Dictionary:
mi·to·chon·dri·on (mī'tə-kŏn'drē-ən)  |
A spherical or elongated organelle in the cytoplasm of nearly all eukaryotic cells, containing genetic material and many enzymes important for cell metabolism, including those responsible for the conversion of food to usable energy. Also called chondriosome.
[New Latin : Greek mitos, warp thread + Greek khondrion, diminutive of khondros, grain, granule.]
mitochondrial mi'to·chon'dri·al (-drē-əl) adj.| 5min Related Video: mitochondrion |
| Sci-Tech Encyclopedia: Mitochondria |
Specialized organelles of all eukaryotic cells that use oxygen (see illustration). Often called the powerhouses of the cell, mitochondria are responsible for energy generation by the process of oxidative phosphorylation. In this process, electrons produced during the oxidation of simple organic compounds are passed along a chain of four membrane-bound enzymes (the electron transport or respiratory chain), finally reacting with and reducing molecular oxygen to water. The movement of the electrons releases energy that is used to build a gradient of protons across the membrane in which the electron transport chain is situated. Like a stream of water that drives the turbines in a hydroelectric plant, these protons flow back through adenosine triphosphate (ATP) synthase, a membrane-bound enzyme that acts as a molecular turbine. Rotation of part of ATP synthase results in storage of energy in the form of ATP, the universal energy currency of the cell.
Electron micrograph of a thin section through the pancreas of a bat, showing a typical mitochondrion in profile. Note how the cristae are formed by extensive folding of the inner membrane. (Courtesy of K. R. Porter)
Besides their role in energy generation, mitochondria house numerous enzymes that carry out steps essential to metabolism. Defects in mitochondrial assembly or function generally have serious consequences for survival of the cell. In humans, mitochondrial dysfunction is the underlying cause of a wide range of degenerative diseases, with energy-demanding cells such as those of the central nervous and endocrine systems, heart, muscle, and kidney being most severely affected.
Mitochondria are bounded by two concentric membranes referred to as the outer and the inner. This creates two distinct compartments, the matrix and the intermembrane space. The outer membrane consists of a bilayer containing about 80% lipid. It is freely permeable to molecules smaller than about 5000 daltons. The inner membrane is also a lipid bilayer. It is extremely rich in protein (about 75%) and is impermeable to even the smallest of ions. The inner membrane contains the enzymes of the electron transport chain and the ATP synthase, together with a set of transporter proteins that regulate the movement of metabolites in and out of the matrix space. Mitochondria of cells that depend on a high level of ATP production are usually extensively folded to produce structures called cristae (see illustration). These greatly increase the surface area of the inner membrane, allowing many more copies of the enzymes of oxidative phosphorylation.
The intermembrane space contains enzymes capable of using some of the ATP that is transported out of the matrix to phosphorylate other nucleotides. The matrix space is packed with a hundred or so water-soluble proteins that form a sort of semisolid gel. They include enzymes of the tricarboxylic acid (Krebs) cycle and enzymes required for the oxidation of pyruvate and fatty acids, comprising steps in the biosynthesis or degradation of amino acids, nucleotides, and steroids.
Both mitochondria and chloroplasts contain DNA and the machinery necessary to express the information stored there. In both cases, the DNAs are relatively small and simple compared with DNA in the nucleus. While chloroplast DNA tends to be very similar in size in all organisms examined, mtDNA varies widely in complexity, from 16–18 kilobases in metazoa to upward of 2000 kilobases in some higher plants. In the case of higher-plant mtDNAs, some extra sequences appear to have been picked up from chloroplast and nuclear DNAs. MtDNA is generally circular, although some linear exceptions are known among the yeasts, algae, and protozoa.
The information content of most mtDNA is limited. This means that most of the several hundred proteins found in these organelles are encoded by genes located in nuclear DNA. These proteins are synthesized in the cytosol and subsequently transported specifically to the respective organelles. The contributions of the two genetic systems are usually closely coordinated, so that cells synthesize organelles of more or less constant composition.
