Cytokinesis is the process that partitions the cytoplasm of a parent cell into daughter cells. It is distinct from mitosis, which separates duplicated chromosomes into daughter nuclei, although the two processes usually overlap in time. Together, nuclear division and cytokinesis complete cell division. Animal cells generally divide by constricting their surface inward, whereas higher plant cells construct a new internal partition called a cell plate. (openstax.org)
Relationship to the cell cycle
Cytokinesis is normally coordinated with the cell cycle so that physical separation does not interfere with chromosome segregation. In many animal cells, visible cleavage begins during anaphase, continues through telophase, and is completed after the daughter nuclei have re-formed. Preparation of the division machinery begins earlier, so cytokinesis is not simply an event that starts after mitosis has finished. (ncbi.nlm.nih.gov)
The processes are nevertheless separable. Nuclear division can occur without cytoplasmic division, producing a multinucleate cell. A well-studied example is the early Drosophila embryo, in which repeated nuclear divisions precede cellularization—the formation of cell boundaries around individual nuclei. Thus, two daughter nuclei do not necessarily indicate that two daughter cells have formed. (ncbi.nlm.nih.gov)
Successful division also requires the distribution of cytoplasmic components, not just nuclear DNA. Organelles, including mitochondria, must be inherited by the daughters; their partitioning is coordinated with cell division but involves mechanisms distinct from chromosome segregation. (ncbi.nlm.nih.gov)
Cytokinesis in animal cells
Animal-cell cytokinesis involves specification of the division plane, assembly and constriction of a contractile ring, formation of an intercellular bridge, and final membrane separation.
Specification of the division plane
The mitotic spindle helps position the division plane between the separating chromosome sets. Signals associated with spindle microtubules organize a narrow zone of contractility at the equatorial cell cortex—the region beneath the plasma membrane. A central regulator is RhoA, a small GTP-binding protein whose localized activity promotes assembly of the cleavage machinery. (pmc.ncbi.nlm.nih.gov)
In cultured human cells, the guanine nucleotide exchange factor ECT2 and the centralspindlin component CYK-4 are required for normal equatorial RhoA localization and contractile-ring assembly. Centralspindlin connects spindle organization with cortical signaling, but this pathway should not be interpreted as requiring all of its components to act exclusively at the spindle midzone: experimental disruption of ECT2 midzone recruitment can leave cytokinesis intact when its membrane-associated functions are preserved. (pmc.ncbi.nlm.nih.gov)
Contractile-ring assembly and furrow ingression
The contractile ring is a dynamic assembly of actin filaments, myosin II, and associated structural and regulatory proteins. It forms beneath the plasma membrane and generates tension that draws the cell surface inward, producing a cleavage furrow. These components belong to the cytoskeleton, but the ring is a transient structure assembled specifically for division. (ncbi.nlm.nih.gov)
The ring is not simply a permanent belt that becomes progressively tighter. Its components reorganize and turn over as constriction proceeds. Membrane remodeling also accompanies cleavage: intracellular vesicles deliver membrane to the dividing surface, helping accommodate the changing geometry of the daughter cells. (ncbi.nlm.nih.gov)
Intercellular bridge and abscission
Furrow ingression does not immediately produce two fully separate cells. The daughters initially remain connected by a narrow, microtubule-containing bridge. Within this bridge, a dense structure called the midbody organizes late cytokinetic events. Abscission is the final severing of the membrane connection, typically at a constricted site beside the midbody rather than through its center. (pmc.ncbi.nlm.nih.gov)
The ESCRT machinery, particularly ESCRT-III proteins, participates in this final membrane-fission step. Live-cell imaging has demonstrated sequential recruitment of ESCRT components to the bridge and accumulation at the eventual separation site. Abscission is therefore mechanistically distinct from the earlier actomyosin-driven constriction of the cell body. (pmc.ncbi.nlm.nih.gov)
Cytokinesis in plant cells
Higher plant cells generally divide through construction of a cell plate, rather than through an animal-like cleavage furrow. This process creates both the membranes separating the daughters and the precursor of a new cell wall. (pubmed.ncbi.nlm.nih.gov)
A plant-specific cytoskeletal apparatus, the phragmoplast, organizes cell-plate formation between the daughter nuclei. It contains opposing microtubule arrays and associated components that support delivery of vesicles to the developing partition. Electron microscopy has revealed a succession of membrane intermediates: vesicles fuse into tubular networks, which develop into a more continuous plate as wall material accumulates. (pmc.ncbi.nlm.nih.gov)
