Cytokinesis 1 Animal cell cytokinesis 1.1 Contractile ring positioning During different, 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 ATP is hydrolysed to move along these actin filaments, constricting the cell membrane to form a cleavage furrow. ConCilliate undergoing cytokinesis, with the cleavage furrow being tinued hydrolysis causes this cleavage furrow to ingress (move inwards), a striking process that is clearly visiclearly visible. ble through a light microscope. Ingression continues until a so-called midbody structure (composed of electrondense, proteinaceous material) is formed and the process of abscission then physically cleaves this midbody into two. Abscission depends on septin filaments beneath the cleavage furrow, which provide a structural basis to ensure the 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). The position at which the contractile ring assembles is dictated by the mitotic spindle.[1] This seems to depend upon the GTPase RhoA, which influences sevAnimal cell telophase and cytokinesis eral downstream effectors (such as the protein kinases ROCK and citron) to promote myosin activation (by inCytokinesis (from the Greek cyto- “cell” (cf. cytology) fluencing the phosphorylation of Myosin regulatory light and kinesis (“motion, movement”) is the process in which chain (rMLC)) and actin filament assembly (by regulating the cytoplasm of a single eukaryotic cell is divided to form formin protein) at a particular region of the cell cortex.[2] two daughter cells. It usually initiates during the early Simultaneous with contractile ring assembly during stages of mitosis, and sometimes meiosis, splitting a mi- prophase, a microtubule based structure termed the totic cell in two, to ensure that chromosome number is central spindle (or spindle midzone) forms when nonmaintained from one generation to the next. After cy- kinetochore microtubule fibres are bundled between the tokinesis two (daughter) cells will be formed that enter spindle poles. A number of different species including interphase to make exact copies of the (parent) original H. sapiens, D. melanogaster and C. elegans require the cell. In animal cells, one notable exception to the nor- central spindle in order to efficiently undergo cytokinemal process of cytokinesis is oogenesis (the creation of sis, although the specific phenotype described when it is an ovum in the ovarian follicle of the ovary), where the absent varies from one species to the next (for example, ovum takes almost all the cytoplasm and organelles, leav- certain Drosophila cell types are incapable of forming a ing very little for the resulting polar bodies, which then cleavage furrow without the central spindle, whereas in die. In plant cells, a dividing structure known as the cell both C. elegans embryos and human tissue culture cells a plate forms within the centre of the cytoplasm and a new cleavage furrow is observed to form and ingress, but then cell wall forms between the two daughter cells. regress before cytokinesis is complete). Seemingly viCytokinesis is distinguished from the prokaryotic process tal for the formation of the central spindle (and therefore of binary fission. efficient cytokinesis) is a heterotetrameric protein com1
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REFERENCES
plex 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.
microscopy. The first components to arrive are pectins, hemicelluloses, and arabinogalactan proteins carried by the secretory vesicles that fuse to form the cell plate.[12] The next component to be added is callose, which is polymerized directly at the cell plate by callose synthases. As the cell plate continues to mature and fuses with the 1.2 Timing cytokinesis parental plasma membrane, the callose is slowly replaced with cellulose, the primary component of a mature cell Cytokinesis must be temporally controlled to ensure that wall [6] . The middle lamella (a glue-like layer containing it occurs only after sister chromatids separate during the pectin) develops from the cell plate, serving to bind the anaphase portion of normal proliferative cell divisions. cell walls of adjoining cells together.[13][14] 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 3 See also the cell cycle.[3][4] Cytokinesis happens only after APC binds with CDC20. This allows for the separation of • Diploid chromosomes and myosin to work simultaneously.
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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 stages of cell plate formation include (1) creation of the phragmoplast, an array of microtubules that guides and supports the formation of the cell plate; (2) trafficking of vesicles to the division plane and their fusion to generate a tubular-vesicular network; (3) continued fusion of membrane tubules and their transformation into membrane sheets upon the deposition of callose, followed by deposition of cellulose and other cell wall components; (4) recycling of excess membrane and other material from the cell plate; and (5) fusion with the parental cell wall [5][6] The phragmoplast is assembled from the remnants of the mitotic spindle, and serves as a track for the trafficking of vesicles to the phragmoplast midzone. These vesicles contain lipids, proteins and carbohydrates needed for the formation of a new cell boundary. Electron tomographic studies have identified the Golgi apparatus as the source of these vesicles,[7][8] but other studies have suggested that they contain endocytosed material as well.[9][10] These tubules then widen and fuse laterally with each other, eventually forming a planar, fenestrated sheet [8] . As the cell plate matures, large amounts of membrane material are removed via clathrin-mediated endocytosis [7] Eventually, the edges of the cell plate fuse with the parental plasma membrane, often in an asymmetrical fashion,[11] thus completing cytokinesis. The remaining fenestrae contain strands of endoplasmic reticulum passing through them, and are thought to be the precursors of plasmodesmata [8] .