Many organisms, including humans, show uniparental inheritance of mitochondrial genes because one parent contributes more cytoplasm to the zygote than the other. In humans, it is the egg cell provided by the mother that contributes the cytoplasm. Human mitochondrial genes are thus inherited maternally.
Recent years have seen a growing interest in human diseases that result from mitochondrial dysfunction. A number of these result from mutations in mtDNA. Others are linked to nuclear genes, whose mutation disturbs oxidative phosphorylation or impairs mitochondrial assembly. Mutations in mtDNA are remarkably frequent and lead to a wide range of degenerative, mainly neuromuscular diseases. Most of these diseases are maternally inherited, but some appear to be spontaneous, possibly resulting from error-prone replication of mtDNA. A striking feature of mtDNA-related diseases is the enormous diversity in clinical presentation. This diversity is attributable to two main factors: (1) Heterogeneity in the mtDNA population. Most human tissues contain many thousands of mtDNA molecules per cell. The severity of clinical symptoms depends on the number of mutated molecules present. (2) Dependence of a particular cell type on mitochondrial function (mainly ATP production). Cells with a high requirement for mitochondrially generated ATP are more severely affected than cells with alternative sources of ATP.
Besides their role in metabolism and energy-linked processes, mitochondria have recently been identified as important players in the initiation of apoptosis (programmed cell death). On one hand, the mitochondrial outer membrane houses a number of members of the Bcl-2 family of apoptosis regulatory proteins. On the other hand, release of certain mitochondrial proteins from the intermembrane space is instrumental in activating specialized proteases called caspases. These catalyze a degradative cascade in the cytoplasm that eventually ends in cell death.
Mammalian mtDNAs accumulate mutations at high rates and evolve correspondingly fast (up to 12–15 times faster than single-copy genes in nuclear DNA and up to 100 times faster for rRNA and tRNA genes). This behavior reflects both a high incidence of mutations and a high probability of their fixation. The first is probably related to oxidative damage to mtDNA by oxygen free radicals produced as by-products of electron transfer through the respiratory chain. The second has been attributed to the lack of efficient DNA repair (mitochondria lack nucleotide-excision repair) and to a relatively high tolerance of many mitochondrial gene products to mutational change.
The rapid rate of sequence evolution of mammalian mtDNAs makes these genomes highly sensitive indicators of recent evolutionary relationships. Unlike their nuclear counterparts, mtDNAs do not undergo recombination during sexual transmission and are strictly maternally inherited. Sequence changes in mtDNA therefore provide a clear record of the history of the female lineages through which this DNA has been transmitted.
| Food and Nutrition: mitochondrion |
(Plural mitochondria.) The subcellular organelles in all cells apart from red blood cells in which the major oxidative reactions of metabolism occur, linked to the formation of ATP from ADP.
| Food and Fitness: mitochondrion |
A sausage-shaped structure within cells that has the specific function of generating energy by using oxygen to break down foodstuffs. Mitochondria are sometimes called the powerhouses of the cell. They contain coloured chemicals (cytochromes) and enzymes needed for aerobic respiration. When mitochondria are tightly packed together, the cytochromes impart a reddish-brown colour to cells (see brown fat). Mitochondria are particularly dense in slow twitch muscle fibres used for activities of long duration. Regular endurance training increases the density of mitochondria within muscle. In addition, training seems to increase the ability of each mitochondrion to generate energy, further improving endurance capacity.
| Dental Dictionary: mitochondria |
Small, rodlike, threadlike, or granular organelles within the cytoplasm that function in cellular metabolism and respiration and occur in varying numbers in all living cells except bacteria, viruses, blue-green algae, and mature erythrocytes.
| Britannica Concise Encyclopedia: mitochondrion |
For more information on mitochondrion, visit Britannica.com.
| Sports Science and Medicine: mitochondrion |
A double-membraned structure within a cell concerned with aerobic metabolism. Mitochondria are sometimes referred to as the ‘powerhouses’ of the cell because they are the sites in which the Krebs cycle and electron transport system generate large amounts of ATP. Muscle fibres of endurance athletes have a higher density of mitochondria than those of non-athletes. Resistance training decreases mitochondrial density, but increases the overall number of mitochondria because of muscle growth.
| Biology Q&A: What are mitochondria? |
A mitochondrion (singular) is a self-replicating, double-membraned
body found in the cytoplasm of all eukaryotic cells. The outer membrane of a
mitochondrion is smooth, while the inner membrane is folded into numerous
layers that are called cristae. Mitochondria are the location for much of the
metabolism necessary for protein synthesis, and the production of both ATP and
lipids.
