Cytokinesis
Cytokinesis is the process by which a cell divides its cytoplasm to produce two daughter cells. As the final step in cell division after mitosis , cytokinesis is a carefully orchestrated process that signals the start of a new cellular generation. The separation of one cell into two is accomplished by a structure called the contractile ring. The contractile ring is a structure believed to operate in a way similar to muscle. A molecular motor, myosin, contracts the actin filaments that form the contractile ring tighter and tighter until the cell is pinched in two. The contraction of the contractile ring has been likened to tightening a purse string to close the top of a pouch. The furrow created by this pinching process is also called the "cleavage furrow," as it is the site at which cleavage of one cell into two cells occurs.Cytokinesis consists of four major steps. The first step is to define the position at which the contractile ring will form. The spindle, the structure responsible for segregating the chromosomes into what will become the daughter cells, also appears to be responsible for defining where the contractile ring forms. The contractile ring forms perpendicular to the long axis of the spindle at its midpoint. Components of the spindle that come in contact with the plasma membrane, called astral microtubules, are believed to transmit a signal to the cell periphery that tells actin and other components of the contractile ring to assemble at that location. Actin and microtubules are both part of the cytoskeleton .
The second step in cytokinesis is to assemble the actin filaments that form the contractile ring. Additional proteins , including the molecular motor myosin, which powers contraction, also assemble in this same domain. The third step is the actual contraction of the contractile ring. In this step, the myosin motor, powered by adenosine triphosphate, moves the actin filaments past each other, much in the same way as myosin interacts with actin to power the contraction of muscle. This step also requires the removal of actin subunits to allow the ring to decrease in size. The final step, breaking and refusion of the plasma membrane, occurs once the ring has contracted to its minimum size. This breaking and fusion finally separates the two daughter cells from each other.
As with each of the steps in mitosis, cytokinesis is highly regulated. If the cell were to divide its cytoplasm prior to the completion of duplication and segregation of the chromosomes, it is unlikely that each of the progeny cells would receive the proper genetic information. Thus the cell employs several regulatory mechanisms to assure that cytokinesis occurs only after all of the chromosomes have been properly segregated. There is, for example, a "spindle checkpoint" that assures that each and every chromosome has attached to the spindle. The entire process of cell division waits at the checkpoint until the conditions of the checkpoint have been satisfied. Once they have been, the process continues and concludes with cytokinesis.
Cytokinesis in Animal Cells
The first step in cytokinesis in animal cells is the contractile ring positioning. The difference between cytokinesis in plants and animals arises from this stage itself. Animal cell cytokinesis begins just after the onset of sister chromatid separation in the anaphase of mitosis. A contractile ring that is made of non-muscle myosin II and actin filaments, assembles equatorially, that is, in the middle of the cell, at the cell cortex. The cell cortex is adjacent to the cell membrane. Myosin II uses the free energy released when ATP is hydrolysed to move along these actin filaments, thus, constricting the cell membrane to form a cleavage furrow towards the middle. The continuing hydrolysis causes this cleavage furrow to ingress, that is, move inwards. This process is visible with the help of a light microscope. Ingression continues until a midbody structure that is composed of electron-dense, proteinaceous material is formed and the process of abscission then physically cleaves this midbody into two.
The abscission depends on septin filaments beneath the cleavage furrow, which provides 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 the interphase. The position at which the contractile ring assembles is dictated by the mitotic spindle. Simultaneous with contractile ring assembly during prophase, a microtubule based structure called the central spindle forms when non-kinetochore microtubule fibers are bundled between the spindle poles. Cytokinesis must be a controlled procedure so as to ensure that it occurs only after sister anaphase separation during normal proliferative cell divisions. To achieve this purpose, many components of the cytokinesis machinery are highly regulated so as to ensure that they are able to perform a specific function at only a specific stage of the cell cycle.
Cytokinesis in Plant Cells
As mentioned earlier, cytokinesis in plants and animals is basically different from each other due to the presence of cell wall, which is one of the plant cells parts. Rather than forming a contractile ring, plant cells form a cell plate in the middle of the cell. The Golgi apparatus releases vesicles that contain cell wall materials. These vesicles fuse at the equatorial plane to form the cell plate. The cell plate initially begins as a fusion tube network, which then becomes a tubulo-vesicular network as more and more components are added.
There is only one exception to the process of cytokinesis, which is in the case of oogenesis, which is the process where an ovum is created in the ovarian follicle of the ovary. Here, the ovum takes up almost all the cytoplasm and organelles, thus, leaving very little for the resulting polar bodies which eventually die. Thus, there is no division of the cytoplasm per se. However, in all other cells, irrespective of whether they are plant cells or animal cells, cytokinesis is the method by which the cell divides into two.
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