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Eukaryotes are organisms consisting of one or more Eukaryotes
eukaryotic cells, the most distinctive feature of Scientific
which is the presence of one or more classification
membrane-bound nuclei. Animals, plants, fungi, and
various other groups collectively referred to as Domain: Eukaryota
Protista are all eukaryotes, varying from
single-celled organisms to truly multicellular Typical kingdoms
forms, in which different cells are specialized Animalia
for different tasks and in general do not survive Fungi
when isolated. In the three-domain system, Plantae
eukaryotes are treated as one domain, referred to "Protista"
as the Eukaryota or Eukarya. Members of the other ΚΚΚ Heterokonts
two domains, the Bacteria and Archaea, are ΚΚΚ Alveolates
microorganisms with prokaryotic cells. ΚΚΚ Euglenozoa
ΚΚΚ Red algae
Cell structure ΚΚΚ inter alia
Eukaryotic cells are generally larger than prokaryotes, with a difference in
volume on 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 a series of DNA strands 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.
Nuclear 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.