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Evolutionary biology
One of the three domains of living organisms: Archaea, Bacteria and Eukaryota. While Archaea are single-celled, they are unlike bacteria given their independent evolutionary history. Archaea differ from Eukaryota in their ribosomal structure and the presence—in some—of introns in the genome, as well as other features (e.g., different membrane composition).
Molecular biology
Archaea are of interest in biotechnology as they have unique biochemical features (e.g., enzymes of theromophiles, such as Taq polymerase, the “workhorse” enzyme of PCR) and are extremely stable at high temperatures. Archaea include metabolic oddities (e.g., extreme halophiles, which live in extremely salty environments), methanogens (which produce methane) and sulphur-dependant extreme thermophiles (which can live in extremely hot environments).

Archaea phyla
• Crenarchaeota
• Euryarchaeota
• Korarchaeota
• Nanoarchaeota
• Thaumarchaeota (recently proposed)
Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.


A group of microorganisms that thrive in the absence of oxygen, produce methane, and live only in bodies of highly concentrated salt water, or in the acidic waters of sulfur springs, at temperatures near 80° Celsius and pH levels as low as 2.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012


one of the three primary groupings (DOMAINS) of ORGANISMS, according to some classification schemes, based on genetic structures and sequences. See CLASSIFICATION. Members of the Archaea are PROKARYOTES and include the extreme HALOPHILES, the thermoacidophiles (organisms that normally grow at high temperatures in acidic environments; see also THERMOPHILIC), and the METHANOGENS.

They differ in a number of ways from other BACTERIA, for example in the structure of their MEMBRANE LIPIDS, TRANSFER RNA molecules and CELL WALL, and in their sensitivity to ANTIBIOTICS. The Archaea is a very diverse group organized into two KINGDOMS, the CRENARCHAEOTA and the EURYARCHAEOTA. It was initially considered to represent the most ancient group of organisms still living. This is reflected in the name, from the Greek archaios, meaning ancient.

Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005
References in periodicals archive ?
nov.: a new taxon of extremely thermophilic archaebacteria," Systematic and Applied Microbiology, vol.
Non-living things (atoms, stellar objects, etc.) move exteriorly; they are "minerals." Archaebacteria and bacteria grow and reproduce; they are operationally plants.
Gupta, "Protein Phylogenies and Signature Sequences: A Reappraisal of Evolutionary Relationships among Archaebacteria, Eubacteria, and Eukaryotes," Microbiology and Molecular Biology Reviews 62 (1998): 1435-91.
Computer analyses of complete genomes suggest that some archaebacteria employ both eukaryotic and eubacterial mechanisms in translation initiation.
Woese therefore proposed that living organisms be classified under three domains: Eubacteria, Archaebacteria, and Eukaryota.
The sequences encompass at least seven groups of methane-producing microbes of a type called archaebacteria. Only about half the groups have relatives in genera known to science.
Messenger RNA molecules (mRNAs) lacking 5' untranslated leader sequences have been observed in eubacteria, archaebacteria, and eukaryotic mitochondria (see 1 and references cited therein).
Although, as previously noted, "Non-Neutral Evolution" has a definite Drosophila bias, it is not as monochromatic as perhaps this review has made it sound: oysters (Beckenbach) and halophilic archaebacteria (Dennis) make appearances, and even vertebrates get a look-in in the final, largely theoretical, chapter by Takahata.
Ever since the proposal of using molecular markers such as 16S (or 18S) rRNA for comparing species similarity, in the late 1970s, followed by the outlining of a third "urkingdom" (the Archaebacteria), our knowledge of organism relatedness has taken a great leap forward.
DeLong, "Methane-consuming archaebacteria in marine sediments," Nature, vol.
Ultrastructural organization of the extremely haloalkalophilic archaebacteria Natronobacterium pharaonis and Natronococcus ocultus.