- Three-domain system
The three-domain system is a
biological classification introduced byCarl Woese in 1990, [cite journal | author = Woese C, Kandler O, Wheelis M | title = Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya. | url=http://www.pnas.org/cgi/reprint/87/12/4576 | journal = Proc Natl Acad Sci U S A | volume = 87 | issue = 12 | pages = 4576–9 | year = 1990 | pmid = 2112744 | doi = 10.1073/pnas.87.12.4576] that divides cellular life forms intoarchaea ,bacteria , andeukaryote domains. In particular, it emphasizes the separation of prokaryotes into two groups, originally called "Eubacteria" and "Archaebacteria". Woese argued that, on the basis of differences in16S rRNA gene s, these two groups and the eukaryotes each arose separately from an ancestor with poorly developed genetic machinery, often called aprogenote . To reflect these primary lines of descent, he treated each as a domain, divided into several different kingdoms.__TOC__
Classification
Although the three-domain system was quickly adopted by most molecular systematists, biologists like
Ernst Mayr criticized him for over-emphasizing the uniqueness of the archaebacteria and ignoring strong genetic similarities between the groups. [cite journal
last=Mayer
first=Ernst
author-link=Ernst Mayer
publication-date=August 18, 1998
date=May 26, 1998
year=1998
title=Two empires or three?
periodical=Proc Natl Acad Sci USA
volume=95
issue=17
pages=9720–9723
url=http://www.pnas.org/cgi/content/full/95/17/9720
pmid=9707542
doi=10.1073/pnas.95.17.9720
journal=Proceedings of the National Academy of Sciences] Subsequent studies have confirmed that the archaea are unusual in the composition of theircell membrane and structure of their flagella. Other significant differences include archaeal systems forDNA replication and transcription which bear distinct similarity to those found in eukaryotes. For instance, archaealRNA polymerase consists of up to 14 subunits, whereas most bacterial RNA polymerases have only 4 subunits. Analysis of these subunits suggests that they are more closely related to those found in eukaryotes. Also, the archaea produce a number ofDNA-binding protein s with similarity to eukaryotichistones .Nevertheless, a minority viewpoint suggests retaining the older
two-empire system (Prokaryota and Eukaryota) and using the word "bacterium" in its earlier meaning of "prokaryote".Which system is preferable depends partly on the relationships of the
organism s in question. Although theprogenote hypothesis is discredited, molecular trees tend to group living things into the three domains, with the eukaryotes placed beside or within the Archaea and the eubacteria forming a separate branch. However, it has been suggested this is an artifact oflong branch attraction and that the root may instead belong among the eubacteria, in which case many eubacterial lines diverged before the archaebacteria did.See also
*
Phylogenetic tree
*Taxonomy
*Two-empire system
*Five-kingdom system
*Monera References
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