Cytokinesis is the final stage of the cell cycle during which a cell physically divides into two daughters through the assembly of new membranes (and cell wall in some cases) between the forming daughters. New membrane assembly can either proceed centripetally behind a contractile apparatus, as in the case of prokaryotes, archaea, fungi, and animals or expand centrifugally, as in the case of higher plants. In this article, we compare the mechanisms of cytokinesis in diverse organisms dividing through the use of a contractile apparatus. While an actomyosin ring participates in cytokinesis in almost all centripetally dividing eukaryotes, the majority of bacteria and archaea (except Crenarchaea) divide using a ring composed of the tubulin-related protein FtsZ. Curiously, despite molecular conservation of the division machinery components, division site placement and its cell cycle regulation occur by a variety of unrelated mechanisms even among organisms from the same kingdom. While molecular motors and cytoskeletal polymer dynamics contribute to force generation during eukaryotic cytokinesis, cytoskeletal polymer dynamics alone appears to be sufficient for force generation during prokaryotic cytokinesis. Intriguingly, there are life forms on this planet that appear to lack molecules currently known to participate in cytokinesis and how these cells perform cytokinesis remains a mystery waiting to be unravelled. V C 2012 Wiley Periodicals, Inc Key Words: cytokinesis, cytoskeleton, contractile apparatus Introduction C ytokinesis is a fundamental cellular process essential for cell proliferation in all life forms. In addition, proper regulation of the site of cytokinesis is important for cell fate establishment in several developmental contexts across kingdoms. All cells, with the exception of plants, use a contractile apparatus placed at a chosen division site to accomplish cytokinesis. Contraction and closure of this cell division apparatus facilitates assembly of new membranes and the completion of cytokinesis. A number of informative review articles in this issue of Cytoskeleton describe in detail mechanisms of cytokinesis in various organisms. In our review article, we compare contractile apparatus-driven cytokinesis regulatory mechanisms across kingdoms, by considering the process in bacteria, archaea, fungi, and animals. We discuss four major aspects of cytokinesis: (1) Composition of the cytokinetic apparatus, (2) Mechanism of positioning of the cytokinetic apparatus, (3) Mechanism of assembly of the cytokinetic apparatus, and (4) Mechanism of force generation by the cytokinetic apparatus. Modes of generation of constrictive forces, other than those by the cytoskeletonbased cytokinetic apparatus, are being discovered in recent times. While these are mentioned briefly in this article, we restrict this review to describe assembly and force generation by cytoskeleton-based cell division apparatuses.
Composition of the Cytokinetic ApparatusThe cytoskeleton that functions in cytokinesis in nearly all cell types fal...