Cytokinesis 200182 221168685 2008-06-23T09:53:27Z Arkuat 29003 got to start clearing out all the eukaryotic confusion from [[binary fission]] [[Image:Cytokinesis-electron-micrograph.jpg|thumb|right|250px|A cell that has almost completed cytokinesis. An arrow points to a [[centrosome]] that can still be seen.]] '''Cytokinesis''' is the process whereby the [[cytoplasm]] of a single [[eukaryotic]] cell is divided to spawn two daughter cells. It usually initiates during the late stages of [[mitosis]], and sometimes [[meiosis]], splitting a binucleate cell in two, to ensure that [[chromosome]] number is maintained from one generation to the next. In animal cells, one notable exception to the normal process of cytokinesis is [[oogenesis]] (the creation of an [[ovum]] in the [[ovarian follicle]] of the [[ovary]]), where the ovum takes almost all the cytoplasm and [[organelles]], leaving very little for the resulting [[polar bodies]], which then die. In plant cells, a dividing structure known as the cell plate forms across the centre of the cytoplasm and a new cell wall forms between the two daughter cells. <ref>{{cite book | last = Maton | first = Anthea | authorlink = | coauthors = Hopkins, Jean Johnson, Susan LaHart, David, Quon Warner, David, Wright, Jill D | title = Cells: Building Blocks of Life | publisher = Prentice Hall | date = 1997 | location = New Jersey | pages = 70-74 | url = | doi = | id = | isbn = 0-13423476-6 }}</ref> Cytokinesis is distinguished from the [[prokaryotic]] process of [[binary fission]]. == Animal cell cytokinesis == During normal proliferative divisions, animal cell cytokinesis begins shortly after the onset of sister [[chromatid]] separation in the anaphase of mitosis. A contractile ring, made of non-muscle [[myosin]] II and [[actin]] filaments, assembles equatorially (in the middle of the cell) at the [[cell cortex]] (adjacent to the cell membrane). Myosin II uses the free energy released when [[Adenosine triphosphate|ATP]] is hydrolysed to move along these actin filaments, constricting the cell membrane to form a cleavage furrow. Continued [[hydrolysis]] causes this cleavage furrow to ingress (move inwards), a striking process that is clearly visible down a [[light microscope]]. Ingression continues until a so-called midbody structure (composed of electron-dense, proteinaceous material) is formed and the process of [[abscission]] then physically cleaves this midbody into two. Abscission depends on [[septins|septin]] filaments beneath the cleavage furrow, which provide a structural basis to ensure completion of cytokinesis. After cytokinesis, non-kinetochore [[microtubules]] reorganize and disappear into a new cytoskeleton as the cell cycle returns to [[interphase]] (see also [[cell cycle]]). ===Contractile ring positioning=== The position at which the contractile ring assembles is dictated by the mitotic spindle<ref>Rappoport R: "Cytokinesis in Animal Cells", Cambridge University Press (1996)</ref>. This seems to depend upon the [[GTPase]] [[RhoA]], which influences several downstream effectors (such as the protein kinases ROCK and citron) to promote [[myosin]] activation (by influencing the phosphorylation of [[Myosin regulatory light chain]] (rMLC)) and [[actin]] filament assembly (by regulating [[formin]] protein) at a particular region of the cell cortex.<ref>Glotzer M: "Animal cell cyctokinesis", Annual Review of Cell Biology 17, 351 (2001)</ref> ===The central spindle=== Simultaneous with contractile ring assembly during anaphase, a microtubule based structure termed the '''central spindle''' (or '''spindle midzone''') forms when non-kinetochore microtubule fibres are bundled between the spindle poles. A number of different species including ''[[H. sapiens]]'', ''[[D. melanogaster]]'' and ''[[C. elegans]]'' require the central spindle in order to efficiently undergo cytokinesis, although the specific [[phenotype]] described when it is absent varies from one species to the next (for example, certain Drosophila cell types are incapable of forming a cleavage furrow without the central spindle, whereas in both ''C. elegans'' [[embryos]] and human [[tissue culture]] cells a cleavage furrow is observed to form and ingress, but then regress before cytokinesis is complete). Seemingly vital for the formation of the central spindle (and therefore efficient cytokinesis) is a heterotetrameric protein complex called '''centralspindlin'''. Along with associated factors (such as SPD-1 in ''C. elegans),'' centralspindlin plays a role in bundling microtubules to form the spindle midzone during anaphase. ===Timing cytokinesis=== Cytokinesis must be temporally controlled to ensure that it occurs only after sister [[chromatid]] separation during normal proliferative cell divisions. To achieve this, many components of the cytokinesis machinery are highly regulated to ensure that they are able to perform a particular function at only a particular stage of the [[cell cycle]].<ref>J. Mishima et al: "Cell cycle regulation of central spindle assembly", Nature 430, 908-913 (2004)</ref><ref>Petronczki et al: "Polo-like kinase 1 triggers the initiation of cytokinesis in human cells by promoting recruitment of the RhoGEF Ect2 to the central spindle", Developmental Cell 12, 713-725 (2007)</ref> == Plant cell cytokinesis == Due to the presence of a [[cell wall]], cytokinesis in plant cells is significantly different from that in animal cells. Rather than forming a contractile ring, plant cells construct a cell plate in the middle of the cell. The [[Golgi apparatus]] releases vesicles containing cell wall materials. These vesicles fuse at the equatorial plane and form a [[cell plate]]. The cell plate begins as a fusion tube network, which then becomes a tubulo-vesicular network (TVN) as more components are added. The TVN develops into a tubular network, which then becomes a fenestrated sheet which adheres to the existing plasma membrane. == Bacterial cell cytokinesis == In bacterial cells, a tubulin-like protein called FtsZ was observed to be distributed equally in the cell, but seen to be forming a ring when cytokinesis takes place. The FtsZ ring becomes narrower by GTP hydrolysis. FtsZ recruits other Fts proteins to the site, among other mureine transpeptidases. It is strongly suggested that the polar regions of a bacterium exclude FtsZ, thereby assuring that the contractile ring forms in the middle of the cell.<ref>J. Lutkenhaus: "FtsZ ring in bacterial cytokinesis", Molecular Microbiology, August 1993</ref> == Further reading == *Cytokinesis in Animal Cells - R. Rappoport (1996), Cambridge University Press *Animal Cell Cytokinesis - Glotzer (2001), Annual Review of Cell Biology 17, 351-86 *The Molecular Requirements for Cytokinesis - Glotzer (2005), Science 307, 1735 *Animal Cytokinesis: from parts list to mechanism - Eggert, Mitchison and Field (2006), Annual Review of Cell Biology 75, 543-66 *[[diploid]] ==References== <references/> {{Cell cycle}} [[Category:Cell cycle]] [[cs:Cytokineze]] [[de:Zellteilung]] [[es:Citocinesis]] [[it:Citocinesi]] [[he:ציטוקינזה]] [[nl:Cytokinese]] [[ja:細胞質分裂]] [[pl:Cytokineza]] [[pt:Citocinese]] [[ru:Цитокинез]] [[simple:Cytokinesis]] [[tr:Sitokinez]]