Contents 1 Classification 2 Niches 3 Alternatives 4 See also 5 References

Classification[edit] The three-domain system includes Eukarya (represented by the Australian green tree frog, left), Bacteria (represented by S. aureus, middle) and Archaea (represented by Sulfolobus, right) The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used five- or six-kingdom systems. This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as archaea appear to be more closely related to eukaryotes than they are to other prokaryotic bacteria. The current system has the following listed kingdoms in the three domains: Domain Archaea – prokaryotic, no nuclear membrane, distinct biochemistry and RNA markers from bacteria, possess unique ancient evolutionary history for which they are considered some of the oldest species of organisms on Earth; traditionally classified as archaebacteria; often characterized by living in extreme environments. Some examples of archaeal organisms are methanogens which produce the gas methane, halophiles which live in very salty water, and thermoacidophiles which thrive in acidic high temperature water. Domain Bacteria – prokaryotic, consists of prokaryotic cells possessing primarily diacyl glycerol diester lipids in their membranes and bacterial rRNA, no nuclear membrane, traditionally classified as bacteria. Most of the known pathogenic prokaryotic organisms belong to bacteria (see [3] for exceptions), and are currently studied more extensively than Archaea. Some examples of bacteria include Cyanobacteria photosynthesizing bacteria that are related to the chloroplasts of eukaryotic plants and algae, Spirochaetes – Gram-negative bacteria that include those causing syphilis and Lyme disease, and Actinobacteria --Gram-positive bacteria including Bifidobacterium animalis which is present in the human large intestine. Domain Eukarya – eukaryotes, organisms that contain a membrane-bound nucleus. An inexhaustive list of eukaryotic organisms includes: Kingdom Fungi or fungi Examples- Saccharomycotina – includes true yeasts Basidiomycota – includes mushrooms Kingdom Plantae or plants Examples: Bryophyta – mosses Magnoliophyta – flowering plants Kingdom Animalia or animals Examples: Arthropoda – includes insects, arachnids, and crustaceans

Niches[edit] Each of the three cell types tends to fit into recurring specialties or roles. Bacteria tend to be the most prolific reproducers, at least in moderate environments. Archaeans tend to adapt quickly to extreme environments, such as high temperatures, high acids, high sulfur, etc. This includes adapting to use a wide variety of food sources. Eukaryotes are the most flexible with regard to forming cooperative colonies, such as in multi-cellular organisms, including humans. In fact, the structure of a Eukaryote is likely to have derived from a joining of different cell types, forming organelles. Parakaryon myojinensis (incertae sedis) is a single-celled organism known by a unique example. "[T]his organism appears to be a life form distinct from prokaryotes and eukaryotes",[4] with features of both.

Alternatives[edit] Parts of the three-domain theory have been challenged by scientists including Ernst Mayr, Thomas Cavalier-Smith and Radhey Gupta.[5][6][7] In particular, Gupta argues that the primary division within prokaryotes should be among those surrounded by a single membrane (monoderm), including gram-positive bacteria and archaebacteria, and those with an inner and outer cell membrane (diderm), including gram-negative bacteria. He claims that sequences of features and phylogenies from some highly conserved proteins are inconsistent with the three-domain theory, and that it should be abandoned despite its widespread acceptance. Recent work has proposed that Eukarya may have actually branched off from the domain Archea. According to Spang et al. Lokiarchaeota forms a monophyletic group with eukaryotes in phylogenomic analyses. The associated genomes also encode an expanded repertoire of eukaryotic signature proteins that are suggestive of sophisticated membrane remodelling capabilities.[8] These data suggest a two-domain system as opposed to the classical three-domain system.

See also[edit] Biology portal Molecular and cellular biology portal Archaea Bacterial phyla Eocyte hypothesis Monera Phylogenetic tree Taxonomy Two-empire system

References[edit] ^ Woese C, Fox G (1977). "Phylogenetic structure of the bacteria domain: the primary kingdoms". Proc Natl Acad Sci USA. 74 (11): 5088–90. Bibcode:1977PNAS...74.5088W. doi:10.1073/pnas.74.11.5088. PMC 432104 . PMID 270744.  ^ a b Woese C, Kandler O, Wheelis M (1990). "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya". Proc Natl Acad Sci USA. 87 (12): 4576–9. Bibcode:1990PNAS...87.4576W. doi:10.1073/pnas.87.12.4576. PMC 54159 . PMID 2112744. Retrieved 11 Feb 2010.  ^ Eckburg, Paul B.; Lepp, Paul W.; Relman, David A. (2003). "Archaea and Their Potential Role in Human Disease". Infection and Immunity. 71 (2): 591–596. doi:10.1128/IAI.71.2.591-596.2003. PMC 145348 . PMID 12540534.  ^ Yamaguchi M, Mori Y, Kozuka Y, Okada H, Uematsu K, Tame A, Furukawa H, Maruyama T, Worman CO, Yokoyama K (2012). "Prokaryote or eukaryote? A unique microorganism from the deep sea". J Electron Microsc (Tokyo). 61 (6): 423–431. doi:10.1093/jmicro/dfs062.  ^ Gupta, Radhey S (1998). "Life's Third Domain (Archaea): An Established Fact or an Endangered Paradigm?: A New Proposal for Classification of Organisms Based on Protein Sequences and Cell Structure". Theoretical Population Biology. 54 (2): 91–104. doi:10.1006/tpbi.1998.1376. PMID 9733652.  ^ Mayr, E. (1998). "Two empires or three?". Proc. Natl. Acad. Sci. USA. 95: 9720–9723. doi:10.1073/pnas.95.17.9720. PMC 33883 . PMID 9707542.  ^ Cavalier-Smith, T (2002). "The neomuran origin of archaebacteria, the negibacterial root of the universal tree and bacterial megaclassification". Int J Syst Evol Microbiol. 52 (1): 7–76. doi:10.1099/00207713-52-1-7. PMID 11837318.  ^ Spang, Anja (2015). "Complex archaea that bridge the gap between prokaryotes and eukaryotes". Nature. 521: 173–179. doi:10.1038/nature14447. PMC 4444528 . PMID 25945739.  Retrieved from "" Categories: Biological classificationHigh-level systems of taxonomyScientific controversiesHidden categories: Pages using div col with deprecated parameters

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