Eukaryote
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Eukaryotes are
organisms consisting of one or more eukaryotic
cellss (as opposed to
prokaryotic cells); such cells contain
membrane-bound
nuclei, as well as
organelles.
Animals,
plants,
fungi, and various other groups collectively referred to as
Protista are all eukaryotes, varying from single-celled organisms to truly multicellular forms, in which different cells are specialized for different tasks and in general do not survive when isolated. They share a common origin, and are usually treated as a
taxon above the rank of
kingdom (either a domain or an empire). Prokaryotic organisms are divided among the domains
Bacteria and
Archaea, or less often grouped as a single empire, depending on the author's view of eukaryotic origins.
Cell structure
Eukaryotic cells are generally larger than prokaryotes, with a difference in volume of the order of a thousand times. The cell includes a variety of different structures, collectively referred to as organelles. The nucleus is characteristically surrounded by a double membrane, with
pores allowing materials to pass in and out. It contains the
genetic material, which is separated into bundles of
DNA called
chromosomes. The remainder of the cell, called the
cytosol, is supported by a
cytoskeleton.
Tube- and sheet-like extensions of the nuclear membrane form the
endoplasmic reticulum, or ER. This includes regions that are smooth and others that are rough, where some of the ribosomes are attached, rather than all being distributed throughout the cell as in prokaryotes.
Proteins synthesized by attached ribosomes enter the ER, and are thereby distributed throughout the cell, generally budding out into
vesicles. In most eukaryotes, there is also a
Golgi apparatus composed of stacked vesicles where proteins are modified. There may also be various vacuoles and other structures. Collectively these make up the
endomembrane system.
Various other structures occur. Aside from a few protists, all eukaryotes have
mitochondria, which are involved in
catabolism and in particular in using
oxygen.
Plants and the various lines of
algae also have
chloroplasts, which conduct
photosynthesis. Both of these are believed to represent derived prokaryotic endosymbionts, in the case of chloroplasts probably incorporated at several different times in different groups (see perhaps
endosymbiotic hypothesis). Many eukaryotes have
flagella or
cilia, projections of the cell surface supported by microtubules that are typically motile or sensory. These arise from basal bodies or kinetosomes, which have a characteristic structure, also found in centrioles which in some groups assist in nuclear division. A few other microtubule supported structures are found, such as the axopodia of some amoeboids.
Reproduction
\nNuclear division is often coordinated with cell division. This generally takes place by
mitosis, a process which allows each daughter nucleus to receive one copy of each chromosome. In most eukaryotes there is also a process of sexual reproduction, typically involving an alternation between
haploid generations, where only one copy of each chromosome is present, and
diploid generations, where two are present, occurring through nuclear fusion (syngamy) and
meiosis. There is considerable variation in this pattern, however.
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