Cytokinesis is the final step in cell division, where a single cell physically separates into two. This process involves the division of the cytoplasm, the jelly-like substance filling the cell, and all its contents. Cytokinesis ensures that each new daughter cell receives a complete set of cellular components necessary for its function. This cytoplasmic division follows the division of the nucleus, which occurs during mitosis or meiosis.
Cytokinesis in Animal Cells
In animal cells that lack a rigid outer boundary, cytokinesis is a process of constriction. The process begins shortly after the genetic material has been separated during anaphase. A specialized structure known as the contractile ring forms just beneath the cell’s surface at its equator. This ring is primarily composed of actin and myosin filaments, the same proteins responsible for muscle contraction.
The contraction of this ring is the driving force behind cell division. As the actin and myosin filaments slide past one another, the ring tightens around the cell’s midsection, similar to pulling a drawstring on a bag. This tightening creates a visible indentation on the cell surface called the cleavage furrow. The furrow gradually deepens as the ring continues to constrict, eventually pinching the parent cell into two separate daughter cells.
Cytokinesis in Plant Cells
The process of cytokinesis in plant cells differs significantly due to the presence of a rigid cell wall that prevents the pinching-in mechanism seen in animal cells. Instead of a contractile ring, plant cells build a new wall from the inside out. This process begins during the late stages of nuclear division, when vesicles produced by an organelle called the Golgi apparatus start to accumulate at the cell’s center. These vesicles are filled with the necessary enzymes, proteins, and glucose to construct a new wall.
These Golgi-derived vesicles are transported along microtubules that form a structure called the phragmoplast. The vesicles align at the former metaphase plate and begin to fuse together. This fusion creates a disc-like structure called the cell plate. The cell plate grows outwards from the center towards the existing cell walls of the parent cell, eventually partitioning the cytoplasm and forming a new plasma membrane and cell wall that separates the two daughter cells.
Coordination with Nuclear Division
The timing and location of cytokinesis are controlled to ensure it occurs only after the cell’s genetic material has been properly duplicated and segregated. This coordination is tightly linked to the final stages of mitosis, specifically anaphase and telophase. The process of cytoplasmic division typically begins as the separated chromosomes arrive at opposite poles of the cell.
Signals originating from the mitotic spindle, the structure that separates the chromosomes, are responsible for determining the precise location of division. In animal cells, these signals dictate where the contractile ring will form, ensuring it is positioned exactly between the two sets of chromosomes. In plant cells, the remnants of the spindle help to guide the formation of the phragmoplast and, consequently, the cell plate.
Consequences of Cytokinesis Failure
Cytokinesis failure results in the formation of a single, larger cell that contains more than one nucleus, as the cytoplasm has not been partitioned despite the completion of nuclear division. Such cells are often referred to as multinucleated or binucleated. The presence of multiple nuclei within one cell can have significant consequences for the organism.
This condition can lead to genomic instability, where the likelihood of mutations and chromosomal abnormalities increases. Such instability is a characteristic feature of many types of cancer cells, and cytokinesis failure is thought to contribute to the development and progression of some tumors. The formation of cells with an abnormal number of chromosomes, a condition called aneuploidy, is a common outcome of failed cytokinesis.