Mitochondrion is a semi-autonomous organelle occurring in eukaryotic cells. Structurally, it consists of two closed membranous sacs (outer and inner membranes) and a matrix, containing a set of enzymes and a genetic apparatus for the expression of its own mitochondrial genome (mtDNA). The metabolic functions of mitochondria include oxygen respiration (at the inner membrane) and the TCA cycle (at the matrix). The mitochondrion is descendant of an ancient alpha-proteobacterium, as showed by extensive phylogenetic analysis of the mtDNA. During eukaryotic evolution, and as a consequence of anaerobic adaptations, mitochondria have originated different organelles as hydrogenosomes and mitosomes.