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Are there differences between prokaryotic and eukaryotic ribosomes?
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| Science Dictionary: mitochondrion |
The cell organelle where much of cellular respiration takes place; the “power plant” of the cell.
| Veterinary Dictionary: mitochondria |
[Gr.] plural of mitochondrion; small, spherical to rod-shaped, membrane-bounded cytoplasmic organelles that are the principal sites of ATP synthesis; they also contain enzymes of the citric acid cycle and for fatty acid oxidation, oxidative phosphorylation, and other biochemical pathways. Mitochondria also contain DNA, RNA and ribosomes; they replicate independently and synthesize some of their own proteins.
| Wikipedia: Mitochondrion |
In cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed organelle found in most eukaryotic cells.[1] These organelles range from 0.5–10 micrometers (μm) in diameter. 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 the chemical energy.[2] 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.[3] Mitochondria have been implicated in several human diseases, including mitochondrial disorders[4] and cardiac dysfunction,[5] and may play a role in the aging process. The word mitochondrion comes from the Greek μίτος or mitos, thread + χονδρίον or chondrion, granule.
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.[6][7] The organelle is composed of compartments that carry out specialized functions. These compartments or regions include the outer membrane, the intermembrane space, the inner membrane, and the cristae and matrix. Mitochondrial proteins vary depending on the tissue and the species. In humans, 615 distinct types of proteins have been identified from cardiac mitochondria;[8] whereas in Murinae (rats), 940 proteins encoded by distinct genes have been reported.[9] The mitochondrial proteome is thought to be dynamically regulated.[10] 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.[11]
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A mitochonstine contains outer and inner membranes composed of phospholipid bilayers and proteins.[6] The two membranes, however, have different properties. Because of this double-membraned organization, there are five distinct compartments within the mitochondrion. There is the outer mitochondrial membrane, the intermembrane space (the space between the outer and inner membranes), the inner mitochondrial membrane, the cristae space (formed by infoldings of the inner membrane), and the matrix (space within the inner membrane).
The outer mitochondrial membrane, which encloses the entire organelle, has a protein-to-phospholipid ratio similar to that of the eukaryotic plasma membrane (about 1:1 by weight). It contains large numbers of integral proteins called porins. These porins form channels that allow molecules 5000 Daltons or less in molecular weight to freely diffuse from one side of the membrane to the other.[6] Larger proteins can enter the mitochondrion if a signaling sequence at their N-terminus binds to a large multisubunit protein called translocase of the outer membrane, which then actively moves them across the membrane.[12] Disruption of the outer membrane permits proteins in the intermembrane space to leak into the cytosol, leading to certain cell death.[13] The mitochondrial outer membrane can associate with the ER membrane, in a structure called MAM (mitochondria-associated ER-membrane). This is important in ER-mitochondria calcium signaling and involved in the transfer of lipids between the ER and mitochondria.[14]
The intermembrane space is the space between the outer membrane and the inner membrane. Because the outer membrane is freely permeable to small molecules, the concentrations of small molecules such as ions and sugars in the intermembrane space is the same as the cytosol.[6] However, as large proteins must have a specific signaling sequence to be transported across the outer membrane, the protein composition of this space is different than the protein composition of the cytosol. One protein that is localized to the intermembrane space in this way is cytochrome c.[13]
The inner mitochondrial membrane contains proteins with five types of functions:[6]
It contains more than 100 different polypeptides, and has a very high protein-to-phospholipid ratio (more than 3:1 by weight, which is about 1 protein for 15 phospholipids). The inner membrane is home to around 1/5 of the total protein in a mitochondrion.[6] In addition, the inner membrane is rich in an unusual phospholipid, cardiolipin. This phospholipid was originally discovered in beef hearts in 1942, and is usually characteristic of mitochondrial and bacterial plasma membranes.[15] Cardiolipin contains four fatty acids rather than two and may help to make the inner membrane impermeable.[6] Unlike the outer membrane, the inner membrane does not contain porins and is highly impermeable to all molecules. Almost all ions and molecules require special membrane transporters to enter or exit the matrix. Proteins are ferried into the matrix via the translocase of the inner membrane (TIM) complex or via Oxa1.[12] In addition, there is a membrane potential across the inner membrane formed by the action of the enzymes of the electron transport chain.
