∞ generated and posted on 2015.12.29 ∞

Highest level of cellular organism taxonomic classification.

The idea of 'Domain' as a taxonomic category sitting above the traditionally highest-level taxa, the kingdom, was brought to the world by the late Carl Woese and colleagues in 1990.

There are three domains, Archaea, Bacteria, and Eukarya. These are the eukaryotes (the domain Eukarya), the "common" bacteria (the domain Bacteria), and a less common or at least less appreciated form of prokaryotes, the Archaea). Beneath the taxonomic level of the domains are kingdoms, with each domain consisting of numerous kingdoms. These encompass all of the cellular organisms and therefore excludes the viruses.

The major taxonomic categoriesthus are Domain, Kingdom, Phylum, Class, Order, Family, and Species. Taxonomic category mnemonics can be found here: .

The distinguishing of the three domains originally was made in terms of the characteristics of what is known as the small-subunit ribosomal RNA, that is the 16S rRNA as found in prokaryotes and the 18S rRNA as found in eukaryotes. From :

Although he wanted to examine the evolution of bacteria, rather than attempt to define taxonomic subgroups based on a preconception that bacteria were phylogenetically cohesive (as they were classified at the time), Woese focused on learning about the evolution of lifeas a whole. By choosing an analytical approach to evaluate evolution across all species, a new view of life emerged as a whole that recognized that all known cellular life contained functionally and structurally conserved small-subunit rRNA (SSu rrNA; 16S rrNA in the Bacteria and the Archaea) and was therefore likely to have evolved from a common ancestor. Strikingly, the resulting universal tree of lifedescribed three distinct taxonomic divisions ('archaebacteria', 'eubacteria' and 'urkaryote'), and in 1990 these were formally proposed as the three domains, 'Archaea', 'Bacteria' and 'Eucarya'. Until Woese determined the first SSu rrNA sequence data (for a methanogen), archaea – which, like bacteria, lack a nucleus – were 'simply' considered to be bacteria; the discovery was so surprising, it even stunned Woese.

Small-subunit ribosomal RNAs are large, but not too large, nucleic acid molecules that are found, in one form or another, in all cellular organisms. The nucleotide sequence of nitrogenous bases in the genes that expressthese rRNA molecules group into three fundamental varieties, i.e., Eukarya, Bacteria, and Archaea.