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 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. Image File history File links An electron micrograph image of a cell that has almost completed cell division and cytokinesis. ... Image File history File links An electron micrograph image of a cell that has almost completed cell division and cytokinesis. ... The centrosome is the main microtubule organizing center (MTOC) of the cell as well as a regulator of cell-cycle progression. ... It has been suggested that Cytoplast be merged into this article or section. ... Mitosis is the process in which a cell duplicates its chromosomes to generate two identical cells. ... Not to be confused with miosis. ... Figure 1: A representation of a condensed eukaryotic chromosome, as seen during cell division. ... Oogenesis or rarely oögenesis is the creation of an ovum (egg cell). ... A human ovum Sperm cells attempting to fertilize an ovum An ovum (plural ova) is a haploid female reproductive cell or gamete. ... Ovarian follicles or Graafian follicles (after Regnier de Graaf) are the roughly spherical cell aggregations in the ovary containing an ovum and from which the egg is released during ovulation. ... For ovary as part of plants see ovary (plants) Ovaries are egg-producing reproductive organs found in female organisms. ... In cell biology, an organelle is one of several structures with specialized functions, suspended in the cytoplasm of a eukaryotic cell. ... The Polar body is a cell structure found inside an ovum. ...

Animal cell cytokinesis

Overview

During normal proliferative divisions, animal cell cytokinesis begins shortly after the onset of sister chromatid separation in the anaphase of mitosis. A contractile ring, comprised of non-muscle myosin II and actin filaments, assembles equatorially 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. Continued hydrolysis causes this cleavage furrow to ingress (move inwards), a stricking 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 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). A chromatid forms one part of a chromosome after it has coalesced for the process of mitosis or meiosis. ... Myosin is a motor protein filament found in muscle tissue. ... G-Actin (PDB code: 1j6z). ... A cell cortex is made of actin microfilaments. ... Adenosine 5-triphosphate (ATP) is a multifunctional nucleotide that is most important as a molecular currency of intracellular energy transfer. ... Hydrolysis is a chemical reaction or process in which a chemical compound reacts with water. ... Abscission (from ab- away from, and scission cutting or severing) is the shedding of a body part. ... Septins are evolutionary conserved proteins with essential functions in cytokinesis, and more subtle roles throughout the cell cycle. ... Microtubules are protein structures found within cells. ... This article or section does not cite its references or sources. ... The cell cycle, or cell-division cycle, is the series of events that take place in an eukaryotic cell between its formation and the moment it replicates itself. ...

Contractile ring positioning

The position at which the contractile ring assembles is dictated in part at least by the mitotic spindle. 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 ^[1]. GTPases are a large family of enzymes that can bind and hydrolyze GTP. The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases. ... RhoA is a small GTPase protein known to regulate the actin cytoskeleton in the formation of stress fibers. ... Myosin is a motor protein filament found in muscle tissue. ... G-Actin (PDB code: 1j6z). ...

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. Human beings are defined variously in biological, spiritual, and cultural terms, or in combinations thereof. ... Binomial name Drosophila melanogaster Meigen, 1830 Drosophila melanogaster Meigen , 1830 (Black-bellied Dew-lover) a dipteran (two-winged) insect, is the species of fruit fly that is commonly used in genetic experiments; it is among the most important model organisms. ... Binomial name Caenorhabditis elegans Wild-type C. elegans hermaphrodite stained to highlight the nuclei of all cells Caenorhabditis elegans () is a free-living nematode (a roundworm), about 1 mm in length, which lives in a temperate soil environment. ...

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 active only at a particular stage of the cell cycle ^{[2] [3]}. A chromatid forms one part of a chromosome after it has coalesced for the process of mitosis or meiosis. ...

Plant cell cytokinesis

Because plant cells have a cell wall, cytokinesis shows significant differences compared with the process in animals 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. A cell wall is a fairly rigid layer surrounding a cell, located external to the cell membrane, that provides the cell with structural support, protection, and a filtering mechanism. ... Diagram of the endomembrane system in a typical eukaryote cell Micrograph of Golgi apparatus, visible as a stack of semicircular black rings near the bottom. ... Cytokinesis in terrestrial plants occurs by cell plate formation. ...

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.^[4]

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

References

1. ^ Glotzer M: "Animal cell cyctokinesis", Annual Review of Cell Biology 17, 351 (2001)
2. ^ J. Mishima et al: "Cell cycle regulation of central spindle assembly", Nature 430, 908-913 (2004)
3. ^ 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)
4. ^ J. Lutkenhaus: "FtsZ ring in bacterial cytokinesis", Molecular Microbiology, August 1993