Figure 1. Mitochondrial structure and function. Mitochondria play an essential role in many aspects of cellular metabolism, but their primary function is the synthesis of adenosine triphosphate (ATP) via the pathways of oxidative phosphorylation. Mitochondria contain two different membranes, the outer and the inner membrane, which are separated by the intermembrane space. The surface area of the inner membrane is greatly increased by a large number of infoldings (cristae), which protrude into the matrix space. The outer membrane contains pore proteins that render it permeable for ions and small molecules. The inner membrane is impermeable and has a very high protein content. It contains the multisubunit complexes comprising the oxidative phosphorylation (OXPHOS) system, as well as various carrier proteins, such as the ATP/adenosine diphosphate (ADP) carrier, which mediate the transport of metabolites in and out of the matrix space. The matrix space is the site of the Krebs cycle, the pyruvate dehydrogenase complex and the enzymes of -oxidation. Matrix proteins also comprise enzymes involved in synthesis or metabolism of amino acids, ketones, urea, pyrimidines, nucleotides and heme. OM: outer membrane; IM: inner membrane; IMS: intermembrane space.

Figure 2. Mitochondrial OXPHOS system. The respiratory chain consists of four high molecular weight complexes (I–IV). Complexes I, III and IV act as proton pumps, generating an electrochemical gradient across the inner membrane, which is then used by the F₁F₀–ATPase complex (complex V) to generate ATP from ADP and phosphate. Both the nuclear and the mitochondrial genome contribute to the biogenesis of these complexes in the mitochondrial inner membrane. Only complex II is made exclusively by nuclear-encoded polypeptide chains. The number of subunits contributed by the nuclear genome and mitochondrial genome are shown. Q: coenzyme Q; C: cytochrome c.

Figure 3. Human mitochondrial genome. The mitochondrial DNA (mtDNA) is a 16569-bp-long, circular, double-stranded molecule. It contains 37 genes, which encode the RNA components of the mitochondrial translational apparatus, that is, the 22 transfer RNA (tRNA) and two ribosomal RNA (rRNA) species (12S rRNA, 16S rRNA), as well as 13 polypeptides, all of which are essential subunits of the OXPHOS complexes. The mtDNA is compact; it contains no introns and only one noncoding region of significant length, the ‘displacement loop’ (D-loop), which harbors the origin of replication of the heavy (H)-strand. The origin of replication of the light (L)-strand is located downstream at two-thirds of the mtDNA length. Replication of the mtDNA occurs independent of the cell-cycle phase and from replication of the nuclear DNA. Synthesis of each mtDNA strand thereby proceeds asynchronously from the two separate origins of replication. Both mtDNA strands are transcribed as polycistronic RNAs from their own promoters. To release functional RNA species (rRNA, tRNA, messenger RNA (mRNA)), these primary transcripts are processed by cleavage and modification. O_H and O_L: origins of H- and L-strand replication; HSP₁ and HSP₂: H-strand promoters; LSP: L-strand promoter; ND (-1, -2, -3, -4, -4L, -5, -6): genes for complex I reduced nicotinamide adenine dinucleotide (NADH) subunits; Cyt b: cytochrome b gene (complex III); COX (I, II, III): complex IV subunit genes; ATP (-6, -8): complex V subunit genes. Arrows: directions of DNA and RNA synthesis. See also