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The three-domain system is a biological classification introduced by Carl Woese ''et al.'' in 1977 that divides cellular life forms into archaea, bacteria, and eukaryote domains. In particular, it emphasizes the separation of prokaryotes into two groups, originally called ''Eubacteria'' (now ''Bacteria'') and ''Archaebacteria'' (now ''Archaea''). Woese argued that, on the basis of differences in 16S rRNA genes, these two groups and the eukaryotes each arose separately from an ancestor with poorly developed genetic machinery, often called a progenote. To reflect these primary lines of descent, he treated each as a domain, divided into several different kingdoms. Woese initially used the term "kingdom" to refer to the three primary phylogenic groupings, and this nomenclature was widely used until the term "domains" was adopted in 1990.〔 == Classification == 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 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 Firmicutes – 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 blue oyster mushrooms * Kingdom Plantae or plants Examples: : * Bryophyta – mosses : * Magnoliophyta – flowering plants * Kingdom Animalia or animals Examples: : * Arthropoda – includes insects, arachnids, and crustaceans : * Chordata – includes vertebrates and, as such, human beings * Kingdom Chromalvaeolate – a group of eukaryotes that represent descent from an organism that had an endosymbiosis between a line related to a bikont and a red alga. However, the monophyly of this group is challenged. Parakaryon myojinensis incertae sedis - a single-celled organism known by a unique example. "()his organism appears to be a life form distinct from prokaryotes and eukaryotes", with features of both. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Three-domain system」の詳細全文を読む スポンサード リンク
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