Mitochondrion
In cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 1–10 micrometers (μm) in size. Mitochondria are sometimes described as "cellular power plants" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. In addition to supplying cellular energy, mitochondria are involved in a range of other processes, such as signaling, cellular differentiation, cell death, as well as the control of the cell cycle and cell growth. Mitochondria have been implicated in several human diseases and may play a role in the aging process. The word mitochondrion comes from the Greek μίτος or mitos, thread + χονδρίον or khondrion, granule. Their ancestry is not fully understood, but, according to the endosymbiotic theory, mitochondria are descended from ancient bacteria, which were engulfed by the ancestors of eukaryotic cells more than a billion years ago.
Several characteristics make mitochondria unique. The number of mitochondria in a cell varies widely by organism and tissue type. Many cells have only a single mitochondrion, whereas others can contain several thousand mitochondria. The organelle is composed of compartments that carry out specialized functions. These compartments or regions include the outer membrane, the intermembrane space, the inner mitochondrial membrane|inner membrane, and the cristae and matrix. In humans, mitochondria contain about 615 distinct types of proteins, depending on the tissue of origin. Although most of a cell's DNA is contained in the cell nucleus, the mitochondrion has its own independent genome. Further, its DNA shows substantial similarity to bacterial genomes.
Contents |
Genome
The human mitochondrial genome is a circular DNA molecule of about 16 kilobases. It encodes 37 genes: 13 for subunits of respiratory complexes I, III, IV, and V, 22 for mitochondrial tRNA, and 2 for rRNA.
As in prokaryotes, there is a very high proportion of coding DNA and an absence of repeats. Mitochondrial genes are transcribed as multigenic transcripts, which are cleaved and polyadenylated to yield mature mRNAs. Not all proteins necessary for mitochondrial function are encoded by the mitochondrial genome; most are coded by genes in the cell nucleus and imported to the mitochondrion. The exact number of genes encoded by the nucleus and the mitochondrial genome differs between species. In general, mitochondrial genomes are circular, although exceptions have been reported. Also, in general, mitochondrial DNA lacks introns, as is the case in the human mitochondrial genome; such as that of yeast and protists, including Dictyostelium discoideum.
While slight variations on the standard code had been predicted earlier, none was discovered until 1979, when researchers studying human mitochondrial genes determined that they used an alternative code. Many slight variants have been discovered since, including various alternative mitochondrial codes. Further, the AUA, AUC, and AUU codons are all allowable start codons.
| Organism | Codon | Standard | Novel |
|---|---|---|---|
| Mammalian | AGA, AGG | Arginine | Stop codon |
| AUA | Isoleucine | Methionine | |
| UGA | Stop codon | Tryptophan | |
| Invertebrates | AGA, AGG | Arginine | Serine |
| AUA | Isoleucine | Methionine | |
| UGA | Stop codon | Tryptophan | |
| Yeast | AUA | Isoleucine | Methionine |
| UGA | Stop codon | Tryptophan | |
| CUA | Leucine | Threonine |
Some of these differences should be regarded as pseudo-changes in the genetic code due to the phenomenon of RNA editing, which is common in mitochondria. In higher plants, it was thought that CGG encoded for tryptophan and not arginine; however, the codon in the processed RNA was discovered to be the UGG codon, consistent with the universal genetic code for tryptophan. Of note, the arthropod mitochondrial genetic code has undergone parallel evolution within a phylum, with some organisms uniquely translating AGG to lysine.
Mitochondrial genomes have far fewer genes than the eubacteria from which they are thought to be descended. Although some have been lost altogether, many have been transferred to the nucleus, such as the respiratory complex II protein subunits. This is thought to be relatively common over evolutionary time. A few organisms, such as the Cryptosporidium, actually have mitochondria that lack any DNA, presumably because all their genes have been lost or transferred. In Cryptosporidium, the mitochondria have an altered ATP generation system that renders the parasite resistant to many classical mitochondrial inhibitors such as cyanide, azide, and atovaquone.
Population genetic studies
The near-absence of genetic recombination in mitochondrial DNA makes it a useful source of information for scientists involved in population genetics and evolutionary biology. Because all the mitochondrial DNA is inherited as a single unit, or haplotype, the relationships between mitochondrial DNA from different individuals can be represented as a gene tree. Patterns in these gene trees can be used to infer the evolutionary history of populations. The classic example of this is in human evolutionary genetics, where the molecular clock can be used to provide a recent date for mitochondrial Eve. This is often interpreted as strong support for a recent modern human expansion out of Africa. Another human example is the sequencing of mitochondrial DNA from Neanderthal bones. The relatively-large evolutionary distance between the mitochondrial DNA sequences of Neanderthals and living humans has been interpreted as evidence for lack of interbreeding between Neanderthals and anatomically-modern humans.
However, mitochondrial DNA reflects the history of only females in a population and so may not represent the history of the population as a whole. This can be partially overcome by the use of paternal genetic sequences, such as the non-recombining region of the Y-chromosome.
External links
- Mitochondria Atlas at University of Mainz
- Mitochondria Research Portal at mitochondrial.net
- Mitochondria: Architecture dictates function at cytochemistry.net
- Mitochondria links at University of Alabama
- Mitochondrion Reconstructed by Electron Tomography at San Diego State University
- Video Clip of Rat-liver Mitochondrion from Cryo-electron Tomography at wadsworth.org
- 3D structures of proteins from inner mitochondrial membrane at University of Michigan
- 3D structures of proteins associated with outer mitochondrial membrane at University of Michigan
License
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Mitochondrion".