4 References [1] Rappoport R: “Cytokinesis in Animal Cells”, Cambridge University Press (1996) [2] Glotzer, M: “Animal cell cytokinesis”, Annual Review of Cell Biology 17, 351-86 (2001) [3] J. Mishima et al.: “Cell cycle regulation of central spindle correctly”, Nature 430, 908-913 (2004) [4] 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) [5] Otegui, M., and Staehelin, L.A. “Cytokinesis in flowering plants: more than one way to divide a cell.” Curr. Opin. Plant Biol. 3, 493-502 (2000) [6] Samuels, A.L., Giddings,T.H.Jr., and Staehelin, L.A. “Cytokinesis in tobacco BY-2 and root tip cells: a new model of cell plate formation in higher plants.” J. Cell Biol. 130, 1345-1357 (1995). [7] Otegui, M.S., Mastronarde, D.N., Kang, B.H., Bednarek, S.Y., and Staehelin, L.A. “Three-dimensional analysis of syncytial-type cell plates during endosperm cellularization visualized by high resolution electron tomography.” Plant Cell 13, 2033-2051 (2001) [8] Segui-Simarro, J.M., Austin, J.R.,2nd, White, E.A., and Staehelin, L.A. “Electron tomographic analysis of somatic cell plate formation in meristematic cells of Arabidopsis preserved by high-pressure freezing.” Plant Cell 16, 836856 (2004) [9] Baluška, F., Liners, F., Hlavačka, A., Schlicht, M., Van Cutsem, P., McCurdy, D.W., and Menzel, D. “Cell wall pectins and xyloglucans are internalized into dividing root cells and accumulate within cell plates during cytokinesis.” Protoplasma 225, 141-55 (2005)
The construction of the new cell wall begins within the [10] Dhonukshe, P., Baluška, F., Schlicht, M., Hlavacka, A., lumen of the narrow tubules of the young cell plate. The Šamaj, J., Friml, J., and W., G.T.,Jr. “Endocytosis of cell order in which different cell wall components are desurface material mediates cell plate formation during plant posited has been determined largely by immuno-electron cytokinesis.” Dev. Cell 10, 137-50 (2006)
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[11] Cutler, S.R., and Ehrhardt, D.W. “Polarized cytokinesis in vacuolate cells of Arabidopsis.” Proc. Natl. Acad. Sci. USA 99, 2812-2817 (2002) [12] Staehelin, L.A., and Moore, I. “The Plant Golgi Apparatus: Structure, Functional Organization and Trafficking Mechanisms.” Annu. Rev. Plant Physiol. Plant Mol. Biol. 46, 261-288 (1995) [13] http://www.jstor.org/stable/2464904 “On the Origin and Nature of the Middle Lamella”. Allen, Charles E. Botanical Gazette, Vol. 32, No. 1 (Jul., 1901), pp. 1-34. [14] Eichorn, Susan, et al. Esau’s Plant Anatomy: Meristems, Cells, and Tissues of the Plant Body: Their Structure, Function, and Development, 3rd Edition. 2006. ISBN 978-0-471-73843-5
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Further reading • The Molecular Requirements for Cytokinesis by M. Glotzer (2005), Science 307, 1735 • “Animal Cytokinesis: from parts list to mechanism” by Eggert, U.S., Mitchison, T.J., Field, C.M. (2006), Annual Review of Cell Biology 75, 543-66 • Campbell Biology (2010), 580-582 • More description and nice images of cell division in plants, with a focus on fluorescence microscopy • Nanninga, Nanne. Cytokinesis in Prokaryotes and Eukaryotes: Common Principles and Different Solutions
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