Mitochondria are membrane enclosed organelles found in most eukaryotic cells. They generate most of the cell's supply of ATP which is used as a source of energy. Mitochondria have their own independent genome, the material of which is known as mitochondrial DNA (mtDNA). Mitochondria contain inner and outer membranes, forming 5 distinct compartments: outermembrane, intermembrane space, inner membrane, cristae space (formed by invaginations of the inner membrane), and the matrix (space within the inner membrane). Much of the activity of the mitochondria occurs within the inner mitochondrial membrane (IMM).

The most important role of mitochondria is the production of ATP. Mitochondria accomplish this feat in the matrix by oxidizing pyruvate and NADH which are produced in the cytosol during glycolysis. Pyruvic acid is first oxidized by NAD⁺ producing NADH and a proton and it is then decarboyxlated producing carbon dioxide and acetyl-CoA. The acetyl-CoA is fed into the citric acid cycle where NADH, FADH₂ and protons are formed. The inner membrane contains 5 complexes that are very important for ATP production. These integral membrane proteins include NADH dehydrogenase (Complex I), succinate dehydrogenase (Complex II), cytochrome c reductase (Complex III or cytochrome b-c1 complex), cytochrome c oxidase ([[Complex IV]]), and ATP synthase (Complex V). Complex I, III, and IV are part of the electron transport chain and the electrons from NADH and FADH₂ are passed along this chain to generate a proton gradient by pumping protons from the matrix to the intermembrane space. The proton gradient is used for the production of ATP in Complex V.

Human mitochondria contain 5 to 10 identical, circular molecules of DNA. Each molecule contains 16,569 base pairs that encode 37 genes including ribosomal RNA (rRNA), transfer RNA (tRNA), and 13 polypeptides. The 13 proteins are an important part of the protein complexes in the inner mitochondrial membrane, forming part of

Mutations in mitochondrial genes can lead to a number of mitochondrial disorders. The muscle or brain are most commonly affected since they rely so heavily on mitochondria for their energy needs. Mitochondrial DNA is inherited from the mother, and the mtDNA is present in 5 to 10 copies per mitochondria. When mitochondria divide, the mtDNA copies are divided randomly among the 2 new mitochondria. If only a few maternally inherited mtDNA copies are defective, mitochondrial division may cause most of the defective mtDNA to reside in just one of the new mitochondria. For this reason, mitochondrial disease often only becomes apparent when the number of affected mitochondria reaches a certain level (threshold experience). Mutations in mtDNA are common because the error checking mechanisms present during nuclear DNA replication are absent. The impact of mitochondrial disease can vary widely, ranging from mild exercise intolerance up to lethal system-wide effects. Some examples of mitochondrial diseases include mitochondrial myopathies, leber hereditary optic neuropathy (LHON), Leigh syndrome, neuropathy/ataxia/retinitis pigmentosa/ptosis (NARP), and [[myoneurogenic gastrointestinal encephalopathy]] (MNGIE).