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Eukaryote

2007 Schools Wikipedia Selection. Related subjects: Organisms

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   Eukaryotes
       Scientific classification

   Domain: Eukaryota
           Whittaker & Margulis, 1978
                Kingdoms
   Animalia - Animals
   Fungi
   Plantae - Plants
   Protista

   A eukaryote ( IPA: [juːˈkærɪɒt]) is an organism with a complex cell or
   cells, in which the genetic material is organized into a membrane-bound
   nucleus or nuclei. Eukaryotes (also spelled "eucaryotes") comprise
   animals, plants, and fungi—which are mostly multicellular—as well as
   various other groups that are collectively classified as protists (many
   of which are unicellular). In contrast, prokaryotes are organisms, such
   as bacteria and archaea, that lack nuclei and other complex cell
   structures. Eukaryotes share a common origin, and are often treated
   formally as a superkingdom, empire, or domain. In the domain system,
   eukaryotes have more in common with archaean prokaryotes than bacterial
   prokaryotes. The name comes from the Greek ευ, meaning good/true, and
   κάρυον, meaning nut, in reference to the cell nucleus.

Structure

   A typical animal cell
   Enlarge
   A typical animal cell

   Eukaryotic cells are generally much larger than prokaryotes. They have
   a variety of internal membranes and structures, called organelles, and
   a cytoskeleton composed of microtubules, microfilaments and
   intermediate filaments, which play an important role in defining the
   cell's organization and shape. Eukaryotic DNA is divided into several
   linear bundles called chromosomes, which are separated by a
   microtubular spindle during nuclear division. In addition to asexual
   cell division ( mitosis), most eukaryotes have some process of sexual
   reproduction via cell fusion ( meiosis), which is not found among
   prokaryotes.
   Detail of the endomembrane system and its components
   Enlarge
   Detail of the endomembrane system and its components

Internal membrane

   Eukaryotic cells include a variety of membrane-bound structures,
   collectively referred to as the endomembrane system. Simple
   compartments, called vesicles or vacuoles, can form by budding off
   other membranes. Many cells ingest food and other materials through a
   process of endocytosis, where the outer membrane invaginates and then
   pinches off to form a vesicle. It is probable that most other
   membrane-bound organelles are ultimately derived from such vesicles.

   The nucleus is surrounded by a double membrane (commonly referred to as
   a nuclear envelope), with pores that allow material to move in and out.
   Various tube- and sheet-like extensions of the nuclear membrane form
   what is called the endoplasmic reticulum or ER, which is involved in
   protein transport and maturation. It includes the Rough ER where
   ribosomes are attached, and the proteins they synthesize enter the
   interior space or lumen. Subsequently, they generally enter vesicles,
   which bud off from the Smooth ER. In most eukaryotes, this
   protein-carrying vesicles are released and further modified in stacks
   of flattened vesicles, called Golgi bodies or dictyosomes.

   Vesicles may be specialized for various purposes. For instance,
   lysosomes contain enzymes that break down the contents of food
   vacuoles, and peroxisomes are used to break down peroxide which is
   toxic otherwise. Many protozoa have contractile vacuoles, which collect
   and expel excess water, and extrusomes, which expel material used to
   deflect predators or capture prey. In multicellular organisms, hormones
   are often produced in vesicles. In higher plants, most of a cell's
   volume is taken up by a central vacuole, which primarily maintains its
   osmotic pressure.
   Mitochondria structure : 1) Inner membrane 2) Outer membrane 3) Crista
   4) Matrix Enlarge
   Mitochondria structure :
   1) Inner membrane
   2) Outer membrane
   3) Crista
   4) Matrix

Mitochondria and plastids

   Mitochondria are organelles found in nearly all eukaryotes. They are
   surrounded by double membranes, the inner of which is folded into
   invaginations called cristae, where aerobic respiration takes place.
   They contain their own DNA and are only formed by the fission of other
   mitochondria. They are now generally held to have developed from
   endosymbiotic prokaryotes, probably proteobacteria. The few protozoa
   that lack mitochondria have been found to contain mitochondrion-derived
   organelles, such as hydrogenosomes and mitosomes.

   Plants and various groups of algae also have plastids. Again, these
   have their own DNA and developed from endosymbiotes, in this case
   cyanobacteria. They usually take the form of chloroplasts, which like
   cyanobacteria contain chlorophyll and produce energy through
   photosynthesis. Others are involved in storing food. Although plastids
   likely had a single origin, not all plastid-containing groups are
   closely related. Instead, some eukaryotes have obtained them from
   others through secondary endosymbiosis or ingestion.

   Endosymbiotic origins have also been proposed for the nucleus, for
   which see below, and for eukaryotic flagella, supposed to have
   developed from spirochaetes. This is not generally accepted, both from
   a lack of cytological evidence and difficulty in reconciling this with
   cellular reproduction.

