoplasm to the nucleus over the course of mammalian evolution. This transfer accounts for the current nuclear location of most of the genes that encode mitochondrial proteins, including most of the proteins required for oxidative phosphorylation. Even eukaryotes that lack mitochondria (such as some protists) contain nuclear genes that encode typical mitochondrial proteins, implying that these eukaryoytes once had mitochondria but subsequently lost them.