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'''Eukaryotes ''' are organisms whose [[cell]]s are organized into complex structures by internal [[membrane|membranes]] and a [[cytoskeleton]]. [[animal|animals]], [[plant|plants]] and [[fungus|fungi]] are all eukaryotes, but [[bacteria]] are not.  

'''Eukaryotes ''' are organisms whose [[cell]]s are organized into complex structures by internal [[membrane]]s and a [[cytoskeleton]]. [[animal]]s, [[plant]]s and [[fungus|fungi]] are all eukaryotes, but [[bacteria]] are not.  

The most characteristic feature of a eukaryotic cell, the nucleus, consists of a [[nucleoplasm]] surrounded by a double nuclear membrane pierced by nuclear pores. <ref>http://biotech.icmb.utexas.edu/search/dict-search.html</ref><ref>http://www.iscid.org/encyclopedia/Eukaryote</ref> The nucleoplasm contains the (linear) chromosomes of the cell, which are organised into [[heterochromatin]], which stains only a little, and euchromatin, which stains more densely. The most important euchromatic area is the nucleolus, in which ribosomes are formed.

The most characteristic feature of a eukaryotic cell, the nucleus, consists of a [[nucleoplasm]] surrounded by a double nuclear membrane pierced by nuclear pores.<ref>http://biotech.icmb.utexas.edu/search/dict-search.html</ref><ref>http://www.iscid.org/encyclopedia/Eukaryote</ref> The nucleoplasm contains the (linear) chromosomes of the cell, which are organised into [[heterochromatin]], which stains only a little, and euchromatin, which stains more densely. The most important euchromatic area is the nucleolus, in which ribosomes are formed.

#εύ-καρυον (eu-karyon) - true [[kernel]].  

#εύ-καρυον (eu-karyon) - true [[kernel]].  

#They can grow 'large' because cytoplasmic streaming allows rapid transport across the cell - 100 μm.  

#They can grow 'large' because cytoplasmic streaming allows rapid transport across the cell - 100 μm.  

[[Image:Animal_cell.png‎ |center||Animal cell: eukaryotic cells are characterised by the possession of a nucleus and membrane bound organelles.]]

[[Image:Animal_cell.png‎|center||Animal cell: eukaryotic cells are characterised by the possession of a nucleus and membrane bound organelles.]]

The role of the nucleus is three-fold:

The role of the nucleus is three-fold:

[[DNA]] storage takes up about 10% of the cell of both prokaryotes (nucleoid area) and eukaryotes (nucleus). However, don't forget that some of the genome lives elsewhere: plasmids, endosymbionts (mitochondria), etc. The nucleus of eukaryotes also protects the genome from the cytoskeleton. Condensation of genome during mitosis is required to withstand these stresses. This is not relevant in prokaryotes, as they lack a cytoskeleton. The existence of the nucleus permits processing of mRNA, and therefore defers translation. Alternative splicing can be performed to generate different proteins from the same RNA primary transcript. mRNA is capped and tailed to permit it to exit the nucleus. rRNA and tRNA are also heavily processed by RNA editing, i.e. base modification (this also occurs to some mRNA in trypanosomes). rRNA modification occurs in the nucleolus, where ribosomes are constructed.

[[DNA]] storage takes up about 10% of the cell of both prokaryotes (nucleoid area) and eukaryotes (nucleus). However, don't forget that some of the genome lives elsewhere: plasmids, endosymbionts (mitochondria), etc. The nucleus of eukaryotes also protects the genome from the cytoskeleton. Condensation of genome during mitosis is required to withstand these stresses. This is not relevant in prokaryotes, as they lack a cytoskeleton. The existence of the nucleus permits processing of mRNA, and therefore defers translation. Alternative splicing can be performed to generate different proteins from the same RNA primary transcript. mRNA is capped and tailed to permit it to exit the nucleus. rRNA and tRNA are also heavily processed by RNA editing, i.e. base modification (this also occurs to some mRNA in trypanosomes). rRNA modification occurs in the nucleolus, where ribosomes are constructed.

