domain biology

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      In biological taxonomy, a domain ( or ) (Latin: regio), also dominion, superkingdom, realm, or empire, is the highest taxonomic rank of all organisms taken together. It was introduced in the three-domain system of taxonomy devised by Carl Woese, Otto Kandler and Mark Wheelis in 1990.
      According to the domain system, the tree of life consists of either three domains, Archaea, Bacteria, and Eukarya, or two domains, Archaea and Bacteria, with Eukarya included in Archaea. In the three-domain model, the first two are prokaryotes, single-celled microorganisms without a membrane-bound nucleus. All organisms that have a cell nucleus and other membrane-bound organelles are included in Eukarya and called eukaryotes.
      Non-cellular life, most notably the viruses, is not included in this system. Alternatives to the three-domain system include the earlier two-empire system (with the empires Prokaryota and Eukaryota), and the eocyte hypothesis (with two domains of Bacteria and Archaea, with Eukarya included as a branch of Archaea).


      Terminology


      The term domain was proposed by Carl Woese, Otto Kandler, and Mark Wheelis (1990) in a three-domain system. This term represents a synonym for the category of dominion (Lat. dominium), introduced by Moore in 1974.


      Development of the domain system


      Carl Linnaeus made the classification "domain" popular in the famous taxonomy system he created in the middle of the eighteenth century. This system was further improved by the studies of Charles Darwin later on but could not classify bacteria easily, as they have very few observable features to compare to the other domains.
      Carl Woese made a revolutionary breakthrough when, in 1977, he compared the nucleotide sequences of the 16s ribosomal RNA and discovered that the rank "domain" contained three branches, not two as scientists had previously thought. Initially, due to their physical similarities, Archaea and Bacteria were classified together and called "archaebacteria". However, scientists now know that these two domains are hardly similar and are internally distinctly different.


      Characteristics of the three domains



      Each of these three domains contains unique ribosomal RNA. This forms the basis of the three-domain system. While the presence of a nuclear membrane differentiates the Eukarya from the Archaea and Bacteria, both of which lack a nuclear envelope, the Archaea and Bacteria are distinct from each other due to differences in the biochemistry of their cell membranes and RNA markers.


      = Archaea

      =

      Archaea are prokaryotic cells, typically characterized by membrane lipids that are branched hydrocarbon chains attached to glycerol by ether linkages. The presence of these ether linkages in Archaea adds to their ability to withstand extreme temperatures and highly acidic conditions, but many archaea live in mild environments. Halophiles (organisms that thrive in highly salty environments) and hyperthermophiles (organisms that thrive in extremely hot environments) are examples of Archaea.
      Archaea are relatively small. They range from 0.1 μm to 15 μm diameter and up to 200 μm long, about the size of bacteria and the mitochondria found in eukaryotic cells. Members of the genus Thermoplasma are the smallest Archaea.


      = Bacteria

      =

      Cyanobacteria and mycoplasmas are two examples of bacteria.
      Even though bacteria are prokaryotic cells like Archaea, their cell membranes are instead made of phospholipid bilayers, with none of the ether linkages that Archaea have. Internally, bacteria have different RNA structures in their ribosomes, hence they are grouped into a different category. In the two- and three-domain systems, this puts them into a separate domain.
      There is a great deal of diversity in the domain Bacteria. That diversity is further confounded by the exchange of genes between different bacterial lineages. The occurrence of duplicate genes between otherwise distantly-related bacteria makes it nearly impossible to distinguish bacterial species, count the bacterial species on the Earth, or organize them into a tree-like structure (unless the structure includes cross-connections between branches, making it a "network" instead of a "tree").


      = Eukarya

      =

      Members of the domain Eukarya – called eukaryotes – have membrane-bound organelles (including a nucleus containing genetic material) and are represented by five kingdoms: Plantae, Protozoa, Animalia, Chromista, and Fungi.


      Exclusion of viruses and prions



      The three-domain system includes no form of non-cellular life. Stefan Luketa proposed a five-dominion system in 2012, adding Prionobiota (acellular and without nucleic acid) and Virusobiota (acellular but with nucleic acid) to the traditional three domains.


      Alternative classifications



      Alternative classifications of life include:

      The two-empire system or superdomain system, proposed by Mayr (1998), with top-level groupings of Prokaryota (or Monera) and Eukaryota.
      The eocyte hypothesis, proposed by Lake et al. (1984), which posits two domains, Bacteria and Archaea, with Eukaryota included as a subordinate clade branching from Archaea.


      See also


      Biological dark matter
      Neomura, which is the two domains of life of Archaea and Eukaryota
      Phylogenetics
      Protein structure
      Realm (virology), an equivalent rank for non-cellular life
      Systematics


      References




      External links


      Learn Biology: Classification-Domains on YouTube

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    What is a domain in biology? - Answers

    Jun 12, 2024 · In Biology, a domain is the highest taxonomic rank of organisms, above the kingdom level. There are three domains of life: Bacteria, Archaea, and Eukarya, each representing a distinct evolutionary ...

    What domain do humans fit in? - Answers

    Jun 20, 2024 · Eukaraya Domain consists of eukaryotes, that is organisms with eukaryotic cells. Humans have eukaryotic cells and are therefore in the Eukarya Domain. ... Taxonomy is the branch of Biology that's ...

    What kingdom and domain does salmon belong to? - Answers

    Jan 14, 2025 · Kingdom is a broader classification level than domain. Organisms in the same kingdom share similar characteristics and traits, but they can belong to different domains, which are higher-level ...

    How many kingdoms are there in domain archaea? - Answers

    Jun 22, 2024 · The six kingdoms are Animalia (Domain Eukarya), Plantae (Domain Eukarya), Fungi (Domain Eukarya), Protista (Domain Eukarya), Archaea (Domain Archaea), and Bacteria (Domain Bacteria). What are the ...

    Which domain is not prokaryotic? - Answers

    Jun 14, 2024 · The Domain Eukaryota is not prokaryotic. Eukaryotic organisms, including plants, animals, fungi, and protists, have cells with a nucleus and other membrane-bound organelles, while prokaryotic ...

    What is the domain of prokaryotes? - Answers

    Jun 15, 2024 · The domain of prokaryotes consists of two major groups: Bacteria and Archaea. Prokaryotes are unicellular organisms that lack a true nucleus and membrane-bound organelles.

    Which domain does monera belong? - Answers

    Jun 9, 2024 · This is a recent change; some Biology textbooks still say Monera. ... Eukarya is a separate domain that includes organisms with eukaryotic cells, which are characterized by having a nucleus and ...

    How are organisms in the domain archaea diffferent from those

    Jun 21, 2024 · How organisms in the domain eukarya from those in the domain archaea? Organisms in the domain Eukarya have membrane-bound organelles, a nucleus, and multiple linear chromosomes, while organisms in ...

    Why are archaea in a different domain from bacteria? - Answers

    Jun 16, 2024 · The Bacteria domain has several shapes, and the Archaea domain generally resembles the bacteria domain. domain containing prokaryotes? Unicellular prokaryotic organisms are actually broken up into ...

    Which domain do organisms live in extreme environments?

    Jun 19, 2024 · Domain Archaea: Prokaryotic organisms that often live in extreme environments. Domain Eukarya: Organisms with eukaryotic cells, including plants, animals, fungi, and protists.