Cytokinesis
From Wikipedia, the free encyclopedia
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. Cytokinesis must happen in both processes. 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.
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[edit] Animal cell cytokinesis
During normal proliferative division, 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 abcission then cleaves this midbody to physically pinch one cell into two. Microtubules (non-kinetochore) then reorganize and disappear into a new cytoskeleton as the cell cycle returns to interphase (see also cell cycle).
Simultaneous with contractile ring assembly, 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 might play a role in bundling microtubules to form the spindle midzone during anaphase.
[edit] 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 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.
During cytokinesis in an animal cell, a contractile ring forms in the center of the dividing cell. The contractile ring contracts, pulling the cell membrane inward. Eventually, the cell is pinched into two daughter cells. In a plant cell, the cell wall prevents the cell membrane from being pulled inward. A structure called a cell plate grows between the two new nuclei. The cell plate develops into a membrane and eventually becomes part of the cell wall of each of the new cells.
The two daughter cells are now completely separated. Each is surrounded by a cell membrane. Each daughter cell has some of its parent cell's cytoplasm. Though daughter cells are genetically identical to their parent cell, they are smaller. After division, cells may enter a period of growth, during which they take in the resources they need to increase the amount of their cytoplasm and to grow to full size. When the cells are fully grown, they are about the same size as the parent cell was before division.
[edit] 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