The nucleus is bound by a double-membrane, which is contiguous through the nuclear pores, known as the nuclear envelope. The pores are required to allow RNA out and membrane lipids in (which is needed for growth during S phase).<ref> Berg, Jeremy M.; Tymoczko, John L.; and Stryer, Lubert."Biochemistry". W. H. Freeman and Co. ; c2002 </ref>

The nucleus is bound by a double-membrane, which is contiguous through the nuclear pores, known as the nuclear envelope. The pores are required to allow RNA out and membrane lipids in (which is needed for growth during S phase).<ref>Berg, Jeremy M.; Tymoczko, John L.; and Stryer, Lubert."Biochemistry". W. H. Freeman and Co. ; c2002</ref>

The inner face of the inner nuclear envelope (INE) is coated by the nuclear lamina, which contains intermediate fibres called lamins A, B and C (at least in mammals). Phosphorylation of lamins by kinases cause nuclear envelope breakdown during prometaphase. Chromosomes occupy definite positions within the nucleus because of the interaction between lamins and telomeres, for example the Rabl conformation in yeast.

The inner face of the inner nuclear envelope (INE) is coated by the nuclear lamina, which contains intermediate fibres called lamins A, B and C (at least in mammals). Phosphorylation of lamins by kinases cause nuclear envelope breakdown during prometaphase. Chromosomes occupy definite positions within the nucleus because of the interaction between lamins and telomeres, for example the Rabl conformation in yeast.

Photosynthetic eukaryotes also have chloroplasts, which are the remains of cyanobacteria that also became endosymbiotic. They have a triple membrane system. The outer membrane surrounds the inner membrane, which itself surrounds the stroma, in which are embedded a number of thylakoid membranes, some of which occur in stacks called grana. Those that are unstacked are termed intergrana. The thylakoids surround another compartment, termed the lumen. Chloroplasts are one of several different sorts of plastid. Others include proplasts (immature plastids), chromoplasts (full of pigments), etioplasts (formed in the dark) and amyloplasts (full of starch). Plastids carry out photosynthesis, converting light energy into ATP and NADH.

Photosynthetic eukaryotes also have chloroplasts, which are the remains of cyanobacteria that also became endosymbiotic. They have a triple membrane system. The outer membrane surrounds the inner membrane, which itself surrounds the stroma, in which are embedded a number of thylakoid membranes, some of which occur in stacks called grana. Those that are unstacked are termed intergrana. The thylakoids surround another compartment, termed the lumen. Chloroplasts are one of several different sorts of plastid. Others include proplasts (immature plastids), chromoplasts (full of pigments), etioplasts (formed in the dark) and amyloplasts (full of starch). Plastids carry out photosynthesis, converting light energy into ATP and NADH.

[[Image:Plant_cell.png|center||Plant cell: The cell wall vs the cell membrane found in animal cells.]]

[[Image:Plant_cell.png|center|Plant cell: The cell wall vs the cell membrane found in animal cells.]]

Eukaryotes have a cytoskeleton, which is composed of a number of interacting fibre types. The first are the actin microfilaments, which, with the motor protein myosin, form the muscles of the cytoskeleton (and in sarcosomes, form actual muscles too). The actin cytoskeleton forms a cortex beneath the plasmalemma of most eukaryotic cells, which is involved in changes to the external shape of the cell (such as the formation of pseudopodia in [[amoebas]]).The second fibre type are intermediate filaments, composed of spectrin, keratin, vimentin, nuclear lamins and other proteins. The intermediate filaments help maintain the relative positions of organelles and give the cell mechanical strength. The third type of cytoskeletal filament are the microtubules, which form the spindle during cell divisions (meiosis and mitosis), and give structure to the cell membrane, forming microvilli. They also form undulipodia, which are waving structures with a typical 9+2 structure, found in cilia (short undulipodia) and flagella (longer ones). The microtubules are composed of tubulin, and are organised by a microtubule organising centre (the centrosome and centrioles in animals), in which the − ends of the microtubules (which grow more slowly than the + ends) are embedded. Vesicles traffic along the microtubules under the influence of motor proteins such as kinesin (which transports items from the −→+ ends, i.e. away from the MTOC) and dynein (which traffics items from +→−). Tubulin is homologous to the FtsZ protein that is involved in bacterial fission.

