Each mitochondrion also has its own chromosome separate from the 23 pairs of chromosomes located within the nucleus of the cell. The mitochondrial chromosome is a small circular DNA that encodes 37 genes not contained within the nuclear genome. Proteins that are made from these genes are located within the mitochondrion, and are necessary for the mitochondrion to function properly. Since the mitochondrion requires several hundred different proteins for its normal function, however, additional mitochondrial proteins are also encoded by the nuclear genome. Mitochondria thus have proteins produced from both the nuclear and the mitochondrial chromosomes. Genetic disorders that alter mitochondrial function can be caused by mutation in one of the 23 pairs of chromosomes located within the nucleus of the cell, or by mutation in the chromosome located within each mitochondrion.