A circular ring of DNA found in mitochondria. In mammals mtDNA makes up less than 1% of the total cellular DNA, but in plants the amount is variable. It codes for ribosomal and transfer RNA but only some mitochondrial proteins (up to 30 proteins in animals), the nuclear DNA being required for encoding most of these. Human mtDNA contains 37 genes, encoding 13 proteins and some RNAs. Mitochondrial DNA is generally inherited via the female line only, although there are exceptions to this (see heteroplasmy). According to the endosymbiont theory, mitochondrial DNA is a remnant of the chromosome of the symbiotic prokaryote ancestor of the mitochondrion. See also mitochondrial eve.