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| Animal Mitochondrial Genomes
The mitochondrial DNA (mtDNA) of most animals is arranged in a circular genome, 15-20 kB in length. Most animal mtDNAs encode 13 proteins, two ribosomal RNAs, and 22 transfer RNAs. The gene organization is compact: except for a region sometimes called the D loop or control region, involved in the initiation of DNA replication and transcription, there is little sequence that does not code for protein or RNA. Most animal mitochondria have no introns. In some animals, all genes and RNAs are transcribed from one strand; in others, they are found on both strands. Each strand may be transcribed as a single large polycistron that is later post-transcriptionally processed into separate RNAs. The tRNAs, which fold into stem-loop structures, may signal this cleavage.
Eleven of the 13 proteins found in animal mtDNA are subunits of the three major membrane-bound enzyme complexes in the respiratory chain. NADH dehydrogenase subunits 1-6 and 4L (ND1-6 and ND4L) are components of the NADH dehydrogenase complex. Cytochrome b (CYTB) is a subunit of the cytochrome b-c1 complex. Cytochrome c oxidase subunits I-III (COX1-3) are part of the cytochrome oxidase complex. In addition, two subunits of ATP synthase, ATP synthase F0 subunits 6 and 8 (ATP6, 8), are encoded in metazoan mtDNA. Other mitochondrial proteins are synthesized in the nucleus, and transported into the mitochondria from the cytoplasm.
Animal mtDNA contains two rRNAs, s-rRNA (12S), and l-rRNA (16S), which encode the RNA components of the small and large subunits of mitochondrial ribosomes. The mitochondrial genomes encode 22 tRNAs, one for each of 18 amino acids, and two each for Serine and Leucine. A single tRNA can read all four codons in a four-codon family. Unlike nuclear tRNA molecules, primary and secondary structure is not well conserved. The mitochondrial genetic code is different from that used by nuclear genes. Furthermore, the translation initiation codon is not limited to AUG, and some termination codons are formed by polyadenylation of the transcript.
The order of genes and RNAs in the mtDNAs have helped phylogenists to understand metazoan evolution. The gene order among mammals and ray-finned fish are identical. However, within other groups of related organisms gene order is quite different (see, for example, sauropsids, and arthropods).
"Nonstandard" genome organization
Of the 105 metazoan mitochondrial reference sequences, 100 are coelomates. All of the coelomate mitochondria encode 13 proteins, 22 tRNAs, and 2 rRNAs. The three pseudocoelomates, all nematodes, as well the one acoelomate, Echinococcus multilocularis (echinococcosis tapeworm), are missing the gene for ATP8. The mitochondrion of the cnidarian Metridium senile (brown sea anemone) contains all genes, plus an extra ORF, but is missing all tRNAs except Trp and Met. Its COX1 and ND5 genes contain introns, both of which encode other genes.
Last modified: November 29, 1999