The inner mitochondrial membrane is compartmentalized into numerous cristae, which expand the surface area of the inner mitochondrial membrane, enhancing its ability to produce ATP. These are not simple random folds but rather invaginations of the inner membrane, which can affect overall chemiosmotic function.[16] In typical liver mitochondria, for example, the surface area, including cristae, is about five times that of the outer membrane. Mitochondria of cells that have greater demand for ATP, such as muscle cells, contain more cristae than typical liver mitochondria.These folds are studded with small round bodies known as F1 particles or oxysomes.
The matrix is the space enclosed by the inner membrane. It contains about 2/3 of the total protein in a mitochondrion.[6] The matrix is important in the production of ATP with the aid of the ATP synthase contained in the inner membrane. The matrix contains a highly-concentrated mixture of hundreds of enzymes, special mitochondrial ribosomes, tRNA, and several copies of the mitochondrial DNA genome. Of the enzymes, the major functions include oxidation of pyruvate and fatty acids, and the citric acid cycle.[6]
Mitochondria have their own genetic material, and the machinery to manufacture their own RNAs and proteins (see: protein biosynthesis). A published human mitochondrial DNA sequence revealed 16,569 base pairs encoding 37 total genes: 22 tRNA, 2 rRNA, and 13 peptide genes.[17] The 13 mitochondrial peptides in humans are integrated into the inner mitochondrial membrane, along with proteins encoded by genes that reside in the host cell's nucleus.
Mitochondria are found in nearly all eukaryotes. They vary in number and location according to cell type. A single highly branched mitochondrion was described in the unicellular alga "Polytomella agilis".[18] Substantial numbers of mitochondria are in the liver, with about 1000–2000 mitochondria per cell making up 1/5th of the cell volume.[6] The mitochondria can be found nestled between myofibrils of muscle or wrapped around the sperm flagellum.[6] Often they form a complex 3D branching network inside the cell with the cytoskeleton. The association with the cytoskeleton determines mitochondrial shape, which can affect the function as well.[19] Recent evidence suggests vimentin, one of the components of the cytoskeleton, is critical to the association with the cytoskeleton.[20]
The most prominent roles of mitochondria are to produce ATP (i.e., phosphorylation of ADP) through respiration, and to regulate cellular metabolism.[7] The central set of reactions involved in ATP production are collectively known as the citric acid cycle, or the Krebs Cycle. However, the mitochondrion has many other functions in addition to the production of ATP.
