Cytokinesis

Cytokinesis is the division of a cell's cytoplasm. It normally follows mitosis or meiosis, which divide the nucleus, and usually begins in late anaphase or telophase. When it is finished, two daughter cells exist, each wrapped in its own plasma membrane and holding a share of the organelles.

Animal and plant cells do it differently. In an animal cell, a contractile ring of actin filaments, driven by the motor protein myosin, forms just under the membrane around the cell's equator, where the chromosomes lined up at metaphase. As the ring tightens it draws the membrane inward to make a groove, the cleavage furrow, which deepens until the cell is pinched in two. A plant cell cannot pinch because of its rigid wall. Instead, vesicles from the Golgi apparatus carrying wall materials travel along microtubules to the middle of the cell and fuse into a disk called the cell plate. The plate spreads outward until it meets the old wall, leaving a new wall between the two cells.

Nuclear division and cytokinesis are separate events, so one can occur without the other. If cytokinesis is skipped, a cell with several nuclei results, as in the early fruit fly embryo. In egg formation, cytokinesis is deliberately unequal, giving one large egg cell and small polar bodies.