In biological taxonomy, a domain (Latin: regio), also superkingdom or empire, is the highest taxonomic rank of organisms in the three-domain system of taxonomy designed by Carl Woese, an American microbiologist and biophysicist. According to the Woese system, introduced in 1990, the tree of life consists of three domains: Archaea, Bacteria, and Eukarya. The first two are all prokaryotic microorganisms, or single-celled organisms whose cells have no nucleus. All life that has a nucleus and membrane-bound organelles, and multicellular organisms, is included in the Eukarya. Stefan Luketa in 2012 proposed a five "dominion" system, adding two more to the above.
Domain or dominionEdit
The term "domain" is proposed by Woese et al. (1990) in his three-domain system. This term represents a synonym for the category of dominion (lat. dominium), introduced by Moore in 1974. However, only Stefan Luketa uses the term "dominion". He placed Prion and Virus under two "dominions" separately.
Characteristics of the three domains of lifeEdit
Each of these three domains of life recognized by biologists today contain unique rRNA. This fact in itself forms the basis of the three-domain system. While the presence of a nuclear membrane differentiates the Eukarya domain from the Archaea and Bacteria domains, both of which lack a nuclear membrane, distinct biochemical and RNA markers differentiate the Archaea and Bacteria domains from each other.
Archaea are prokaryotic cells, typically characterized by membrane lipids that are branched hydrocarbon chains attached to glycerol by ether linkages. The presence of these ether linkages in Archaea adds to their ability to withstand extreme temperatures and highly acidic conditions, but many archea live in mild environments. Halophiles, organisms that thrive in highly salty environments, and hyperthermophiles, organisms that thrive in extremely hot environments, are examples of Archaea. Archaea evolved many cell sizes, but all are relatively small. Their size ranges from 0.1 to 15 μ diameter and up to 200 μ long. They are about the size of bacteria or similar to the size of a mitochondrion in a eukaryotic cell. Members of the genus Thermoplasma are the smallest of the archaea.
Even though bacteria are prokaryotic cells just like Archaea, their membranes are made of unbranched fatty acid chains attached to glycerol by ester linkages. Cyanobacteria and mycoplasmas are two examples of bacteria. They characteristically do not have ether linkages like Archaea, and they are grouped into a different category—and hence a different domain. There is a great deal of diversity in this domain, and between that and horizontal gene transfer, it is next to impossible to determine how many species of bacteria exist on the planet.
Members of the domain Eukarya—called eukaryotes—have membrane-bound organelles (including a nucleus containing genetic material) and are represented by four kingdoms: Plantae, Protista, Animalia, and Fungi.
Exclusion of virusesEdit
None of the three systems currently include non-cellular life. As of 2011 there is talk about Nucleocytoplasmic large DNA viruses possibly being a fourth branch domain of life, a view supported by researchers in 2012.
Alternative classifications of life so far proposed include:
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- "The Scientific Taxonomy and Classification of all Creatures". Retrieved 19 October 2015.
- Luketa S. (2012). "New views on the megaclassification of life" (PDF). Protistology. 7 (4): 218–237.
- Moore R.T. (1974). "Proposal for the recognition of super ranks" (PDF). Taxon. 23 (4): 650–652. doi:10.2307/1218807.
- Cox, C. J.; Foster, P. G.; Hirt, R. P.; Harris, S. R.; Embley, T. M. (2008). "The archaebacterial origin of eukaryotes". Proc Natl Acad Sci USA. 105 (51): 20356–61. Bibcode:2008PNAS..10520356C. doi:10.1073/pnas.0810647105. PMC . PMID 19073919.
- Ciccarelli FD, Doerks T, von Mering C, Creevey CJ, Snel B, Bork P (2006). "Toward automatic reconstruction of a highly resolved tree of life" (PDF). Science. 311 (5765): 1283–7. Bibcode:2006Sci...311.1283C. doi:10.1126/science.1123061. PMID 16513982.
- Nasir, A; Kim, KM; Caetano-Anolles, G (2012). "Giant viruses coexisted with the cellular ancestors and represent a distinct supergroup along with superkingdoms Archaea, Bacteria and Eukarya". BMC Evol. Biol. 12: 156. doi:10.1186/1471-2148-12-156. PMC . PMID 22920653.
- Mayr, Ernst (1998). "Two empires or three?". Proc Natl Acad Sci USA. 95 (17): 9720–9723. Bibcode:1998PNAS...95.9720M. doi:10.1073/pnas.95.17.9720. PMC . PMID 9707542. Retrieved 5 September 2011.
- Cavalier-Smith, T. (2004). "Only six kingdoms of life" (PDF). Proc. R. Soc. Lond. B. 271 (1545): 1251–62. doi:10.1098/rspb.2004.2705. PMC . PMID 15306349. Retrieved 2010-04-29
- Archibald, John M. (23 December 2008). "The eocyte hypothesis and the origin of eukaryotic cells". PNAS. 105 (51): 20049–20050. doi:10.1073/pnas.0811118106.
- Lake, James A.; Henderson, Eric; Oakes, Melanie; Clark, Michael W. (June 1984). "Eocytes: A new ribosome structure indicates a kingdom with a close relationship to eukaryotes". PNAS. 81 (12): 3786–3790. doi:10.1073/pnas.81.12.3786. PMC . PMID 6587394.
- Williams, Tom A.; Foster, Peter G.; Cox, Cymon J.; Embley, T. Martin (December 2013). "An archaeal origin of eukaryotes supports only two primary domains of life". Nature. 504 (7479): 231–236. doi:10.1038/nature12779. PMID 24336283.
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