A dominant role for the mitochondria is the production of ATP, as reflected by the large number of proteins in the inner membrane for this task. This is done by oxidizing the major products of glucose, pyruvate, and NADH, which are produced in the cytosol.[7] This process of cellular respiration, also known as aerobic respiration, is dependent on the presence of oxygen. When oxygen is limited, the glycolytic products will be metabolized by anaerobic respiration, a process that is independent of the mitochondria.[7] The production of ATP from glucose has an approximately 13-fold higher yield during aerobic respiration compared to anaerobic respiration.[21] Recently it has been shown that plant mitochondria can produce a limited amount of ATP without oxygen by using the alternate substrate nitrite.[22]
Each pyruvate molecule produced by glycolysis is actively transported across the inner mitochondrial membrane, and into the matrix where it is oxidized and combined with coenzyme A to form CO2, acetyl-CoA, and NADH.[7]
The acetyl-CoA is the primary substrate to enter the citric acid cycle, also known as the tricarboxylic acid (TCA) cycle or Krebs cycle. The enzymes of the citric acid cycle are located in the mitochondrial matrix, with the exception of succinate dehydrogenase, which is bound to the inner mitochondrial membrane as part of Complex II.[23] The citric acid cycle oxidizes the acetyl-CoA to carbon dioxide, and, in the process, produces reduced cofactors (three molecules of NADH and one molecule of FADH2) that are a source of electrons for the electron transport chain, and a molecule of GTP (that is readily converted to an ATP).[7]
The redox energy from NADH and FADH2 is transferred to oxygen (O2) in several steps via the electron transport chain. These energy-rich molecules are produced within the matrix via the citric acid cycle but are also produced in the cytoplasm by glycolysis. Reducing equivalents from the cytoplasm can be imported via the malate-aspartate shuttle system of antiporter proteins or feed into the electron transport chain using a glycerol phosphate shuttle.[7] Protein complexes in the inner membrane (NADH dehydrogenase, cytochrome c reductase, and cytochrome c oxidase) perform the transfer and the incremental release of energy is used to pump protons (H+) into the intermembrane space. This process is efficient, but a small percentage of electrons may prematurely reduce oxygen, forming reactive oxygen species such as superoxide.[7] This can cause oxidative stress in the mitochondria and may contribute to the decline in mitochondrial function associated with the aging process.[24]
As the proton concentration increases in the intermembrane space, a strong electrochemical gradient is established across the inner membrane. The protons can return to the matrix through the ATP synthase complex, and their potential energy is used to synthesize ATP from ADP and inorganic phosphate (Pi).[7] This process is called chemiosmosis, and was first described by Peter Mitchell[25][26] who was awarded the 1978 Nobel Prize in Chemistry for his work. Later, part of the 1997 Nobel Prize in Chemistry was awarded to Paul D. Boyer and John E. Walker for their clarification of the working mechanism of ATP synthase.[27]
Under certain conditions, protons can re-enter the mitochondrial matrix without contributing to ATP synthesis. This process is known as proton leak or mitochondrial uncoupling and is due to the facilitated diffusion of protons into the matrix. The process results in the unharnessed potential energy of the proton electrochemical gradient being released as heat.[7] The process is mediated by a proton channel called thermogenin, or UCP1.[28] Thermogenin is a 33kDa protein first discovered in 1973.[29] Thermogenin is primarily found in brown adipose tissue, or brown fat, and is responsible for non-shivering thermogenesis. Brown adipose tissue is found in mammals, and is at its highest levels in early life and in hibernating animals. In humans, brown adipose tissue is present at birth and decreases with age.[28]
The concentrations of free calcium in the cell can regulate an array of reactions and is important for signal transduction in the cell. Mitochondria can transiently store calcium, a contributing process for the cell's homeostasis of calcium.[30] In fact, their ability to rapidly take in calcium for later release makes them very good "cytosolic buffers" for calcium.[31][32][33] The endoplasmic reticulum (ER) is the most significant storage site of calcium, and there is a significant interplay between the mitochondrion and ER with regard to calcium.[34] The calcium is taken up into the matrix by a calcium uniporter on the inner mitochondrial membrane.[35] It is primarily driven by the mitochondrial membrane potential.[30] Release of this calcium back into the cell's interior can occur via a sodium-calcium exchange protein or via "calcium-induced-calcium-release" pathways.[35] This can initiate calcium spikes or calcium waves with large changes in the membrane potential. These can activate a series of second messenger system proteins that can coordinate processes such as neurotransmitter release in nerve cells and release of hormones in endocrine cells.
Mitochondria play a central role in many other metabolic tasks, such as:
Some mitochondrial functions are performed only in specific types of cells. For example, mitochondria in liver cells contain enzymes that allow them to detoxify ammonia, a waste product of protein metabolism. A mutation in the genes regulating any of these functions can result in mitochondrial diseases.