Cytoskeletal structures

   Many eukaryotes have long slender motile cytoplasmic projections,
   called flagella. These are composed mainly of tubulin and shorter
   cilia, both of which are variously involved in movement, feeding, and
   sensation. These are entirely distinct from prokaryotic flagella. They
   are supported by a bundle of microtubules arising from a basal body,
   also called a kinetosome or centriole, characteristically arranged as
   nine doublets surrounding two singlets. Flagella also may have hairs,
   or mastigonemes, and scales connecting membranes and internal rods.
   Their interior is continuous with the cell's cytoplasm.

   Centrioles are often present even in cells and groups that do not have
   flagella. They generally occur in groups of one or two, called
   kinetids, that give rise to various microtubular roots. These form a
   primary component of the cytoskeletal structure, and are often
   assembled over the course of several cell divisions, with one flagellum
   retained from the parent and the other derived from it. Centrioles may
   also be associated in the formation of a spindle during nuclear
   division.

   Some protists have various other microtubule-supported organelles.
   These include the radiolaria and heliozoa, which produce axopodia used
   in flotation or to capture prey, and the haptophytes, which have a
   peculiar flagellum-like organelle called the haptonema.

Reproduction

   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.

   Eukaryotes have a smaller surface to volume area ratio than
   prokaryotes, and thus have lower metabolic rates and longer generation
   times. In some multicellular organisms, cells specialized for
   metabolism will have enlarged surface areas, such as intestinal vili.

Origin and evolution

   The origin of the eukaryotic cell was a milestone in the evolution of
   life, since they include all complex cells and multi-cellular
   organisms. The timing of this series of events is hard to determine;
   Knoll (1992) suggests they developed approximately 1.6 - 2.1 billion
   years ago. Fossils that are clearly related to modern groups start
   appearing around 1200 million years ago, in the form of a red alga.

   rRNA trees constructed during the 1980s and 1990s left most eukaryotes
   in an unresolved "crown" group (not technically a true crown), which
   was usually divided by the form of the mitochondrial cristae. The few
   groups that lack mitochondria branched separately and so the absence
   was believed to be primitive, but this is now considered an artifact of
   long branch attraction and they are known to have lost them
   secondarily.

   Trees based on actin and other molecules have painted a different and
   more complete picture. Most eukaryotes are now included in several
   supergroups:
   Opisthokonts Animals, fungi, choanoflagellates, etc.
   Amoebozoa Most lobose amoebae and slime moulds
   Rhizaria Foraminifera, Radiolaria, and various other amoeboid protozoa
   Excavates Various flagellate protozoa
   Primoplantae (or Archaeplastida) Land plants, green, red algae, and
   glaucophytes
   Heterokonts Brown algae, diatoms, water molds, etc.
   Alveolates Ciliates, Apicomplexa, dinoflagellates, etc.

   The heterokonts and alveolates may be part of a larger group that is
   ancestrally photosynthetic, called the chromalveolates, but this
   remains contentious. Otherwise the relationships between the different
   supergroups are mostly uncertain, and in particular there is dispute
   about where the root of the evolutionary tree belongs, and as a result
   what the earliest eukaryotes were like. A few small protist groups have
   not been related to any of the major supergroups, notably the
   centrohelids, apusozoans, and ebriids.

   Eukaryotes are closely related to archaea, at least in terms of nuclear
   DNA and genetic machinery. In other respects, such as membrane
   composition, they are similar to eubacteria. Three main explanations
   for this have been proposed:
     * Eukaryotes resulted from the complete fusion of two or more cells,
       the cytoplasm forming from a eubacterium and the nucleus from an
       archaeon ( alternatively a virus).
     * Eukaryotes developed from Archaea, and acquired their eubacterial
       characteristics from the proto-mitochondrion.
     * Eukaryotes and Archaea developed separately from a modified
       eubacterium.

   The origin of the endomembrane system and mitochondria are also
   disputed. The phagotrophic hypothesis states the membranes originated
   with the development of endocytosis and later specialized; mitochondria
   were acquired by ingestion, like plastids. The syntrophic hypothesis
   states that the proto-eukaryote relied on the proto-mitochondrion for
   food, and so ultimately grew to surround it; the membranes originate
   later, in part thanks to mitochondrial genes (the hydrogen hypothesis
   is one particular version).

   A new hypothetical theory thinks of the Eucarya ancestor as the
   Urkaryote. This ancestor is only the precursor of the Eucarya and is
   not the ancestor of the Archaea or Bacteria. The commmon ancestor of
   all three of these domains is a more primitive organism with a variety
   of different cell structures (less vs more complex structures,
   pseudomurein vs peptidoglycan cell wall, rRNA differences, lipid
   bilayer membrane vs phospholipid bilayer membrane, nucleoid region vs
   defined nucleus, differences in metabolic processes, Enther Dnodroff
   Pathway vs Glycolysis Pathway, etc).

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