In a typical somatic division, the cell plate begins internally and expands toward the cell periphery. The active region of the phragmoplast moves outward with its growing edge. The plate ultimately joins the parental plasma membrane, and its wall material becomes continuous with the existing wall. This is often described as centrifugal, or inside-out, division, in contrast to the inward progression of an animal cleavage furrow. (pubmed.ncbi.nlm.nih.gov)
Vesicle fusion is essential to this process. In Arabidopsis, the cytokinesis-specific syntaxin KNOLLE localizes to the division plane and is required for normal cell-plate formation. Its identification provided molecular evidence that plant cytokinesis depends on specialized membrane-fusion machinery, not merely on the deposition of a passive barrier. (pubmed.ncbi.nlm.nih.gov)
Cytokinesis in fungi
Many fungi combine an actomyosin contractile ring with construction of a wall partition, or septum. In fission yeast, the ring constricts at the leading edge of the growing septum. Experiments show that the ring helps coordinate the geometry of septum growth, illustrating how cytoskeletal contraction and wall synthesis can function as a coupled system. (pmc.ncbi.nlm.nih.gov)
A ring in a wall-bearing cell should therefore not automatically be understood as operating exactly like an animal-cell ring. Studies of fission yeast have distinguished ring tension and organization from the accompanying growth of the septum; the mechanical contribution of each depends on the cellular setting. (pmc.ncbi.nlm.nih.gov)
Asymmetric division
Cytokinesis need not produce daughters of equal size or identical cytoplasmic composition. In asymmetric cell division, displacement of the spindle and division plane can generate unequal daughters, while the prior localization of cellular components determines which daughter inherits particular developmental regulators. The resulting difference reflects coordinated cell polarity, spindle positioning, and cytoplasmic partitioning rather than necessarily indicating a division error. (ncbi.nlm.nih.gov)
Regulation and failure
Late cytokinesis includes safeguards that coordinate physical separation with chromosome segregation. In human cells, persistent chromatin bridges can sustain Aurora B kinase activity and delay abscission. This abscission checkpoint stabilizes the connection between daughters while unresolved chromatin remains in the division region. (pubmed.ncbi.nlm.nih.gov)
Cytokinesis can fail through defective furrow formation, regression of an existing furrow, or failure of final abscission. When chromosome segregation has occurred but the cell does not separate, the result can be a binucleate cell containing the combined chromosome complement of the intended daughters. This differs from a segregation error in which individual chromosomes are distributed unequally between otherwise separate cells. (pubmed.ncbi.nlm.nih.gov)
Experimental investigation
Cytokinesis is studied through live-cell imaging, electron microscopy, genetic perturbation, biochemical analysis, and measurements of cellular mechanics. Fluorescently tagged proteins reveal when division factors arrive and disappear; depletion or mutation tests their necessity; and ultrastructural imaging resolves membrane and cytoskeletal intermediates that light microscopy cannot distinguish. These complementary approaches have been central to identifying RhoA regulation, ESCRT-dependent abscission, and the architecture of the plant cell plate. (pmc.ncbi.nlm.nih.gov)
Interpretation requires attention to the model organism and experimental method. For example, introducing fluorescently tagged ESCRT proteins at excessive levels can itself interfere with abscission. Likewise, observations from wall-bearing yeast, cultured animal cells, and plant tissues describe related biological problems but not interchangeable mechanical systems. (pmc.ncbi.nlm.nih.gov)
References
- 2 The Cell Cycle — Biology 2eopenstax.org
- Cytokinesis — Molecular Biology of the Cellncbi.nlm.nih.gov
- An ECT2–centralspindlin complex regulates the localization and function of RhoApmc.ncbi.nlm.nih.gov
- Plasma Membrane Association but Not Midzone Recruitment of RhoGEF ECT2 Is Essential for Cytokinesispmc.ncbi.nlm.nih.gov
- Dynamics of endosomal sorting complex required for transport (ESCRT) machinery during cytokinesis and its role in abscissionpmc.ncbi.nlm.nih.gov
- The Arabidopsis KNOLLE protein is a cytokinesis-specific syntaxinpubmed.ncbi.nlm.nih.gov
- Electron Tomographic Analysis of Somatic Cell Plate Formation in Meristematic Cells of Arabidopsis Preserved by High-Pressure Freezingpmc.ncbi.nlm.nih.gov
- Cytokinesis in tobacco BY-2 and root tip cells: a new model of cell plate formation in higher plantspubmed.ncbi.nlm.nih.gov
- The contractile ring coordinates curvature-dependent septum assembly during fission yeast cytokinesispmc.ncbi.nlm.nih.gov
- Mechanism of Cytokinetic Contractile Ring Constriction in Fission Yeastpmc.ncbi.nlm.nih.gov
- Aurora B-mediated abscission checkpoint protects against tetraploidizationpubmed.ncbi.nlm.nih.gov
- Inhibition of ESCRT-II–CHMP6 interactions impedes cytokinetic abscission and leads to cell deathpmc.ncbi.nlm.nih.gov