Cytokinesis

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Cytokinesis illustration
Ciliate undergoing cytokinesis, with the cleavage furrow being clearly visible.

Cytokinesis (

eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of nuclear division in mitosis and meiosis. During cytokinesis the spindle apparatus partitions and transports duplicated chromatids into the cytoplasm of the separating daughter cells. It thereby ensures that chromosome number and complement are maintained from one generation to the next and that, except in special cases, the daughter cells will be functional copies of the parent cell. After the completion of the telophase and cytokinesis, each daughter cell enters the interphase of the cell cycle
.

Particular functions demand various deviations from the process of symmetrical cytokinesis; for example in

polar bodies, which in most species die without function, though they do take on various special functions in other species.[1]
Another form of mitosis occurs in tissues such as liver and skeletal muscle; it omits cytokinesis, thereby yielding multinucleate cells (see syncytium).

Plant cytokinesis differs from animal cytokinesis, partly because of the rigidity of plant cell walls. Instead of plant cells forming a cleavage furrow such as develops between animal daughter cells, a dividing structure known as the cell plate forms in the cytoplasm and grows into a new, doubled cell wall between plant daughter cells. It divides the cell into two daughter cells.

Cytokinesis largely resembles the

binary fission, but because of differences between prokaryotic and eukaryotic cell structures and functions, the mechanisms differ. For instance, a bacterial cell has a Circular chromosome (a single chromosome in the form of a closed loop), in contrast to the linear, usually multiple, chromosomes of eukaryote. Accordingly, bacteria construct no mitotic spindle in cell division. Also, duplication of prokaryotic DNA takes place during the actual separation of chromosomes; in mitosis, duplication takes place during the interphase before mitosis begins, though the daughter chromatids don't separate completely before the anaphase
.

Etymology and pronunciation

The word "cytokinesis" (

combining forms of cyto- + kine- + -sis, Neo-Latin from Classical Latin and Ancient Greek, reflecting "cell" and kinesis ("motion, movement"). It was coined by Charles Otis Whitman in 1887.[4]

Origin of this term is from Greek κύτος (kytos, a hollow), Latin derivative cyto (cellular), Greek κίνησις (kínesis, movement).

Animal cell

Animal cell telophase and cytokinesis

Animal cell cytokinesis begins shortly after the onset of sister chromatid separation in the anaphase of mitosis. The process can be divided to the following distinct steps: anaphase spindle reorganization, division plane specification, actin-myosin ring assembly and contraction, and abscission.[5] Faithful partitioning of the genome to emerging daughter cells is ensured through the tight temporal coordination of the above individual events by molecular signaling pathways.

Anaphase spindle reorganization

Animal cell cytokinesis starts with the stabilization of microtubules and reorganization of the mitotic spindle to form the central spindle. The

embryos and human tissue culture
cells a cleavage furrow is observed to form and ingress, but then regress before cytokinesis is complete). The process of mitotic spindle reorganization and central spindle formation is caused by the decline of CDK1 activity during anaphase.[5] The decline of CDK1 activity at the metaphase-anaphase transition leads to dephosphorylating of inhibitory sites on multiple central spindle components. First of all, the removal of a CDK1 phosphorylation from a subunit of the CPC (the chromosomal passenger complex) allows its translocalization to the central spindle from the centromeres, where it is located during metaphase. Besides being a structural component of the central spindle itself, CPC also plays a role in the phosphoregulation of other central spindle components, including PRC1 (microtubule-bundling protein required for cytokinesis 1) and MKLP1 (a kinesin motor protein). Originally inhibited by CDK1-mediated phosphorylation, PRC1 is now able to form a homodimer that selectively binds to the interface between antiparallel microtubules, facilitating spatial organization of the microtubules of the central spindle. MKLP1, together with the Rho-family GTPase activating protein CYK-4 (also termed MgcRacGAP), forms the centralspindlin complex. Centralspindlin binds to the central spindle as higher-order clusters. The centralspindlin cluster formation is promoted by phosphorylation of MLKP1 by Aurora B, a component of CPC. In short, the self-assembly of central spindle is initiated through the phosphoregulation of multiple central spindle components by the decline of CDK1 activity, either directly or indirectly, at the metaphase-anaphase transition. The central spindle may have multiple functions in cytokinesis including the control of cleavage furrow positioning, the delivery of membrane vesicles to the cleavage furrow, and the formation of the midbody structure that is required for the final steps of division.[6]

