Cytokinesis

The eukaryotic somatic cell cycle is defined by a sequential order of tasks a dividing cell has to complete it must replicate its DNA, segregate its chromosomes, grow, and divide. The cell cycle can be divided into four discrete phases. DNA replication is restricted to S phase (DNA synthesis phase), which is preceded by a gap phase called G1 and followed by a gap phase called G2. During mitosis (M phase) the sister chromatids are segregated into two new daughter nuclei and mitosis is completed by the division of the cytoplasm termed cytokinesis (Fig. 1). 

Role of Microtubules in Cytokinesis and in Plant Cell Wall Formation... 

Role of the Cytoskeleton in Cell Division Formation of the Mitotic Spindle, Mitosis, and Cytokinesis Drug Effects on Microtubules Mlcrofllaments Actin Filaments Structure and Composition... 

Microtubules are universally present in eukaryotes from protozoa to the cells of higher animals and plants (Porter, 1966 Hardham and Gunning, 1978 Lloyd, 1987), but they are absent in mammalian erythrocytes and in prokaryotes. Microtubules participate in a number of cellular functions including the maintenance of cell shape and polarity, mitosis, cytokinesis, the positioning of organelles, intracellular transport to specific domains, axoplasmic transport, and cell locomotion. The diversity of microtubule fimctions suggests that not all microtubules are identical and that different classes of microtubules are present in different cell types or are localized in distinct domains in the same cell type (Ginzburg et al., 1989). 

Cytokinesis (cell division) in animal cells involves the progressive formation in telophase of a furrow between the two daughter cells in the equator of the mitotic spindle. Immunofluorescent staining of the cortical cytoplasm at the site of the contraction ring reveals an abundance of actin as well as myosin, a-actinin, and filamin (Fishkind and Wang, 1995). Cytokinesis is highly sensitive to actin-myosin inhibitors such as cytochalasin and phalloidin. 

The cytochalasins are a group of metabolites produced by certain fungi (e.g., Helminthosporium dermatoideum) that inhibit actin-dependent types of cell movement such as leukocyte locomotion, phagocytosis, cytokinesis, the retraction of... 

Cytokinesis begins with astral relaxation of the cell cortex, perhaps triggered by the mitotic spindle, followed by the accumulation in a circumferential equatorial band of actin filaments and associated myosin molecules to form a contractile... 

Cell migration and cytoplasmic movement involve predominantly actin filaments in the locomotion of neutrophilic granulocytes, both actin filaments and microtubules in the elongation of neuronal growth cones and migration of neurites, and both actin and myosin in cytokinesis and the contraction of skeletal and cardiac muscle. 

Fishkind, D.J. Wang, Y-L. (1995). New horizons for cytokinesis. Curr. Opin. Cell Biol. 7, 23-31. 

Two other questions beg answers. First, it is known from studies on Dic-tyostelium and other lower organisms that myosin-II is essential for cytokinesis (Kom and Hammer, 1988). Based on these data, it is assumed that myosin-II is essential for mammalian cells to divide correctly. However, there is no direct evidence from studies on mammalian cells to support such a notion. Given the importance of cell division in normal development and cancer, it is essential to determine whether myosin-II plays a comparable role in mammalian cell division. 

Microtubule-Based Motor Proteins The Meiotic and Mitotic Spindles Microfilament-Based Intracellular Motility Cytokinesis... 

It is most unlikely that the sole functions of mysoin-Il in nonmuscle cells are to provide the contractile force to bisect cells during cytokinesis and for the contractility of stress fibers. Myosin-II is present in a variety of cell types at moderate concentrations in tissues such as brain, which are almost totally non-mitotic and do... 

The processes of meiosis and mitosis involve many motile events, from the separation of the daughter chromosomes to the final act of cell separation at cytokinesis (Wadsworth, 1993). DNA replication itself may be considered as a motile event, because the polymerase complex moves along the linear DNA. One of the most obvious motile events is the separation of the chromosomes along the mitotic spindle at anaphase. Details of the structure and polarity of microtubules in the spindle apparatus in meiosis and mitosis are known through electron and light microscopy, but it is not yet clear whether the chromosomes are pushed, pulled or... 

Cytokinesis is the separation of daughter cells at the completion of the microtubule directed separation of duplicate chromosomes at mitosis or meiosis. This is usually accomplished by a purse string mechanism, whereby daughter cells become separated by the gradual constriction of a structure composed of actin microfilaments and myosins-II (Fujiwara and Pollard, 1976 Satterwhite and Pollard, 1992). 

A second type of cytokinesis has come to light in mutants of Dictyostelium lacking mysoin-II. These cells are able to crudely separate by traction mediated cytokinesis, a process by which the bi- (or multi-) nucleate cells tear themselves apart in an attempt to crawl in two opposite directions. 

Franck, Z., Footer, M., Bretscher, A. (1990). Microinjection of villin in cultured cells induces rapid and long-lasting changes in cell morphology but does not inhibit cytokinesis, cell motility or membrane ruffling. J. Cell Biol. Ill, 2475-2485. 

Fig. 2.3 The development of polarity and asymmetric division in Saccharomyces cerevisiae. The diagram is reproduced in a slightly simplified form from the work of Lew Reed (1995) with the permission of Current Opinion in Genetics and Development, (a) The F-actin cytoskeleton strands = actin cables ( ) cortical actin patches, (b) The polarity of growth is indicated by the direction of the arrows (arrows in many directions signifies isotropic growth), (c) 10-nm filaments which are assembled to form a ring at the neck between mother and bud. (d) Construction of the cap at the pre-bud site. Notice that the proteins of the cap become dispersed at the apical/isotropic switch, first over the whole surface of the bud, then more widely. Finally, secretion becomes refocussed at the neck in time for cytokinesis, (e) The status and distribution of the nucleus and microtubules of the spindle. Notice how the spindle pole body ( ) plays an important part in orientation of the mitotic spindle.

Whether the DNA replication checkpoint directly affects the Plxl activation pathway for Cdc25C has not yet been established. It is possible that the replication checkpoint arrests the cell cycle prior to initiation of the Plxl kinase cascade. Further characterization of upstream components of the cascade should reveal whether it is directly regulated by replication checkpoint activation. Such a characterization will also have importance for other M phase events, inasmuch as Plxl also regulates bipolar spindle formation, APC activation and cytokinesis (Qian et al 1998, 1999). These multiple functions of Plxl are associated with changes in localization of Plxl, and are most likely mediated by protein—protein interaction with the polo box motif in the non-catalytic C-terminal half of Plxl. 

Nurse In that context, in budding yeast there is nuclear migration and there are astral microtubules captured by the bud tip. The bud tip will have also been marking where cytokinesis will have been initiated. There might be some links there. 

Induction of DNA damage to the cleavingXenopus eggdoes not block cytokinesis... 

DNA damage in the context of the embryo does not inhibit cell division as seen by the synchronous and timely occurrence of cytokinesis. In a similar fashion as a cell cycle checkpoint was uncovered in extracts following inhibition of DNA... 

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