a This highlights the importance of the mitochondrial genome in contributing polypeptide subunits to the five enzyme complexes that comprise the oxidative phosphorylation (OXPHOS) system within the inner mitochondrial membrane — the site of ATP synthesis. The reoxidation of reducing equivalents (NADH (reduced flavin adenine dinucleotide)) that are produced by the oxidation (reduced nicotinamide adenine dinucleotide) and FADH₂ of carbohydrates (the tricarboxylic acid (TCA) cycle) and fatty acids (β-oxidation) is coupled to the generation of an electrochemical gradient across the inner mitochondrial membrane, which is harnessed by the ATP synthase to drive the formation of ATP. b A map of the human mitochondrial genome. The genes that encode the subunits of complex I (ND1–ND6 and ND4L) are shown in blue; cytochrome c oxidase (COI–COIII) is shown in red; cytochrome b of complex III is shown in green; and the subunits of the ATP synthase (ATPase 6 and 8) are shown in yellow. The two ribosomal RNAs (rRNAs; 12S and 16S, shown in purple) and 22 tRNAs, indicated by black lines and denoted by their single letter code, which are required for mitochondrial protein synthesis are also shown. The displacement loop (D-loop), or non-coding control region, contains sequences that are vital for the initiation of both mtDNA replication and transcription, including the proposed origin of heavy-strand replication (shown as O_H). The origin of light-strand replication is shown as O_L.

Transverse tissue sections that are reacted for both cytochrome c oxidase (COX) and succinate dehydrogenase (SDH) activities sequentially, with COX-positive cells shown in brown and COX-deficient cells shown in blue. a Skeletal muscle from a patient with a heteroplasmic mitochondrial tRNA point mutation. The section shows a typical ‘mosaic’ pattern of COX activity, with many muscle fibres harbouring levels of mutated mtDNA that are above the crucial threshold to produce a functional enzyme complex. b Cardiac tissue (left ventricle) from a patient with a homoplasmic tRNA mutation that causes hypertrophic cardiomyopathy, which demonstrates an absence of COX in most cells. c A section of cerebellum from a patient with an mtDNA rearrangement that highlights the presence of COX-deficient neurons. d,e Tissues that show COX deficiency that is due to clonal expansion of somatic mtDNA mutations within single cells — a phenomenon that is seen in both post-mitotic cells (d; extraocular muscles) and rapidly dividing cells (e; colonic crypt) in ageing humans.

During the production of primary oocytes, a selected number of mitochondrial DNA (mtDNA) molecules are transferred into each oocyte. Oocyte maturation is associated with the rapid replication of this mtDNA population. This restriction-amplification event can lead to a random shift of mtDNA mutational load between generations and is responsible for the variable levels of mutated mtDNA observed in affected offspring from mothers with pathogenic mtDNA mutations. Mitochondria that contain mutated mtDNA are shown in red, those with normal mtDNA are shown in green.

The prevention of mutated maternal mtDNA transmission could be achieved by nuclear transfer techniques. In theory, this could be carried out at the germinal vesicle stage (a), when clearly identifiable structures are visible. The germinal vesicle contains nuclear chromosomes and could be transferred to an oocyte that contains normal mtDNA (green). Alternatively, pronuclear transfer can be carried out at the fertilized oocyte stage (b); this involves removal of the male and female pronuclei from the oocyte that contains mutated mtDNA (red) and their subsequent transfer to an oocyte that has normal mtDNA (green).