Cytokinesis is the physical process where the cytoplasm of a single eukaryotic cell divides to form two distinct daughter cells. This event is a part of cell division, enabling growth, tissue repair, and reproduction. It ensures that after the cell’s genetic material has been separated, the surrounding cellular machinery is also partitioned, providing each new cell with the components to function. The process marks the final step in the creation of new cells.
The Role of Cytokinetics in the Cell Cycle
The cell cycle is an ordered series of events culminating in cell growth and division into two daughter cells. This cycle has two main parts: interphase, where the cell grows and replicates its DNA, and the M phase, where the cell divides. The M phase consists of two overlapping processes: mitosis, the division of the nucleus, and cytokinesis, the division of the cytoplasm. Cytokinesis is the concluding event of the cell cycle, ensuring a full complement of cytoplasm surrounds each new nucleus.
While often discussed with mitosis, cytokinesis is a distinct process. Its initiation begins during the later stages of nuclear division, anaphase or telophase, when the duplicated chromosomes have arrived at opposite poles of the cell. The precise timing ensures that division of the cytoplasm only occurs after the genetic material has been segregated. This coordination guarantees the creation of two genetically identical and viable daughter cells.
The Mechanism in Animal Cells
In animal cells and other eukaryotes that lack a rigid cell wall, cytokinesis proceeds through the formation of a cleavage furrow. This process begins with a shallow groove on the cell surface at the cell’s equator, precisely between the two newly separated nuclei. The furrow gradually deepens, eventually pinching the cell in two. This mechanism is an “outside-in” process of division.
The driving force behind the cleavage furrow is the contractile ring, a band of protein filaments that assembles just beneath the plasma membrane. This ring is composed of actin filaments and the motor protein myosin II, the same proteins responsible for muscle contraction. The RhoA signaling pathway regulates the ring’s assembly and function, ensuring it forms at the correct time and location.
The contraction of this ring functions much like tightening a drawstring on a purse. Myosin II molecules use energy to pull on the actin filaments, constricting the ring’s diameter. This constriction pulls the plasma membrane inward, deepening the cleavage furrow. The process continues until the connection between the two forming cells is a narrow intercellular bridge, which is severed in a final step called abscission, resulting in two separate cells.
The Mechanism in Plant Cells
Cytokinesis in plant cells differs due to the rigid cell wall, which prevents the cell from pinching inward. Instead of a cleavage furrow, plant cells build a new partition from the inside out. This division begins with the formation of a structure called the cell plate in the center of the cell, at the equatorial plane.
The cell plate originates from vesicles derived from the Golgi apparatus. These membrane-bound sacs are transported along microtubules to the cell’s equator, where they align and fuse. This fusion creates an initial, disc-like membrane structure that expands outward toward the edges of the parent cell.
As more vesicles arrive and fuse, the cell plate grows, and its contents form the middle lamella, a pectin-rich layer that cements the new cell walls together. The membranes of the fused vesicles become the new plasma membranes for each daughter cell. The structure continues to expand until it fuses with the existing cell wall of the parent cell, completing the physical separation into two daughter cells.
Consequences of Cytokinetic Failure
When cytokinesis fails to occur after mitosis is complete, the result is a single cell containing two or more nuclei, known as a multinucleated cell. This occurs because the genetic material has divided, but the cytoplasm has not. These cells are often larger than their normal counterparts and represent an error in the cell division process.
The formation of multinucleated cells can lead to genetic instability. If a multinucleated cell attempts to divide again, the presence of multiple spindles can lead to an incorrect segregation of chromosomes. This can result in aneuploidy, a condition where the daughter cells receive an abnormal number of chromosomes.
This error in cell division is implicated in the development of various diseases. For instance, the genomic instability caused by failed cytokinesis and subsequent aneuploidy is a feature of many types of cancer cells. Uncontrolled cell proliferation, combined with genetic errors, can contribute to tumor progression.