<P> Animal cell cytokinesis starts with the stabilization of microtubules and reorganization of the mitotic spindle to form the central spindle . 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). The process of mitotic spindle reorganization and central spindle formation is caused by the decline of CDK1 activity during anaphase . The decline of CDK1 activity at the metaphase - anaphase transition leads to dephosphorylating of inhibitory sites on multiple central spindle components . First of all, the removal of a CDK1 phosphorylation from a subunit of the CPC (the chromosomal passenger complex) allows its translocalization to the central spindle from the centromeres, where it is located during metaphase . Besides being a structural component of the central spindle itself, CPC also plays a role in the phosphoregulation of other central spindle components, including PRC1 (microtubule bundling protein required for cytokinesis 1) and MKLP1 (a kinesin motor protein). Originally inhibited by CDK1 - mediated phosphorylation, PRC1 is now able to form a homodimer that selectively binds to the interface between antiparallel microtubules, facilitating spatial organization of the microtubules of the central spindle . MKLP1, together with the Rho - family GTPase activating protein CYK - 4 (also termed MgcRacGAP), forms the centralspindlin complex . Centralspindlin binds to the central spindle as higher - order clusters . The centralspindlin cluster formation is promoted by phosphorylation of MLKP1 by Aurora B, a component of CPC . In short, the self - assembly of central spindle is initiated through the phosphoregulation of multiple central spindle components by the decline of CDK1 activity, either directly or indirectly, at the metaphase--anaphase transition . The central spindle may have multiple functions in cytokinesis including the control of cleavage furrow positioning, the delivery of membrane vesicles to the cleavage furrow, and the formation of the midbody structure that is required for the final steps of division . </P> <P> The second step of animal cell cytokinesis involves division plane specification and cytokinetic furrow formation . Precise positioning of the division plane between the two masses of segregated chromosomes is essential to prevent chromosome loss . Meanwhile, the mechanism by which the spindle determines the division plane in animal cells is perhaps the most enduring mystery in cytokinesis and a matter of intense debate . There exist three hypotheses of furrow induction . The first is the astral stimulation hypothesis, which postulates that astral microtubules from the spindle poles carry a furrow - inducing signal to the cell cortex, where signals from two poles are somehow focused into a ring at the spindle . A second possibility, called the central spindle hypothesis, is that the cleavage furrow is induced by a positive stimulus that originates in the central spindle equator . The central spindle may contribute to the specification of the division plane by promoting concentration and activation of the small GTPase RhoA at the equatorial cortex . A third hypothesis is the astral relaxation hypothesis . It proposes that postulates that active actin--myosin bundles are distributed throughout the cell cortex, and inhibition of their contraction near the spindle poles results in a gradient of contractile activity that is highest at the midpoint between poles . In other words, astral microtubules generate a negative signal that increases cortical relaxation close to the poles . Genetic and laser - micromanipulation studies in C. elegans embryos haven shown that the spindle sends two redundant signals to the cell cortex, one originating from the central spindle, and a second signal deriving from the spindle aster, suggesting the involvement of multiple mechanisms combined in the positioning of the cleavage furrow . The predominance of one particular signal varies between cell types and organisms . And the multitude and partial redundancy of signals may be required to make the system robust and to increase spatial precision . </P> <P> At the cytokinesis furrow, it is the actin - myosin contractile ring that drives the cleavage process, during which cell membrane and wall grow inward, which eventually pinches the mother cell in two . The key components of this ring are the filamentous protein actin and the motor protein myosin II . The contractile ring assembles equatorially (in the middle of the cell) at the cell cortex (adjacent to the cell membrane). Rho protein family (RhoA protein in mammalian cells) is a key regulator of contractile ring formation and contraction in animal cells . The RhoA pathway promotes assembly of the actin - myosin ring by two main effectors . First, RhoA stimulates nucleation of unbranched actin filaments by activation of Diaphanous - related formins . This local generation of new actin filaments is important for the contractile ring formation . This actin filament formation process also requires a protein called profilin, which binds to actin monomers and helps load them onto the filament end . Second, RhoA promotes myosin II activation by the kinase ROCK, which activates myosin II directly by phosphorylation of the myosin light chain and also inhibits myosin phosphatase by phosphorylation of the phosphatase - targeting subunit MYPT . Besides actin and myosin II, the contractile ring contains the scaffolding protein anillin . Anillin binds to actin, myosin, RhoA and CYK - 4, and thereby links the equatorial cortex with the signals from the central spindle . It also contributes to the linkage of the actin - myosin ring to the plasma membrane . Another protein, septin, has also been speculated to serve as a structural scaffold on which the cytokinesis apparatus is organized . Following its assembly, contraction of the actin - myosin ring leads to ingression of the attached plasma membrane, which partitions the cytoplasm into two domains of emerging sister cells . The force for the contractile processes is generated by movements along actin by the motor protein myosin II . 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 striking process that is clearly visible through a light microscope . </P> <P> The cytokinetic furrow ingresses until a so - called midbody structure (composed of electron - dense, proteinaceous material) is formed, where the actin - myosin ring has reached a diameter of about 1--2 μm . Most animal cell types remain connected by an intercellular bridge for up to several hours until they are split by an actin - independent process termed abscission, the last step of cytokinesis . The process of abscission physically cleaves the aforementioned midbody into two . Abscission proceeds by removal of cytoskeletal structures from the intercellular bridge, constriction of the cell cortex, and plasma membrane fission . The intercellular bridge is filled with dense bundles of antiparallel microtubules that derive from the central spindle . These microtubules overlap at the midbody, which is generally thought as a targeting platform for the abscission machinery . Microtubule severing protein spastin is largely responsible for the disassembly of microtubule bundles inside the intercellular bridge . Complete cortical constriction also requires removal of the underlying cytoskeletal structures . Actin filament disassembly during late cytokinesis depends on the PKCε--14 - 3 - 3 complex, which inactivates RhoA after furrow ingression . Actin disassembly is further controlled by the GTPase Rab35 and its effector, the phosphatidylinositol - 4, 5 - bisphosphate 5 - phosphatase OCRL . Understanding the mechanism by which the plasma membrane ultimately splits requires further investigation . </P>

When does a cell membrane pinch together to divide