What is cytokinesis?

Cytokinesis in Animal Cell 

1. In a typical animal cell, cytokinesis occurs during the mitosis process, however, there may be some types of cells such as osteoclasts that can go through the mitosis process without cytokinesis taking place.
2. Cytokinesis in the animal cell occurs by centripetal cleavage.
3. Animal Cell cytokinesis is also called cleavage cytokinesis.
4. The cytokinesis process begins during anaphase and concludes during telophase, which takes place entirely at the time the next interface begins. 
5. The first visible change of cytokinesis in animal cells becomes apparent when a dividing groove appears on the cell surface. 
6. This groove quickly becomes more pronounced and expands around the cell until it breaks completely through the middle.
7. In animal cells and many eukaryotic cells, the structure that accompanies the cytokinesis process is known as the “contractile ring,” a dynamic set consisting of actin filaments, myosin II filaments, and many structural and regulatory proteins. 
8. It is installed under the plasma membrane of the cell and contracts to divide it into two parts.
9. The constriction or furrow deepens centripetal and the mid-body provides the raw material for the formation of a new membrane. The furrow ultimately divides the mother cell into two daughter cells. 

Cytokinesis in plant cells

1. Cytokinesis in plant cells occurs by two methods, cleavage cytokinesis, and cell plate cytokinesis.
2. Cleavage cytokinesis is found in some lover plants. In that, Plasmalemma invaginates in the equatorial plane of the spindle. Invagination or cleavage deepens centripetally till it divides the cell protoplast into two daughter protoplasts.
3. There is a centripetal deposition of wall material (pectins, hemicellulose, and cellulose) in the furrow. lt produces a double wall.
4. Cell Plate Cytokinesis: It is the common method of plant cell cytokinesis. The equatorial part of the spindle enlarges and gets digitated with microtubules to form a complex called phragmoplast.
5. Is carried out after the formation of a septum known as a phragmoplast, which arises from the accumulation of vesicles that come from the Golgi Apparatus and contain material from the cell wall.
6. Once the phragmoplast contacts the cell walls, the septum will form in the middle part of the cell, making cell division possible.
7. The biggest problem a cell undergoing the cytokinesis process must face is ensuring that this process occurs at the right time and place. 
8. Since cytokinesis must not occur early during the mitosis phase, or it may interrupt the correct partitioning of the chromosomes.

Basic Stages of Cytokinesis: 

1. Mitotic spindles and cell division

1. Mitotic spindles in animal cells are not only responsible for separating the chromosomes, they also specify the location of the contractile ring and therefore the plane of cell division. 
2. The contractile ring has an invariable shape in the plane of the metaphase plate. When it is at the correct angle, it extends along the axis of the mitotic spindle, ensuring that division occurs between the two sets of separate chromosomes.
3. The part of the mitotic spindle that specifies the plane of division can vary depending on the cell type. 
4. The relationship between the micro spindle tubules and the location of the contractile ring has been extensively studied by scientists. 
5. They have manipulated fertilized eggs of marine vertebrate animals to observe the speed with which the grooves appear in the cells without interrupting the growth process.
6. When the cytoplasm is clear, the spindle can be more easily seen, as well as the moment in real-time when it is located in a new position in the early anaphase state.

Read: Somatic Cell

2. Asymmetric division

1. In most cells, cytokinesis occurs symmetrically. In most animals, for example, the contractile ring forms around the equator line of the stem cell, so that the resulting two daughter cells are the same size and have similar properties. 
2. This symmetry is possible thanks to the location of the mitotic spindle, which tends to focus on the cytoplasm with the help of the astral microtubules and the proteins that pull them from one side to the other.
3. Within the cytokinesis process, many variables must work synchronously for it to be successful. 
4. However, when one of these variables changes, the cells can divide asymmetrically, producing two daughter cells of different sizes and with dissimilar cytoplasmic content.
5. Usually, the two daughter cells are destined to develop differently. For this to be possible, the stem cell must segregate some destination-determining components to one side of the cell and then locate the division plane so that the indicated daughter cell inherits these components at the time of division. 
6. To position division asymmetrically, the mitotic spindle must be moved in a controlled manner within the cell that is about to divide.
7. This spindle movement is driven by changes in regional areas of the cell cortex and by localized proteins that help displace one of the spindle poles with the help of astral microtubules.