Mitochondria have many features in common with prokaryotes. As a result, they are believed to be originally derived from endosymbiotic prokaryotes.
A mitochondrion contains DNA, which is organized as several copies of a single, circular chromosome. This mitochondrial chromosome contains genes for redox proteins such as those of the respiratory chain. The CoRR Hypothesis proposes that this Co-location is required for Redox Regulation. The mitochondrial genome codes for some RNAs of ribosomes, and the twenty-two tRNAs necessary for the translation of messenger RNAs into protein. The circular structure is also found in prokaryotes, and the similarity is extended by the fact that mitochondrial DNA is organized with a variant genetic code similar to that of Proteobacteria.[40] This suggests that their ancestor, the so-called proto-mitochondrion, was a member of the Proteobacteria.[40] In particular, the proto-mitochondrion was probably related to the rickettsia.[41] However, the exact relationship of the ancestor of mitochondria to the alpha-proteobacteria and whether the mitochondria was formed at the same time or after the nucleus, remains controversial.[42]
The ribosomes coded for by the mitochondrial DNA are similar to those from bacteria in size and structure.[43] They closely resemble the bacterial 70S ribosome and not the 80S cytoplasmic ribosomes which are coded for by nuclear DNA.
The endosymbiotic relationship of mitochondria with their host cells was popularized by Lynn Margulis.[44] The endosymbiotic hypothesis suggests that mitochondria descended from bacteria that somehow survived endocytosis by another cell, and became incorporated into the cytoplasm. The ability of these bacteria to conduct respiration in host cells that had relied on glycolysis and fermentation would have provided a considerable evolutionary advantage. In a similar manner, host cells with symbiotic bacteria capable of photosynthesis would have had an advantage. The incorporation of symbiotes would have increased the number of environments in which the cells could survive. This symbiotic relationship probably developed 1.7[45]-2[46] billion years ago.
A few groups of unicellular eukaryotes lack mitochondria: the microsporidians, metamonads, and archamoebae.[47] These groups appear as the most primitive eukaryotes on phylogenetic trees constructed using rRNA information, suggesting that they appeared before the origin of mitochondria. However, this is now known to be an artifact of long-branch attraction – they are derived groups and retain genes or organelles derived from mitochondria (e.g., mitosomes and hydrogenosomes).[1]
The human mitochondrial genome is a circular DNA molecule of about 16 kilobases.[48] It encodes 37 genes: 13 for subunits of respiratory complexes I, III, IV and V, 22 for mitochondrial tRNA (for the 20 standard amino acids, plus an extra gene for leucine and serine), and 2 for rRNA.[48] One mitochondrion can contain two to ten copies of its DNA.[49]
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 the corresponding proteins are imported into the mitochondrion.[50] 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.[51] In general, mitochondrial DNA lacks introns, as is the case in the human mitochondrial genome;[50] however, introns have been observed in some eukaryotic mitochondrial DNA,[52] such as that of yeast[53] and protists,[54] including Dictyostelium discoideum.[55]
In animals the mitochondrial genome is typically a single circular chromosome that is approximately 16-kb long and has 37 genes. The genes while highly conserved may vary in location. Curiously this pattern is not found in the human body louse (Pediculus humanus). Instead this mitochondrial genome is arranged in 18 minicircular chromosomes each of which is 3–4 kb long and has one to three genes.[56] This pattern is also found in other sucking lice but not in chewing lice. Recombination has been shown to occur between the minichromosomes. The reason for this difference is not known.
