Mitochondria are subcellular organelles that constitute a metabolic compartment separated from the general cytoplasm by a double layered membrane. The outer membrane serves to regulate access of proteins and metabolites to the mitochondrial compartment, and the convoluted inner mitochondrial membrane is the site of several multicomponent enzyme systems. These include the respiratory chain complexes, which generate the vital ATP through a series of oxidation reactions. Other enzyme systems present within the mitochondria include those of the Krebs cycle (tricarboxylic acid cycle), those involved in part of the urea cycle, and many of the enzymes required for the oxidation of fatty acids and amino acids.

One benefit conferred by the separation of this metabolic compartment from the rest of the cell may be the protection of other cell systems, including nuclear DNA, from oxidative damage by hydroxyl radicals generated during oxidative phosphorylation. This may account for the much greater mutation rate within the mitochondrial genome than in the cell nucleus, leading to either point mutations or rearrangements such as large deletions. The protection from these conferred on the cell by mitochondria may explain not only their evolutionary origin as endosymbionts but also their persistence as discrete organelles within eukaryotes.

In higher organisms, many of the genes encoding enzymatic proteins have become incorporated into the nuclear genome, but a minimal …