Domains of life

Eukaryota (represented by the Australian green tree frog, left),

Bacteria (represented by Staphylococcus aureus, middle) and

In biological taxonomy, a domain (Latin: regio) is the highest taxonomic rank of organisms in the three-domain system of taxonomy designed by Carl Woese, an American microbiologist and biophysicist. According to the Woese system, introduced in 1990, the tree of life (biology) consists of three domains: Archaea (a term which Woese created), Bacteria, and Eukarya. The first two are all prokaryotic microorganisms, or single-celled organisms whose cells have no nucleus. All life that has a nucleus and membrane-bound organelles, and multicellular organisms, is included in the Eukarya. Stefan Luketa in 2012 proposed the five-domain system of life with both cellular and non-cellular organisms.

The term "domain" is proposed by Woese et al. (1990) in his three-domain system. This term represents a synonym for the category of dominion (lat. dominium), introduced by Moore in 1974. However, only S. Luketa uses the term dominion.

Each of these three domains of life recognized by biologists today contain unique rRNA. This fact in itself forms the basis of the three-domain system. While the presence of a nuclear membrane differentiates the Eukarya domain from the Archaea and Bacteria domains, both of which lack a nuclear membrane, distinct biochemical and RNA markers differentiate the Archaea and Bacteria domains from each other.

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