The Cytokinesis Is the process of partitioning the cytoplasm of a cell Which gives rise to daughter cells during the process of cell division.
It occurs both in mitosis and in meiosis and is common in animal cells. In the case of some plants and fungi, cytokinesis does not take place, as these organisms never divide their cytoplasm. The cycle of cell reproduction culminates with the partition of the cytoplasm by means of the cytokinesis process.
In a typical animal cell, cytokinesis occurs during the mitosis process, however, there may be some types of cells such as osteoclasts that can pass through the mitosis process without cytokinesis (Biology-Online.org, 2017 ).
The cytokinesis process begins during the anaphase and concludes during the telophase, taking place entirely at the moment the next interface starts.
The first visible change of cytokinesis in animal cells becomes evident when a cleavage furrow appears on the cell surface. This groove quickly becomes more pronounced and expands around the cell until it completely through the middle.
In animal cells and many eukaryotic cells, the structure that accompanies the cytokinesis process is known as the"contractile ring,"a Dynamic array composed of actin filaments, myosin II filaments and many structural and regulatory proteins. It is installed underneath the plasma membrane of the cell and contracted to divide it into two parts.
The biggest problem facing a cell that goes through the process of cytokinesis is the assurance that this process occurs at the right time and place. Since, cytokinesis should not occur early during the mitosis phase or it may interrupt the proper partitioning of the chromosomes.
Mitotic spindles and cell division
Mitotic spindles in animal cells are not only responsible for separating the resulting chromosomes, they also specify the contractile ring location and hence the plane of cell division.
The contractile ring has an invariable shape in the plane of the metaphase plate. When it is at the right angle, it extends along the axis of the mitotic spindle, ensuring that the division is given between the two sets of separate chromosomes.
The part of the mitotic spindle that specifies the plane of the division may vary depending on the type of cell. The relationship between the micro tubules of the spindle and the location of the contractile ring has been widely studied by scientists.
They have manipulated fertile eggs from marine vertebrates to observe the speed with which the furrows appear in the cells without the growth process being interrupted (Guertin, Trautmann, & McCollum, 2002).
When the cytoplasm is clear, the spindle can be more easily seen, as well as the real-time momentum in which it is located in a new position in the early anaphase state.
Asymmetric division
In most cells, cytokinesis occurs symmetrically. In most animals, for example, the contractile ring is formed around the equator line of the stem cell , So that the two resulting daughter cells have the same size and similar properties.
This symmetry is possible thanks to the location of the mitotic spindle, which tends to focus on the cytoplasm with the help of the micro astral tubules and the proteins that pull them from side to side.
Within the process of cytokinesis there are many variables that must work synchronously to make it successful. However, when one of these variables changes, the cells can be divided asymmetrically, producing two daughter cells of different size and with a dissimilar cytoplasmic content (Education, 2014).
Usually, the two daughter cells are destined to develop differently. For this to be possible, the stem cell must segregate some determinant components of the fate to one side of the cell and then locate the division plane so that the daughter cell indicated inherits these components at the time of division.
To position the division asymmetrically, the mitotic spindle must be moved in a controlled manner within the cell that is about to be divided.
Apparently this movement of the spindle is directed by changes in regional zones of the cellular cortex and by localized proteins that help to displace one of the poles of the spindle with the help of the micro astral tubules.
Contractile ring
To the extent that astral microtubules become longer and less dynamic in their physical response, the contractile ring begins to be created beneath the plasmatic membrane .
However, much of the preparation for cytokinesis occurs earlier in the mitosis process, even before the cytoplasm begins to divide.
During the interphase, the actin and myosin II filaments combine and form a cortical network, and even in some cells, generate large cytoplasmic bundles called stress fibers.
To the extent that a cell initiates the mitosis process, these arrays are disarranged and much of the actin is rearranged and the myosin II filaments are released.
As the chromatids separate during the anaphase, myosin II begins to accumulate rapidly to create the contractile ring. Even in some cells, it is necessary to use proteins from the kinase family to regulate the composition of both the mitotic spindle and the contractile ring.
When the contractile ring is fully armed, it contains many proteins different from actin and myosin II. The superimposed matrices of actin filaments and bipolar myosin II generate the necessary force to divide the cytoplasm into two parts, in a process similar to that of smooth muscle cells (Rappaport, 1996).
However, the way in which the contractile ring contracts is still a mystery. Apparently, it does not operate on account of a cord mechanism with filaments of actin and myosin II moving one above the other, as would the skeletal muscles.
Since, when the ring contracts, it retains its same rigidity throughout the process. This means that the number of filaments decreases in the range in which the ring closes (Alberts, et al., 2002).
Distribution of organelles in daughter cells
The mitosis process must ensure that each of the daughter cells receives the same amount of chromosomes. However, when a eukaryotic cell divides, each daughter cell must also inherit a series of essential cellular components, including organelles locked within the cell membrane.
Cell organelles such as mitochondria and Chloroplasts Can not be generated spontaneously from their individual components, can only arise from the growth and division of preexisting organelles.
Similarly, cells can not make a new endoplasmic reticulum, unless part of it is present within the cell membrane.
Some organelles such as mitochondria and chloroplasts are present in a large number within the stem cell, in order to ensure that the two daughter cells successfully inherit them.
The endoplasmic reticulum during the cell-interface period is found continuously along with the cell membrane and is organized by the micro-cytoskeletal tubule (Brill, Hime, Scharer-Schuksz, & Fuller, 2000).
After entering the phase of mitosis, the reorganization of the micro tubules releases the endoplasmic reticulum, which fragments to the extent that the core wrap also breaks. The Golgi apparatus is probably also fragmented, although in some cells it appears to be distributed across the reticulum to subsequently emerge in the telophase.
Mitosis without cytokinesis
Although cell division is usually followed by the division of the cytoplasm, there are some exceptions. Some cells go through several processes of cell division without the cytoplasm being broken.
For example, the fruit fly embryo undergoes 13 stages of nuclear division before cytoplasmic division takes place, resulting in a large cell with up to 6000 nuclei.
This arrangement mostly aims to accelerate the early development process because cells do not have to take that long to go through all stages of cell division that involves cytokinesis.
After this rapid nuclear division takes place, the cells are created around each nucleus in a single process of cytokinesis, known as celurization. The contractile rings are formed on the surface of the cells, and the plasma membrane extends inwardly and is adjusted to enclose each nucleus
The process of mitosis without cytokinesis also occurs in some types of mammalian cells, such as osteoclasts, trophoblasts, and some hepatocytes and cardiac muscle cells. These cells, for example, grow multinuclearly, as would some fungi or fruit fly (Zimmerman, 2012).
References
- Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular Biology of the Cell. 4th edition. New York: Garland Science.
- Biology-Online.org. (March 12, 2017). Biology Online . Retrieved from Cytokinesis: biology-online.org.
- Brill, J.A., Hime, G.R., Scharer-Schuksz, M., & Fuller, &. (2000).
- Education, N. (2014). Nature Education . Retrieved from"cytokinesis: nature.com".
- Guertin, D.A., Trautmann, S., & McCollum, D. (June 2002). Retrieved from Cytokinesis in Eukaryotes: ncbi.nlm.nih.gov.
- Rappaport, R. (1996). Cytokinesis in Animal Cells. New York: Cambridge University Press.
- Zimmerman, A. (2012). Mitosis / Cytokinesis. Academic Press.