The cytokinesis is the process of partitioning the cytoplasm of a cell resulting in two daughter cells during cell division. It occurs in both mitosis and 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 through the process of cytokinesis.
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.
The cytokinesis process begins during anaphase and ends during telophase, taking place completely at the moment when the next interface starts.
The first visible change in cytokinesis in animal cells becomes evident when a division groove appears on the cell surface. This groove quickly becomes more pronounced and expands around the cell until it part completely in the middle.
In animal cells and many eukaryotic cells, the structure that accompanies the process of cytokinesis is known as the “contractile ring”, a dynamic ensemble composed of actin filaments, myosin II filaments, and many structural and regulatory proteins. It settles below the plasma membrane of the cell and contracts to divide it into two parts.
The biggest problem faced by a cell undergoing cytokinesis is ensuring that this process occurs at the right time and place. Since, cytokinesis must not occur early during the mitosis phase or it may interrupt the correct partition 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 location of the contractile ring and therefore the plane of cell division.
The contractile ring has an invariable shape in the plane of the metaphase plate. When at the correct angle, it runs along the axis of the mitotic spindle, ensuring that division occurs between the two separate sets of chromosomes.
The part of the mitotic spindle that specifies the plane of division can vary depending on the type of cell. The relationship between the microtubules of the spindle and the location of the contractile ring has been extensively studied by scientists.
They have manipulated fertilized eggs of marine vertebrate animals in order to observe the speed with which the grooves appear in the cells without interrupting the growth process.
When the cytoplasm is clear, the spindle can be more easily seen, as well as the moment in real time in which it is located in a new position in the early anaphase state.
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 astral microtubules and the proteins that pull them from one side to the other.
Within the cytokinesis process there are many variables that must work synchronously for it to be successful. However, when one of these variables changes, cells can divide asymmetrically, producing two daughter cells of different sizes and with dissimilar cytoplasmic content.
Usually, the two daughter cells are destined to develop differently. For this to be possible, the stem cell must secrete some fate-determining components to one side of the cell and then locate the plane of division so that the indicated daughter cell 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 divide.
Apparently, this movement of the spindle is driven by changes in regional areas of the cell cortex and by localized proteins that help shift one of the spindle poles with the help of the astral microtubules.
As the astral microtubules become longer and less dynamic in their physical response, the contractile ring begins to form under the plasma membrane.
However, much of the preparation for cytokinesis occurs earlier in the mitosis process, even before the cytoplasm begins to divide.
During the interface, the actin and myosin II filaments combine to form a cortical network, and even in some cells, they generate large cytoplasmic bundles called stress fibers.
As a cell initiates the process of mitosis, these arrangements break down and much of the actin is rearranged and the myosin II filaments are released.
As the chromatids separate during anaphase, myosin II begins to accumulate rapidly to create the contractile ring. 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.
When the contractile ring is fully armed, it contains many proteins other than actin and myosin II. The superimposed matrices of bipolar actin and myosin II filaments generate the force necessary to divide the cytoplasm into two parts, in a process similar to that carried out by smooth muscle cells.
However, the way in which the contractile ring contracts is still a mystery. Apparently, 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.
Since, when the ring contracts, it maintains its same rigidity throughout the process. This means that the number of filaments decreases as the ring closes.
Organelle distribution in daughter cells
The mitosis process must ensure that each of the daughter cells receives the same number of chromosomes. However, when a eukaryotic cell divides, each daughter cell must also inherit a number of essential cellular components, including the organelles enclosed within the cell membrane.
Cellular organelles such as mitochondria and chloroplasts cannot be generated spontaneously from their individual components, they can only arise from the growth and division of pre-existing organelles.
Similarly, cells cannot 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 numerous forms within the stem cell, in order to ensure that the two daughter cells inherit them successfully.
The endoplasmic reticulum during the period of cellular interface is continuously together with the cell membrane and is organized by the cytoskeletal microtubule.
After entering the mitosis phase, the reorganization of the microtubules releases the endoplasmic reticulum, which is fragmented as the envelope of the nucleus is also broken. The Golgi apparatus is probably also fragmented, although in some cells it appears that it was distributed through the reticulum and later emerged in the telophase.
Mitosis without cytokinesis
Although cell division is usually followed by division of the cytoplasm, there are some exceptions. Some cells go through various processes of cell division without the cytoplasm being broken.
For example, the fruit fly embryo goes through 13 stages of nuclear division before cytoplasmic division takes place, resulting in a large cell with up to 6,000 nuclei.
This arrangement is mostly aimed at speeding up the early development process, as cells don’t have to take as long to go through all the stages of cell division that cytokinesis involves.
After this rapid nuclear division takes place, cells are created around each nucleus in a single process of cytokinesis, known as celurization. Contractile rings form on the surface of cells, and the plasma membrane extends inward and tightens 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 heart muscle cells. These cells, for example, grow in a multinuclear way, as would those of some fungi or the fruit fly.
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