Eukaryotes have a cytoskeleton, which is composed of a number of interacting fibre types. The first are the actin microfilaments, which, with the motor protein myosin, form the muscles of the cytoskeleton (and in sarcosomes, form actual muscles too). The actin cytoskeleton forms a cortex beneath the plasmalemma of most eukaryotic cells, which is involved in changes to the external shape of the cell (such as the formation of pseudopodia in [[amoebas]]).The second fibre type are intermediate filaments, composed of spectrin, keratin, vimentin, nuclear lamins and other proteins. The intermediate filaments help maintain the relative positions of organelles and give the cell mechanical strength. The third type of cytoskeletal filament are the microtubules, which form the spindle during cell divisions (meiosis and mitosis), and give structure to the cell membrane, forming microvilli. They also form undulipodia, which are waving structures with a typical 9+2 structure, found in cilia (short undulipodia) and flagella (longer ones). The microtubules are composed of tubulin, and are organised by a microtubule organising centre (the centrosome and centrioles in animals), in which the − ends of the microtubules (which grow more slowly than the + ends) are embedded. Vesicles traffic along the microtubules under the influence of motor proteins such as kinesin (which transports items from the −→+ ends, i.e. away from the MTOC) and dynein (which traffics items from +→−). Tubulin is homologous to the FtsZ protein that is involved in bacterial fission.

The rough ER differs from the smooth ER morphologically. The RER's cytoplasmic surface is studded with ribosomes, and it has flattened cisternae (rather than tubular ones). The RER is denser than SER due to presence of ribosomes. Consequently, disrupted ER 'microsomes' can be separated using density gradient centrifugation.

The rough ER differs from the smooth ER morphologically. The RER's cytoplasmic surface is studded with ribosomes, and it has flattened cisternae (rather than tubular ones). The RER is denser than SER due to presence of ribosomes. Consequently, disrupted ER 'microsomes' can be separated using density gradient centrifugation.

[[Image:Density_gradient_er.png|center||SER microsomes are less dense than RER microsomes, so can be separated by centrifugation.]]

[[Image:Density_gradient_er.png|center|SER microsomes are less dense than RER microsomes, so can be separated by centrifugation.]]

Other proteins are destined for the endolysosome system. Material engulfed from outside by [[phagocytosis]] (solids) and pinocytosis (liquids) are packaged into phagosomes, and internal organelles destined for destruction are packed into autophagosomes. Endolysosomes containing digestive enzymes fuse with the phagosomes and digest their contents. The fused organelles produced are the cells disposal system, and are called lysosomes. Very large lysosomes are found in plants where they help to bulk the cell with water. Here they are called vacuoles, and are surrounded by a membrane called to tonoplast. Other vacuoles are involved in osmoregulation: for example contractile vacuoles in Amoeba.

Other proteins are destined for the endolysosome system. Material engulfed from outside by [[phagocytosis]] (solids) and pinocytosis (liquids) are packaged into phagosomes, and internal organelles destined for destruction are packed into autophagosomes. Endolysosomes containing digestive enzymes fuse with the phagosomes and digest their contents. The fused organelles produced are the cells disposal system, and are called lysosomes. Very large lysosomes are found in plants where they help to bulk the cell with water. Here they are called vacuoles, and are surrounded by a membrane called to tonoplast. Other vacuoles are involved in osmoregulation: for example contractile vacuoles in Amoeba.

[[Category:Molecular Biology]]

[[Category:Molecular Biology]]

[[Category:Cellular Biology]]

[[Category:Cellular Biology]]

[[Category:microbiology]]

[[Category:Microbiology]]