10.2.5. Cytokinesis

Mitosis accomplishes not only the segregation of duplicated chromosomes into daughter nuclei (karyokinesis), but the cell itself is divided into two daughter cells by the separation of cytoplasm called cytokinesis at the end of which cell division gets completed (Figure 10.2 e).

Cytokinesis is the process that occurs at the end of mitosis. While mitosis deals with the separation of the cell's genetic material into two nuclei, cytokinesis is responsible for dividing the cytoplasm. This is crucial because it ensures that both new daughter cells have the necessary cytoplasm and organelles they need to operate effectively.

You can think of cytokinesis like splitting a pizza after cutting it into slices. Each slice represents a new part of the pizza that needs its own toppings (in this case, organelles). Just as each slice must be complete for someone to enjoy their piece of pizza, each daughter cell must have all its components to function.

In an animal cell, this is achieved by the appearance of a furrow in the plasma membrane. The furrow gradually deepens and ultimately joins in the centre dividing the cell cytoplasm into two.

In animal cells, cytokinesis occurs through the formation of a cleavage furrow. As the cell prepares to divide, the membrane begins to pinch inwards at the center. This pinching continues until the membrane separates completely, forming two individual daughter cells. This mechanism is important in animal cells due to their flexible membranes.

Imagine a drawstring bag: when you pull the strings (like the components inside the cell pulling together), the bag closes in at the middle, separating the contents into two halves. This represents how the animal cell divides, with the cleavage furrow acting like the string tightening around the center.

Plant cells however, are enclosed by a relatively inextensible cell wall, therefore they undergo cytokinesis by a different mechanism. In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls.

Because plant cells have a rigid cell wall, they cannot simply pinch in half like animal cells. Instead, they form a structure called the cell plate in the middle of the cell. Vesicles that contain the materials for the new wall fuse to form this plate, which then expands outward until it fuses with the existing cell walls, completing the division.

Think of building a fence around a yard. You start at a central point (the cell plate) and build outward towards each edge of the yard (the existing walls of the cell). This method ensures that the new fence not only divides the yard but also integrates with the existing boundaries.

At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells.

During cytokinesis, not only is the cytoplasm divided, but important cellular organelles are shared between the two new cells. Mitochondria and plastids are essential for energy production and photosynthesis, respectively, so it is crucial that both daughter cells receive a fair distribution of these components to function properly.

Imagine giving out supplies to two different classrooms. Just like each class needs markers and paper to function well, each new cell needs organelles to perform its essential roles. Ensuring both classrooms receive an adequate amount is like ensuring both daughter cells receive the necessary organelles.

In some organisms karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises leading to the formation of syncytium (e.g., liquid endosperm in coconut).

In certain scenarios, after the nucleus divides (karyokinesis), the cytoplasm does not split, resulting in a cell with multiple nuclei. This condition is seen in some types of fungi and the liquid endosperm of coconut, where it is beneficial for the production of large amounts of nutrients.

Think of a large office building where multiple departments (nuclei) share the same physical space (cytoplasm) without walls separating them. This can enhance communication and resource sharing, just like multinucleate cells can share resources more efficiently than separate cells.

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