Mitochondrial DNA
Mitochondrial DNA was discovered in the 1960s by Margit M. K. Nass and Sylvan Nass by electron microscopy as DNase-sensitive threads inside mitochondria, and by Ellen Haslbrunner, Hans Tuppy and Gottfried Schatz by biochemical assays on highly purified mitochondrial fractions.
Extrachromosomal DNA
The mitochondria present in eukaryotic cells contain multiple copies of mitochondrial DNA referred to as mtDNA which is housed within the mitochondrial matrix. In multicellular animals, including humans, the circular mtDNA chromosome contains 13 genes that encode proteins that are part of the electron transport chain and 24 genes that produce RNA necessary for the production of mitochondrial proteins; these genes are broken down into 2 rRNA genes and 22 tRNA genes. The size of an animal mtDNA plasmid is roughly 16.6 kb and although it contains genes for tRNA and mRNA synthesis, proteins produced as a result of nuclear genes are still required in order for the mtDNA to replicate or for mitochondrial proteins to be translated. There is only one region of the mitochondrial chromosome that does not contain a coding sequence and that is the 1 kb region known as the D-loop to which nuclear regulatory proteins bind. The number of mtDNA molecules per mitochondria varies from species to species as well as between cells with different energy demands. For example, muscle and liver cells contain more copies of mtDNA per mitochondrion than blood and skin cells do. Due to the proximity of the electron transport chain within the mitochondrial inner membrane and the production of reactive oxygen species (ROS), and due to the fact that the mtDNA molecule is not bound by or protected by histones, the mtDNA is more susceptible to DNA damage than nuclear DNA. In cases where mtDNA damage does occur, the DNA can either be repaired via base excision repair pathways, or the damaged mtDNA molecule is destroyed (without causing damage to the mitochondrion since there are multiple copies of mtDNA per mitochondrion).