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According to the domain system, the tree of life consists of either three domains, Archaea, Bacteria, and Eukarya, [1] or two domains, Archaea and Bacteria, with Eukarya included in Archaea. [ 3 ] [ 4 ] In the three-domain model , the first two are prokaryotes , single-celled microorganisms without a membrane-bound nucleus .
The eukaryotes (/ j uː ˈ k ær i oʊ t s,-ə t s / yoo-KARR-ee-ohts, -əts) [4] constitute the domain of Eukaryota or Eukarya, organisms whose cells have a membrane-bound nucleus. All animals , plants , fungi , seaweeds , and many unicellular organisms are eukaryotes.
It consists of all bacteria, which are prokaryotes (lacking nucleus), thus, Domain Bacteria is made up solely of prokaryotic organisms. [62] [63] Some examples are: Cyanobacteria – photosynthesising bacteria related to the plastids of eukaryotes. [64] Spirochaetota – Gram-negative bacteria involved in human diseases like syphilis and lyme ...
Finally, at the highest rank all of these are grouped together with all other organisms possessing cell nuclei in the domain Eukarya. The International Code of Zoological Nomenclature defines rank as: "The level, for nomenclatural purposes, of a taxon in a taxonomic hierarchy (e.g. all families are for nomenclatural purposes at the same rank ...
The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used five- or six-kingdom systems.This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as Archaea appear to be more closely related to eukaryotes than they are to other prokaryotes – bacteria-like organisms with no cell nucleus.
Human taxonomy is the classification of the human species within zoological taxonomy. The systematic genus , Homo , is designed to include both anatomically modern humans and extinct varieties of archaic humans .
SAR or Harosa is a highly diverse clade of eukaryotes, often considered a supergroup, [2] that includes stramenopiles (heterokonts), alveolates, and rhizarians. [3] [4] [5] It is a node-based taxon, including all descendants of the three groups' last common ancestor, [6] and comprises most of the now-rejected Chromalveolata. [2]
Opisthokont characteristics include synthesis of extracellular chitin in exoskeleton, cyst/spore wall, or cell wall of filamentous growth and hyphae; the extracellular digestion of substrates with osmotrophic absorption of nutrients; and other cell biosynthetic and metabolic pathways.