cytokinesis(redirected from cytocleisis)
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, process of nuclear division in a living cell by which the carriers of hereditary information, or the chromosomes, are exactly replicated and the two copies distributed to identical daughter nuclei.
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The physical partitioning of a plant or animal cell into two daughter cells during cell reproduction. There are two modes of cytokinesis: by a constriction (the cleavage furrow in animal cells and some plant cells) or from within by an expanding cell plate (the phragmoplast of many plant cells). In either mode, cytokinesis requires only a few minutes, beginning at variable times after the segregation of chromosomes during mitosis (nuclear division). In the vast majority of cases the resulting daughter cells are completely separated. Since they are necessarily smaller cells as a result of cytokinesis, most cells grow in volume between divisions.
Occasionally, cytokinesis is only partial, permitting nutrients and metabolites to be shared between cells. Should cytokinesis fail to occur at all, mitosis may cause more than one nucleus to accumulate. Such a cell is a syncytium. Some tissues normally contain syncytia, for example, binucleate cells in the liver and multinucleate plant endosperm. Some whole organisms such as slime molds are syncytial.
Cytokinesis is precisely and indispensably linked to mitosis, yet the timing and actual mechanisms are distinct. The plane of cell partitioning is perpendicular to the axis of mitosis and coincides with the plane previously occupied by the chromosomes at metaphase. Despite the reliability of this correlation, the chromosomes themselves are not essential for cytokinesis. Experiments performed upon living cells have shown that it is the cell's machinery for chromosome separation, the mitotic apparatus, that provides the essential positional signal to other parts of the cytoplasm which initiates cytokinesis. Subsequently, the mitotic apparatus is no longer involved in cytokinesis and can be destroyed or even sucked out without affecting cytokinesis. See Chromosome, Mitosis
A cleavage furrow develops by circumferential contraction of the peripheral cytoplasm, usually at the cell's equator. The mechanism of furrowing is very similar among a wide diversity of cell types in lower and higher animals and certain plants. The physical forces of contraction exhibited by a cleavage furrow are evidently greater than the forces of resistance elsewhere. Electron microscopic analysis of the peripheral cytoplasm beneath the cleavage furrow consistently reveals a specialization called the contractile ring. This transient cell organelle is composed of numerous long, thin protein fibers oriented circumferentially within the plane of furrowing. These microfilaments are about 5 nanometers in thickness, appear to attach to the cell membrane, and are known to be composed of actin intermixed with myosin. Both of these proteins are intimately involved in force generation in muscle cells. Thus, the present theory of cytokinesis by furrowing implicates the contractile ring as a transient, localized intracellular “muscle” that squeezes the cell in two. See Muscle proteins
In plant cells the dominant mode of cytokinesis involves a phragmoplast, a structure composed of fibrous and vesicular elements that resemble parts of the mitotic apparatus. Microtubules (the fibers) appear to convey a stream of small membranous vesicles toward the midline where they fuse into a pair of partitioning cell membranes. Cellulose cell walls are subsequently secreted between these membranes to solidify the separation between daughter cells. This mode of cytokinesis is well suited to plant cells whose stiff cell walls cannot participate in furrowing. Surprisingly, however, there are instances among the algae where cleavage furrows are the normal mode of cytokinesis. Occasionally, both cleavage furrows and phragmoplasts are employed in the same cell. See Cell walls (plant), Plant cell
the division of a plant or animal cell; usually the final process in mitosis (seeMITOSIS).
The plane along which division occurs always crosses the mitotic spindle midway between the poles. Plant cells, which have a rigid cell wall, divide by way of a process called cell-plate formation, in which the plate, fusing with the side walls of the mother cell, splits the cell into two daughter cells. In animal cells, cytokinesis is accomplished by the formation of a division furrow, which develops around the outside of the cell and slowly divides the cytoplasm into two parts by ingression. The formation of the furrow is associated primarily with changes in the surface, or cortical, layer of the cell. Division of the cell body probably proceeds with the participation of the mitotic apparatus, which determines the plane in which the furrow appears, and the chromosomes (in the absence of chromosomes, cytokinesis slows down and incomplete furrows form). Both of these structures are believed to affect cytokinesis indirectly and only during the early stages of division. It is possible the chromosomes secrete chemical substances that affect the properties of the cortical layer.
The absence of cytokinesis during the final stage of mitosis (the telophase) occurs relatively frequently, resulting in the formation of binuclear cells.
M. E. ASPIZ