Cytokinesis is the physical process that divides the cytoplasm of a single parent cell into two new daughter cells. This action represents the final, non-nuclear stage of cell replication, completing the division process that began with the duplication of genetic material. The primary purpose of this cytoplasmic separation is to ensure that each daughter cell receives a full complement of organelles and cytoplasm necessary to function independently.
The Timing and Context of Cytokinesis
Cytokinesis takes place within the M-phase of the cell cycle, following the nuclear division known as karyokinesis. While nuclear division (mitosis or meiosis) separates the duplicated chromosomes, cytokinesis separates the rest of the cell. The two are distinct processes but are temporally coupled to ensure proper segregation of all cellular components.
The process typically begins during the late stages of anaphase or the beginning of telophase. This timing is precise, as starting too early would interfere with the separation of chromosomes by the spindle apparatus. Cytokinesis continues through telophase, the stage where the two new nuclei reform at opposite poles of the cell.
The Mechanism in Animal Cells
Cytokinesis in animal cells involves pinching the cell membrane inward, a process known as cleavage furrow formation. This furrow first appears as a slight indentation along the cell’s equator, perpendicular to the axis of the mitotic spindle. The inward pull is generated by the contractile ring, a structure that assembles just beneath the plasma membrane.
The contractile ring is a dynamic assembly composed primarily of filamentous actin and the motor protein myosin II. This composition is similar to muscle contraction proteins, allowing for an active shortening of the ring’s circumference. Actin filaments serve as structural tracks, while myosin II motors use energy to slide these filaments past one another.
As the myosin II motors pull on the actin filaments, the ring constricts like a tightening drawstring. This contraction generates a powerful inward force, driving the cleavage furrow deeper into the cytoplasm. Regulatory proteins, such as RhoA, organize the ring’s assembly at the correct equatorial position.
The constant contraction of this actomyosin ring eventually squeezes the cell into two separate compartments. The final step, known as abscission, involves severing the thin cytoplasmic bridge connecting the two nascent daughter cells, completing the physical division.
The Mechanism in Plant Cells
The mechanism of cytokinesis in plant cells is fundamentally different from that in animal cells due to the presence of a rigid cell wall. Instead of pinching inward, plant cells construct a new cell wall and plasma membrane from the center outward to divide the cytoplasm. This construction is directed by the phragmoplast, a unique cytoskeletal structure.
The phragmoplast is an array of microtubules and actin filaments that forms in the central plane between the two newly formed nuclei during late anaphase or telophase. It forms from the remnants of the mitotic spindle and acts as a scaffold, guiding components to the division site.
Vesicles, primarily derived from the Golgi apparatus, are transported along the phragmoplast microtubules to the cell’s equator. These vesicles carry the materials needed to build both the new plasma membrane and the cell wall, fusing with each other to form the cell plate.
The initial fusion creates a tubulovesicular network that rapidly expands outward toward the side walls of the parent cell. As more vesicles fuse, the cell plate matures into a continuous, double-layered membrane structure, which becomes the new plasma membranes for the two daughter cells, separated by the primary cell wall formed by the vesicle contents.
The Biological Importance of Cytokinesis
Accurate cytokinesis is necessary for the successful reproduction of cells and the maintenance of a healthy organism. This process ensures that both daughter cells receive a roughly equal share of organelles, cytoplasm, and a full set of chromosomes. This proper partitioning supports growth, tissue repair, and the replacement of damaged or dead cells.
When cytokinesis fails to execute correctly, it can lead to severe cellular abnormalities. A common outcome is the formation of a single cell containing two nuclei (a binucleated cell) or a single large cell with a duplicated set of chromosomes (a tetraploid cell). Although some healthy tissues contain a percentage of these cells, their frequent occurrence is problematic.
Failed or improper cytokinesis directly contributes to genomic instability, a hallmark of many diseases. Tetraploid cells resulting from division failure are prone to further errors in subsequent divisions, leading to an incorrect number of chromosomes, or aneuploidy. This genomic instability can promote the development of tumor cells and is a factor in cancer progression.