Cytokinesis

Cytokinesis is the process that partitions the cytoplasm of a parent cell into two daughter cells, marking the final phase of cell division after mitosis and karyokinesis. It ensures that each daughter inherits organelles, cytosol, and a complete set of chromosomes prior to cell growth. In animal cells cytokinesis begins with the formation of a contractile actin-myosin ring at the cell cortex, just beneath the plasma membrane. As the ring contracts, a cleavage furrow ingresses from the cell equator, constricting the cell membrane until the cytoplasm is divided. This contractile ring is composed mainly of actin filaments and myosin II and is regulated by small GTPases such as RhoA, along with signaling from the spindle midzone and central spindle.

In plant cells, rigid cell walls prevent cleavage furrow formation. Instead, membrane-bound vesicles derived from the Golgi apparatus coalesce at the center of the cell to form a cell plate. The cell plate expands outward as more vesicles fuse, guided by microtubules of the phragmoplast, ultimately fusing with the existing plasma membranes and forming two separate cell walls separating the daughter cells.

Cytokinesis is coordinated with mitosis to ensure accurate chromosome segregation. In animal cells, abscission may involve the ESCRT-III complex to sever the final membrane bridge between daughter cells. Plant cytokinesis concludes with the maturation of the cell plate into a separating cell wall.

Errors in cytokinesis can produce binucleate or polyploid cells and are associated with developmental abnormalities and diseases, including cancer, though many defects are buffered by cell cycle checkpoints.