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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 ?
Halobacterium halobium Mn-SOD gene: archaebacterial and eubacterial features.
(2) The karyomastigont organellar system (a chromosome-containing membrane-bounded nucleus with a proteinaceous connector that attaches to the centriole-kinetosomes at the base of the undulipodia) evolved in response to selection pressures that tended to separate attached spirochetes (eubacteria that became undulipodia) from their sulfidogenic archaebacterial partners (the rest of the cytoplasm).
"Under our hypothesis, the cytoskeleton results, like many other features, from the genetic complexity conferred by the forced integration of eubacterial genes into archaebacterial chromosomes," he says.
Shen, "Newly discovered archaebacterial flap endonucleases show a structure- specific mechanism for DNA substrate binding and catalysis resembling human flap endonuclease-1," Journal of Biological Chemistry, vol.
As it happens, the majority of the archaebacterial sequences are not consistent with currently accepted views of the Tree of Life which cluster the archaebacteria (Archaea) with eukaryotes (Eukarya).