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Eukaryotic cells are some 10,000 times larger than prokaryotic cells by volume, have their DNA organised in a nucleus, and contain membrane-bound organelles. The division between prokaryotes and eukaryotes has been considered the most important distinction or difference among organisms.
The development of microscopy revealed important distinctions between those organisms whose cells do not have a distinct nucleus (prokaryotes) and organisms whose cells do have a distinct nucleus . In 1937 Édouard Chatton introduced the terms "prokaryote" and "eukaryote" to differentiate these organisms.
Some eukaryotic cells also use flagella—and they can be found in some protists and plants as well as animal cells. Eukaryotic flagella are complex cellular projections that lash back and forth, rather than in a circular motion. Prokaryotic flagella use a rotary motor, and the eukaryotic flagella use a complex sliding filament system.
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]
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.
The origin of the eukaryotic cell, or eukaryogenesis, is a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. The last eukaryotic common ancestor (LECA) is the hypothetical origin of all living eukaryotes, [ 71 ] and was most likely a biological population , not a single ...
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.
The reason for this is the differences in species concepts between the bacteria and macro-organisms, the difficulties in growing/characterising in pure culture (a prerequisite to naming new species, vide supra) and extensive horizontal gene transfer blurring the distinction of species.