Genetic novelties in mitochondrial genomes of multicellular animals

Mitochondrial genomes of multicellular animals are mostly small, circular molecules in which 13 protein genes, two ribosomal-RNA genes and 22 transfer-RNA genes are closely packed. Substantial rearrangements of genes have only occurred between phylogenetically distant organisms. However, a wealth of genetic novelties are found among these genomes that include modified genetic codes, unorthodox translation initiation codons, and structurally modified RNA components of the mitochondrion's translation system.