3. Contractile Ring Formation: 

1. As the astral microtubules become longer and less dynamic in their physical response, the contractile ring begins to create below the plasma membrane. 
2. However, much of the preparation for cytokinesis occurs earlier in the mitosis process, even before the cytoplasm begins to divide.
3. During the interface, the actin and myosin II filaments combine and form a cortical network, and even in some cells, generate large cytoplasmic bundles called stress fibers. 
4. As a cell begins the mitosis process, these arrangements break down and much of the actin is rearranged and the myosin II filaments are released.
5. As the chromatids separate during anaphase, myosin II begins to accumulate rapidly to create the contractile ring. 
6. In some cells, it is even necessary to use proteins from the kinase family to regulate the composition of both the mitotic spindle and the contractile ring.
7. When the contractile ring is fully armed, it contains many different proteins than actin and myosin II. 
8. The superimposed matrices of the bipolar actin and myosin II filaments generate the necessary force to divide the cytoplasm into two parts, in a process similar to that performed by smooth muscle cells.
9. However, how the contractile ring contracts is still a mystery. It does not operate on behalf of a cord mechanism with actin and myosin II filaments moving on top of each other, as skeletal muscles would. 
10. Since, when the ring contracts, it maintains its same rigidity throughout the process. This means that the number of filaments decreases as the ring is closed.

4. Distribution of organelles in daughter cells : 

1. The mitosis process must ensure that each of the daughter cells receives the same number of chromosomes. 
2. However, when a eukaryotic cell divides, each daughter cell must also inherit several essential cellular components, including organelles enclosed within the cell membrane. 
3. Cellular organelles such as mitochondria and chloroplasts cannot be spontaneously generated from their components, they can only arise from the growth and division of pre-existing organelles. 
4. Similarly, cells cannot make a new endoplasmic reticulum unless part of it is present within the cell membrane.
5. Some organelles such as mitochondria and chloroplasts are present in large numbers within the stem cell, to ensure that the two daughter cells successfully inherit them. 
6. The endoplasmic reticulum during the period of the cellular interface is found continuously along with the cell membrane and is organized by the cytoskeletal microtubule. 
7. After entering the mitosis phase, the reorganization of the microtubules releases the endoplasmic reticulum, which fragments as the nucleus envelope also breaks. 
8. The Golgi apparatus probably also fragments, although in some cells it appears to have spread throughout the reticulum and then emerge at telophase.

Mitosis without cytokinesis

1. Although cell division is usually followed by division of the cytoplasm, there are some exceptions. 
2. Some cells go through various processes of cell division without the cytoplasm being broken.
3. For example, the fruit fly embryo goes through 13 stages of nuclear division before cytoplasmic division occurs, resulting in a large cell with up to 6,000 nuclei.
4. This arrangement is mostly aimed at speeding up the early development process since cells don’t have to take that long to go through all the stages of cell division that cytokinesis involves.
5. After this rapid nuclear division takes place, cells are created around each nucleus in a single cytokinesis process, known as cellularization. 
6. Contractile rings form on the surface of cells, and the plasma membrane extends inward and adjusts to enclose each nucleus
7. The process of mitosis without cytokinesis also occurs in some types of mammalian cells, such as osteoclasts, trophoblasts, and some hepatocytes and heart muscle cells. 
8. These cells, for example, grow in a multinuclear way, as would those of some fungi or the fruit fly.

Frequently Asked Questions on Cytokinesis