The evolutionary precursors of today’s mitochondria gave up their independent existence as bacteria about 2 billion years ago in order to live on symbiotically inside other cells (endosymbiosis). Meanwhile they supply most of the energy for nearly all the cells in our body: Most of the energy in nutrients is utilized inside the mitochondria by oxidation to form the ATP molecule that can be used everywhere in the cell as an all-purpose energy supplier. The symbiosis can also be seen in the fact that the genes for nearly all the 750–1,000 different mitochondrial proteins are localized in the nucleus. The proteins encoded by these genes are produced in the cell cytosol and then imported into the mitochondria.
However, a few sub-units of the protein apparatus of the respiratory chain, in which the final steps of ATP production take place, are still directly produced in the mitochondria. The genes for these 13 proteins and for the tRNA molecules necessary for genetic transcription (and also for two other RNA molecules) are located inside the mitochondria on a circular DNA molecule, the mitochondrial DNA (mtDNA).
Unlike nuclear DNA, which is present in every cell exactly twice (one set of chromosomes from the father and one from the mother) there are about 1,000 to 2,000 mtDNA molecules in every cell. What happens when some of these mtDNA molecules are mutated, how the mtDNA molecules are distributed to the daughter cells upon cell division, and what happens to the mtDNA when an oocyte is fertilized and develops further are exciting questions that are being researched by scientists like Christian Kukat and other research groups all over the world.