While slight variations on the standard code had been predicted earlier,[57] none was discovered until 1979, when researchers studying human mitochondrial genes determined that they used an alternative code.[58] Many slight variants have been discovered since,[59] including various alternative mitochondrial codes.[60] 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.[61] Of note, the arthropod mitochondrial genetic code has undergone parallel evolution within a phylum, with some organisms uniquely translating AGG to lysine.[62]
Mitochondrial genomes have far fewer genes than the bacteria 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.[48] 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.[63] 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.[63]
Mitochondria divide by binary fission similar to bacterial cell division; unlike bacteria, however, mitochondria can also fuse with other mitochondria.[48][64]. The regulation of this division differs between eukaryotes. In many single-celled eukaryotes, their growth and division is linked to the cell cycle. For example, a single mitochondrion may divide synchronously with the nucleus. This division and segregation process must be tightly controlled so that each daughter cell receives at least one mitochondrion. In other eukaryotes (in humans for example), mitochondria may replicate their DNA and divide mainly in response to the energy needs of the cell, rather than in phase with the cell cycle. When the energy needs of a cell are high, mitochondria grow and divide. When the energy use is low, mitochondria are destroyed or become inactive. In such examples, and in contrast to the situation in many single celled eukaryotes, mitochondria are apparently randomly distributed to the daughter cells during the division of the cytoplasm.
An individual's mitochondrial genes are not inherited by the same mechanism as nuclear genes. At fertilization of an egg cell by a sperm, the egg nucleus and sperm nucleus each contribute equally to the genetic makeup of the zygote nucleus. In contrast, the mitochondria, and therefore the mitochondrial DNA, usually comes from the egg only. The sperm's mitochondria enter the egg but do not contribute genetic information to the embryo.[65] Instead, paternal mitochondria are marked with ubiquitin to select them for later destruction inside the embryo.[66] The egg cell contains relatively few mitochondria, but it is these mitochondria that survive and divide to populate the cells of the adult organism. Mitochondria are, therefore, in most cases inherited down the female line, known as maternal inheritance. This mode is seen in most organisms including all animals. However, mitochondria in some species can sometimes be inherited paternally. This is the norm among certain coniferous plants, although not in pine trees and yew trees.[67] It has been suggested that it occurs at a very low level in humans.[68]
Uniparental inheritance leads to little opportunity for genetic recombination between different lineages of mitochondria, although a single mitochondrion can contain 2–10 copies of its DNA.[49] For this reason, mitochondrial DNA usually is thought to reproduce by binary fission. What recombination does take place maintains genetic integrity rather than maintaining diversity. However, there are studies showing evidence of recombination in mitochondrial DNA. It is clear that the enzymes necessary for recombination are present in mammalian cells.[69] Further, evidence suggests that animal mitochondria can undergo recombination.[70] The data are a bit more controversial in humans, although indirect evidence of recombination exists.[71][72] If recombination does not occur, the whole mitochondrial DNA sequence represents a single haplotype, which makes it useful for studying the evolutionary history of populations.
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.[73] 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.[74][75] This is often interpreted as strong support for a recent modern human expansion out of Africa.[76] 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.[77]
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.[76] In a broader sense, only studies that also include nuclear DNA can provide a comprehensive evolutionary history of a population.[78]
With their central place in cell metabolism, damage - and subsequent dysfunction - in mitochondria is an important factor in a wide range of human diseases. Mitochondrial disorders often present as neurological disorders, but can manifest as myopathy, diabetes, multiple endocrinopathy, or a variety of other systemic manifestations.[79] Diseases caused by mutation in the mtDNA include Kearns-Sayre syndrome, MELAS syndrome and Leber's hereditary optic neuropathy.[80] In the vast majority of cases, these diseases are transmitted by a female to her children, as the zygote derives its mitochondria and hence its mtDNA from the ovum. Diseases such as Kearns-Sayre syndrome, Pearson's syndrome, and progressive external ophthalmoplegia are thought to be due to large-scale mtDNA rearrangements, whereas other diseases such as MELAS syndrome, Leber's hereditary optic neuropathy, myoclonic epilepsy with ragged red fibers (MERRF), and others are due to point mutations in mtDNA.[79]
