Cytokinesis

Cytokinesis is the final stage of cell division, when the cytoplasm is divided to produce two separate daughter cells. It usually follows mitosis, which divides the nucleus, and its mechanism differs between animals, plants and bacteria.

After chromosomes separate, a ring of actin filaments and the motor myosin assembles under the membrane at the cell's equator, at a position set by the spindle. As the ring contracts, it pulls the membrane inward, forming a cleavage furrow, like pulling a drawstring. A narrow bridge, the midbody, remains for a time, and a final step called abscission cuts it, completing the separation.

Plant cells have a stiff cell wall and cannot pinch in. Instead, vesicles from the Golgi apparatus are guided by a framework of microtubules, the phragmoplast, to the middle of the cell, where they fuse into a disc-shaped cell plate. This grows outward until it joins the parent walls and forms a new wall between the daughters.

Bacteria divide by binary fission, in which a ring of the tubulin-like protein FtsZ marks the site of division. If cytokinesis fails in animal cells, a cell with two nuclei results, and repeated failures can produce cells with extra chromosomes, as in polyploidy, which is often seen in cancer. The process is controlled as a part of the cell cycle and cell division, and involves the centrosome in organizing the spindle.

• Mitosis — the nuclear division that precedes it
• Actin — the filament in the contractile ring
• Binary Fission — cell division in bacteria