A semi-autonomous intracellular organelle, one or more (usually many) of which occur in most types of eukaryotic cell (being absent e.g. in protozoa of the orders Diplomonadida and Pelobiontida); RESPIRATION and the reactions of the TCA CYCLE occur within the mitochondrion. Mitochondria differ in size (<1 µm to >10 µm), shape, and number, according to the type of cell in which they occur and to the physiological state of that cell; a mitochondrion may be spherical, ovoid, elongated, calyciform or irregularly shaped, and some are branched.
Structure and composition. A mitochondrion consists of two closed membranous sacs, one fitting closely within the other, and an amorphous matrix (enclosed by the inner membrane) which contains the mitochondrial DNA and enzymes of the TCA cycle. The mitochondrial inner membrane is extensively corrugated and forms plate-like, tubular, or finger-like structures (cristae) which project into the matrix—often more or less perpendicular to the longitudinal axis of the mitochondrion; the inner membrane contains components of the ELECTRON TRANSPORT CHAIN, PROTON ATPASES and TRANSPORT SYSTEMS for e.g. nucleotides, inorganic phosphate, and Ca2+. (See also ION TRANSPORT.) The mitochondrial outer membrane contains PORINS (see also VDAC).
Mitochondrial DNA (mtDNA) is typically in the form of covalently closed circular, double-stranded molecules (differing from the nuclear DNA e.g. in base composition and buoyant density); mtDNA is linear in e.g. certain ciliates (including Paramecium), Physarum polycephalum, and Hansenula mrakii. Mitochondrial genes employ a GENETIC CODE (q.v.) which differs in some respects from the ‘universal’ code, and some fungal mitochondrial genes contain introns (see SPLIT GENE). Genetic recombination can occur between mtDNAs. (See also PETITE MUTANT.)
Origin and semi-autonomy of mitochondria. It is generally believed that mitochondria are formed by the division or fragmentation of pre-existing mitochondria—or (see later) by the development of promitochondria—and that these organelles incorporate new material (i.e., grow) during interdivision periods. The components of mitochondria are encoded partly by the cell’s nuclear DNA and partly by the mtDNA. [Nuclear genes encoding mitochondrial proteins in yeast: TIBS (1985) 10 192–194.] Thus, in Saccharomyces cerevisiae, mtDNA encodes e.g. tRNAs, two types of rRNA, one ribosomal protein, the apoprotein of cytochrome b in Complex III, and a protein designated ‘var1’. Certain mitochondrial components are synthesized in the mitochondrion itself. However, while a mitochondrion can synthesize DNA and RNA, and can carry out protein synthesis, many of the proteins needed for these processes are encoded by nuclear genes, synthesized on cytoplasmic ribosomes, and incorporated into the mitochondrion; control of the synthesis of these proteins may be largely at the transcriptional level.
Mitochondrial protein synthesis differs from cytoplasmic protein synthesis e.g. in that it is sensitive to those agents (e.g. CHLORAMPHENICOL, ERYTHROMYCIN) which inhibit bacterial PROTEIN SYNTHESIS; it is not sensitive to CYCLOHEXIMIDE; and it is characterized by the incorporation of N-formylmethionine as the first amino acid in a polypeptide chain. These features have lent support to a popular hypothesis which supposes that mitochondria have their evolutionary origins in endosymbiotic prokaryotes. [Mitochondrial origins: PNAS (1985) 82 4443–4447.] (An alternative hypothesis supposes that mitochondria evolved from plasmids.)
In at least some facultatively fermentative organisms (including e.g. Saccharomyces cerevisiae) cells growing under anaerobic conditions do not contain functional mitochondria; such cells contain promitochondria: organelles which resemble mitochondria but which lack components of the electron transport chain. On exposure to aerobic conditions promitochondria apparently develop into functional mitochondria; reversion to promitochondria occurs if anaerobic growth is resumed.
In e.g. certain fungi mitochondrial defects can alter the nature of the cell’s metabolism (see e.g. PETITE MUTANT and POKY MUTANT), and can affect the expression of nuclear genes which are involved e.g. in determining the nature of cell-surface antigens [TIBS (1982) 7 147–151]; inhibition of mitochondrial protein synthesis can affect both meiotic and apomictic sporulation in Saccharomyces cerevisiae [Yeast (1985) 1 39–47].
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