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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.
[3] [4] 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.
Much of gene structure is broadly similar between eukaryotes and prokaryotes. These common elements largely result from the shared ancestry of cellular life in organisms over 2 billion years ago. [3] Key differences in gene structure between eukaryotes and prokaryotes reflect their divergent transcription and translation machinery.
In 1937 Édouard Chatton introduced the terms "prokaryote" and "eukaryote" to differentiate these organisms. [9] In 1938, Herbert F. Copeland proposed a four-kingdom classification by creating the novel Kingdom Monera of prokaryotic organisms; as a revised phylum Monera of the Protista, it included organisms now classified as Bacteria and Archaea.
Eukaryotes represent a small minority of the number of organisms, but given their generally much larger size, their collective global biomass is much larger than that of prokaryotes. The eukaryotes seemingly emerged within the Asgard archaea, and are closely related to the Heimdallarchaeia. [5]
Both eukaryotes and prokaryotes contain ribosomes which produce proteins as specified by the cell's DNA. Prokaryote ribosomes are smaller than those in eukaryote cytoplasm, but similar to those inside mitochondria and chloroplasts , one of several lines of evidence that those organelles derive from bacteria incorporated by symbiogenesis .
Prokaryotic cells probably transitioned into eukaryotic cells between 2.0 and 1.4 billion years ago. [31] This was an important step in evolution. In contrast to prokaryotes, eukaryotes reproduce by using mitosis and meiosis. Sex appears to be a ubiquitous and ancient, and inherent attribute of eukaryotic life. [32]
Cell division in prokaryotes (binary fission) and eukaryotes (mitosis and meiosis). The thick lines are chromosomes, and the thin blue lines are fibers pulling on the chromosomes and pushing the ends of the cell apart. The cell cycle in eukaryotes: I = Interphase, M = Mitosis, G 0 = Gap 0, G 1 = Gap 1, G 2 = Gap 2, S = Synthesis, G 3 = Gap 3.