Mitochondrial DNA: The DNA of the mitochondria. Abbreviated mtDNA. There are 2 to 10 copies of the mtDNA genome in each mitochondrion. The mtDNA molecule is double-stranded and circular. It is very small compared to the chromosomes in the nucleus, and so it contains only a limited number of genes. It is specialized in the information it carries, and it encodes a number of the subunits in the mitochondrial respiratory-chain complex that the cell needs in order to respire. It also contains genes for some ribosomal RNAs and transfer RNAs. Mutations in mtDNA can cause disease. These mutations often impair the function of oxidative-phosphorylation enzymes in the respiratory chain. This is especially manifest in tissues with a high energy expenditure, such as those of the brain and muscle. All mtDNA comes from the oocyte at fertilization. Therefore, inherited mtDNA mutations are transmitted from the mother to both male and female offspring.