Mitochondrion, (pl., Mitochondria)

An organelle that occurs in the cytoplasm of all eukaryotes, capable of self-replicating. Each mitochondrion is surrounded by a double membrane. The inner membrane is highly invaginated, with projections called cristae that are tubular or lamellar. Mitochrondria are the sites of oxidative phosphorylation which result in the formation of ATP. Mitochondria contain distinctive ribosomes, transfer RNAs, and aminoacyl-tRNA synthetases. Mitochondria depend upon genes within the nucleus of the cells they inhabit for many essential mRNAs. Proteins translated from mRNAs in the cytoplasm are imported into the mitochondrion. Mitochondria are thought to be endosymbionts derived from aerobic bacteria that associated with primitive eukaryotes and have their own circular DNA molecules. The genetic code of mitochondria differs slightly from the universal genetic code. Mitochondria are transferred primarily through the egg, and thus maternally inherited.