Mitochondria

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Electron micrograph of a mitochondrion from mammalian lung tissue showing its matrix and membranes.
Animal mitochondrion diagram en (edit).svg
Schematic of typical animal cell, showing subcellular components. Organelles:
(1) Nucleolus
(2) Nucleus
(3) Ribosomes
(4) Vesicle
(5) Rough endoplasmic reticulum (ER)
(6) Golgi apparatus
(7) Cytoskeleton
(8) Smooth ER
(9) Mitochondria
(10) Vacuole
(11) Cytoplasm
(12) Lysosome
(13) Centrioles within centrosome

Mitochondria [1] are organelles, or parts of a cell.

Their main job is energy conversion. They oxidise glucose to produce energy that the cell can use. The process is called cellular respiration, which makes ATP, a molecule which the cell can use as a source of energy. [2]

In addition to supplying cellular energy, mitochondria are involved in a range of other processes, such as signalling, cellular differentiation, cell death, as well as the control of the cell division cycle and cell growth.[3]

Structure[change | edit source]

A mitochondrion contains outer and inner membranes. These are made of phospholipid double layers and proteins.[4] The two membranes have different properties. Because of this double-membraned organization, there are five distinct compartments within the mitochondrion. They are:

  1. the outer mitochondrial membrane,
  2. the intermembrane space (the space between the outer and inner membranes),
  3. the inner mitochondrial membrane,
  4. the cristae space (formed by infoldings of the inner membrane), and
  5. the matrix (space within the inner membrane).

Function[change | edit source]

Mitochondria's main role in the cell is to take glucose, and use the energy stored in its chemical bonds to make ATP in a process called cellular respiration. There are 3 main steps to this process: glycolysis, the citric acid cycle or Krebs cycle, and ATP Synthesis .This ATP then is released from the mitochondria, and broken down by the other organelles of the cell to power their own functions. [5]

DNA[change | edit source]

It is thought that mitochondria were once independent bacteria, and became part of the eukaryotic cells by being engulfed, a process called endosymbiosis.[6]

Although most of a cell's DNA is contained in the cell nucleus, the mitochondrion has its own independent genome, called mDNA. Further, its DNA shows substantial similarity to bacterial genomes.[7]

Inheritance[change | edit source]

Mitochondria divide by binary fission similar to bacterial cell division. In single-celled eukaryotes, division of mitochondria is linked to cell division. This division must be 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 in response to the energy needs of the cell, rather than in phase with the cell cycle.

An individual's mitochondrial genes are not inherited by the same mechanism as nuclear genes. The mitochondria, and therefore the mitochondrial DNA, usually comes from the egg only. The sperm's mitochondria enter the egg, but are marked for later destruction.[8] 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 true for all animals, and most other organisms. However, mitochondria is inherited paternally in some conifers, though not in pines or yews.[9]

A single mitochondrion can contain 2–10 copies of its DNA.[10] For this reason, mitochondrial DNA usually is thought to reproduce by binary fission, so producing exact copies. However, there is some evidence that animal mitochondria can undergo recombination.[11] If recombination does not occur, the whole mitochondrial DNA sequence represents a single haploid genome, which makes it useful for studying the evolutionary history of populations.

Population genetic studies[change | edit source]

The near-absence of recombination in mitochondrial DNA makes it a useful for population genetics and evolutionary biology.[12] All the mitochondrial DNA is inherited as a single haploid unit, the relationships between mitochondrial DNA from different individuals can be seen 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 where the molecular clock can be used to give a date for the so-called mitochondrial Eve.[13][14] This is often interpreted as strong support for the spread of modern humans out of Africa.[15] 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 is evidence for a general lack of interbreeding between Neanderthals and anatomically-modern humans.[16]

However, mitochondrial DNA only reflects the history of females in a population. It may not represent the history of the population as a whole. To some extent, paternal genetic sequences from the Y-chromosome can be used.[15] In a broader sense, only studies that also include nuclear DNA can provide a comprehensive evolutionary history of a population.[17]

Related pages[change | edit source]

References[change | edit source]

  1. single = 'mitochondrion'; from Greek mitos, a thread + khondrion, granule
  2. Henze K. & Martin W. 2003. Evolutionary biology: essence of mitochondria. Nature 426: 127–8.
  3. McBride HM, Neuspiel M, Wasiak S (2006). "Mitochondria: more than just a powerhouse". Curr. Biol. 16 (14): R551. doi:10.1016/j.cub.2006.06.054. PMID 16860735.
  4. Alberts, Bruce et al 1994. Molecular biology of the cell. New York: Garland. ISBN 0-8153-3218-1
  5. Campbell, Reece, et al (2008). Biology: Concepts and connections..
  6. Lake, James A. Evidence for an early prokaryote symbiogenesis. Nature 460 967–971.
  7. Andersson SG, Karlberg O, Canbäck B, Kurland CG (January 2003). "On the origin of mitochondria: a genomics perspective". Philos. Trans. R. Soc. Lond., B, Biol. Sci. 358 (1429): 165–77; discussion 177–9. doi:10.1098/rstb.2002.1193. PMC 1693097. PMID 12594925.
  8. Sutovsky, P., et al. (1999). "Ubiquitin tag for sperm mitochondria". Nature 402: 371–372. doi:10.1038/46466. Discussed in Science News.
  9. Mogensen HL (1996). "The hows and whys of cytoplasmic inheritance in seed plants". American Journal of Botany 83 (3): 247. doi:10.2307/2446172.
  10. Wiesner RJ, Ruegg JC, Morano I (1992). "Counting target molecules by exponential polymerase chain reaction, copy number of mitochondrial DNA in rat tissues". Biochim Biophys Acta. 183 (2): 553–559. PMID 1550563.
  11. Lunt DB, Hyman BC (15 May 1997). "Animal mitochondrial DNA recombination". Nature 387 (6630): 247. doi:10.1038/387247a0. PMID 9153388.
  12. Castro JA, Picornell A, Ramon M (1998). "Mitochondrial DNA: a tool for populational genetics studies". Int Microbiol. 1 (4): 327–32. PMID 10943382.
  13. Cann RL, Stoneking M, Wilson AC (1987 January). "Mitochondrial DNA and human evolution". Nature. 325 (6099): 31–36. doi:10.1038/325031a0. PMID 3025745.
  14. Torroni A et al (2006). "Harvesting the fruit of the human mtDNA tree". Trends Genet. 22 (6): 339–45. doi:10.1016/j.tig.2006.04.001. PMID 16678300.
  15. 15.0 15.1 Garrigan D, Hammer MF (2006). "Reconstructing human origins in the genomic era". Nat. Rev. Genet. 7 (9): 669–80. doi:10.1038/nrg1941. PMID 16921345.
  16. Krings M. et al (1997). "Neandertal DNA sequences and the origin of modern humans". Cell 90 (1): 19–30. doi:10.1016/S0092-8674(00)80310-4. PMID 9230299.
  17. Harding RM et al (1997 April). "Archaic African and Asian lineages in the genetic ancestry of modern humans". Am J Hum Genet. 60 (4): 772–89. PMC 1712470. PMID 9106523.