In other diseases, defects in nuclear genes lead to dysfunction of mitochondrial proteins. This is the case in Friedreich's ataxia, hereditary spastic paraplegia, and Wilson's disease.[81] These diseases are inherited in a dominance relationship, as applies to most other genetic diseases. A variety of disorders can be caused by nuclear mutations of oxidative phosphorylation enzymes, such as coenzyme Q10 deficiency and Barth syndrome.[79] Environmental influences may interact with hereditary predispositions and cause mitochondrial disease. For example, there may be a link between pesticide exposure and the later onset of Parkinson's disease.[82][83]
Other pathologies with etiology involving mitochondrial dysfunction include schizophrenia, bipolar disorder, dementia, Alzheimer's disease, Parkinson's disease, epilepsy, stroke, cardiovascular disease, retinitis pigmentosa, and diabetes mellitus.[84][85] A common thread thought to link these seemingly-unrelated conditions is cellular damage causing oxidative stress. How exactly mitochondrial dysfunction fits into the etiology of these pathologies is yet to be elucidated.[citation needed]
Given the role of mitochondria as the cell's powerhouse, there may be some leakage of the high-energy electrons in the respiratory chain to form reactive oxygen species. This can result in significant oxidative stress in the mitochondria with high mutation rates of mitochondrial DNA.[86] A vicious cycle is thought to occur, as oxidative stress leads to mitochondrial DNA mutations, which can lead to enzymatic abnormalities and further oxidative stress. A number of changes occur to mitochondria during the aging process.[87] Tissues from elderly patients show a decrease in enzymatic activity of the proteins of the respiratory chain.[88] Large deletions in the mitochondrial genome can lead to high levels of oxidative stress and neuronal death in Parkinson's disease.[89] Hypothesized links between aging and oxidative stress are not new and were proposed over 50 years ago;[90] however, there is much debate over whether mitochondrial changes are causes of aging or merely characteristics of aging. One notable study in mice demonstrated shortened lifespan but no increase in reactive oxygen species despite increasing mitochondrial DNA mutations, suggesting that mitochondrial DNA mutations can cause lifespan shortening by other mechanisms.[91] As a result, the exact relationships between mitochondria, oxidative stress, and aging have not yet been settled.
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| Translations: Mitochondrion |
Dansk (Danish)
n. - mitokondrie
Français (French)
n. - mitochondrie
Deutsch (German)
n. - (Med.) Mitochondrium
Español (Spanish)
n. - mitocondrio
中文（简体）(Chinese (Simplified))
生物的线粒体
中文（繁體）(Chinese (Traditional))
n. - 生物的線粒體
עברית (Hebrew)
n. - גוף קטן ביצור חד-תאי המכיל אנזימים לנשימה ולייצור אנרגיה
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 | chondrioid (microbiology) |
| crista |
| organelle |
 | What is the function of mitochondrions? Read answer... |
| What color is a mitochondrion? Read answer... |
| What is a mitochondrion made of? Read answer... |
 | What goes in and out of the mitochondrion? |
| What is the mass of a mitochondrion? |
| What is the function of a mitochondrion? |
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved. Read more |
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| Food and Nutrition. A Dictionary of Food and Nutrition. Copyright © 1995, 2003, 2005 by A. E. Bender and D. A. Bender. All rights reserved. Read more |
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| Food and Fitness. Food and Fitness: A Dictionary of Diet and Exercise. Copyright © 1997, 2003 by Oxford University Press. All rights reserved. Read more |
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| Dental Dictionary. Mosby's Dental Dictionary. Copyright © 2004 by Elsevier, Inc. All rights reserved. Read more |
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| Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved. Read more |
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| Sports Science and Medicine. The Oxford Dictionary of Sports Science & Medicine. Copyright © Michael Kent 1998, 2006, 2007. All rights reserved. Read more |
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| Biology Q&A. The Handy Biology Answer Book. 2004 ©Visible Ink Press. All rights reserved. Read more |
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| Science Dictionary. The New Dictionary of Cultural Literacy, Third Edition Edited by E.D. Hirsch, Jr., Joseph F. Kett, and James Trefil. Copyright © 2002 by Houghton Mifflin Company. Published by Houghton Mifflin. All rights reserved. Read more |
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| Veterinary Dictionary. Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved. Read more |
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| Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Mitochondrion". Read more |
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