Mitochondria are small cellular bodies or organelles, cylindrical and delimited by a double membrane, which are found in countless specimens in the cytosol of eukaryote cells. Cellular respiration occurs within mitochondria. It is a central process of metabolism that uses as materials oxygen and the products of glucose degradation, generating carbonic gas, water and energy, the latter in the form of adenosine triphosphate molecules (ATP). Mitochondria, descendants from ancient aerobic bacteria adopted as symbiotic guests by eukaryote cells, have no nuclei. They reproduce asexually, independently from the cell in whose cytosol they reside, and are randomly distributed into the two parts of that cell when it divides. They are transmitted to the host new generation by maternal lineage, since most of the sperm cytosol –contained in its tail– does not penetrate the ovum.
The mitochondrial genome is the most compact and efficient that we know of. To synthesize its proteins it uses its own ribosomes, smaller than those of the host cell. Curiously, they employ a genetic code which differs in a few codons from the universal one. However, the action of this genome is tightly coordinated with that of the host's nucleus, where many genes of the original bacterium were transferred and, consequently, some mitochondrial enzymes are codified. The mitochondrial genome exists in multiple copies, whose number varies at different times in the organelle's life. The number of mitochondria themselves also varies, according to the type of cell they inhabit. A human fertilized egg contains around one hundred thousand. Differentiated cells, on their part, contain as many as their energy requirements demand at each particular moment. Among the neediest of them are neurons, as well as heart and muscle cells. It is not surprising that mitochondria constitute the greatest part of the human-cell volume: approximately 80%. Their mutations occur rather frequently, due, among other factors, to the fact that mitochondria have developed neither enough repair mechanisms for their DNA nor a protecting histon sheath like the one surrounding nuclear DNA. Another very important instability factor is proximity to the respiratory chain, a source of free radicals capable of attacking nucleotides. Unfavorable mitochondrial mutations wreak havoc in diverse body organs, the seriousness of the resulting generalized illness being proportional to the organism's age.
Mitochondrial DNA is constituted by two circular strands: H (heavy) and L (light). Each molecule resides in the external crannies of the mitochondrial internal membrane. The greater weight of the external strand is due to its adjunction to this membrane. An initiating bubble in the heavy chain, called D-loop, seems to be a control mechanism. It is a polymorphic region with two hyper-variable regions, extremely useful to follow maternal lineages. The fact that mutations in that zone are evolution neutral has allowed its use as a sort of “evolutionary clock” in diverse paleontological research programs, such as date determination of the mitochondrial Eve or the departure from Africa by modern man, and the routes and times of their progressive occupation of the Earth's surface.
Everything affecting energy production has, of course, catastrophic consequences in the life of the cell. In particular, it is recognized that mitochondria perform an orchestra-conductor roll in the programmed death of cells, by means of diverse apoptosis-triggering mechanisms.