Division plane specification

The second step of animal cell cytokinesis involves division plane specification and cytokinetic furrow formation. Precise positioning of the division plane between the two masses of segregated chromosomes is essential to prevent chromosome loss. Meanwhile, the mechanism by which the spindle determines the division plane in animal cells is perhaps the most enduring mystery in cytokinesis and a matter of intense debate. There exist three hypotheses of furrow induction.[6] The first is the astral stimulation hypothesis, which postulates that astral microtubules from the spindle poles carry a furrow-inducing signal to the cell cortex, where signals from two poles are somehow focused into a ring at the spindle. A second possibility, called the central spindle hypothesis, is that the cleavage furrow is induced by a positive stimulus that originates in the central spindle equator. The central spindle may contribute to the specification of the division plane by promoting concentration and activation of the small GTPase RhoA at the equatorial cortex. A third hypothesis is the astral relaxation hypothesis. It postulates that active actin-myosin bundles are distributed throughout the cell cortex, and inhibition of their contraction near the spindle poles results in a gradient of contractile activity that is highest at the midpoint between poles. In other words, astral microtubules generate a negative signal that increases cortical relaxation close to the poles. Genetic and laser-micromanipulation studies in C. elegans embryos have shown that the spindle sends two redundant signals to the cell cortex, one originating from the central spindle, and a second signal deriving from the spindle aster, suggesting the involvement of multiple mechanisms combined in the positioning of the cleavage furrow. The predominance of one particular signal varies between cell types and organisms. And the multitude and partial redundancy of signals may be required to make the system robust and to increase spatial precision.[5]

Actin-myosin ring assembly and contraction

At the cytokinesis

light microscope
.

Abscission

For the use of this term in plant and animal shedding, see Abscission.

The cytokinetic furrow ingresses until a midbody structure (composed of electron-dense, proteinaceous material) is formed, where the actin-myosin ring has reached a diameter of about 1–2 μm. Most animal cell types remain connected by an intercellular cytokinetic bridge for up to several hours until they are split by an actin-independent process termed abscission, the last step of cytokinesis.[5][8]

The process of abscission physically cleaves the midbody into two. Abscission proceeds by removal of cytoskeletal structures from the cytokinetic bridge, constriction of the cell cortex, and plasma membrane fission. The intercellular bridge is filled with dense bundles of antiparallel microtubules that derive from the central spindle. These microtubules overlap at the midbody, which is generally thought to be a targeting platform for the abscission machinery.

The microtubule severing protein spastin is largely responsible for the disassembly of microtubule bundles inside the intercellular bridge. Complete cortical constriction also requires removal of the underlying cytoskeletal structures. Actin filament disassembly during late cytokinesis depends on the PKCε–14-3-3 complex, which inactivates RhoA after furrow ingression. Actin disassembly is further controlled by the GTPase Rab35 and its effector, the phosphatidylinositol-4,5-bisphosphate 5-phosphatase OCRL. Understanding the mechanism by which the plasma membrane ultimately splits requires further investigation.

Timing cytokinesis

Cytokinesis must be temporally controlled to ensure that it occurs only after sister chromatids separate during the anaphase portion of normal proliferative cell divisions. To achieve this, many components of the cytokinesis machinery are highly regulated to ensure that they are able to perform a particular function at only a particular stage of the cell cycle.[9][10] Cytokinesis happens only after APC binds with CDC20.[citation needed] This allows for the separation of chromosomes and myosin to work simultaneously.

After cytokinesis, non-kinetochore

microtubules reorganize and disappear into a new cytoskeleton as the cell cycle returns to interphase (see also cell cycle
).

Plant cell

Due to the presence of a

microtubules that guides and supports the formation of the cell plate; (2) trafficking of vesicles to the division plane and their fusion to generate a tubular-vesicular network; (3) continued fusion of membrane tubules and their transformation into membrane sheets upon the deposition of callose, followed by deposition of cellulose and other cell wall components; (4) recycling of excess membrane and other material from the cell plate; and (5) fusion with the parental cell wall[11][12]

The

vesicles to the phragmoplast midzone. These vesicles contain lipids, proteins and carbohydrates needed for the formation of a new cell boundary. Electron tomographic studies have identified the Golgi apparatus as the source of these vesicles,[13][14] but other studies have suggested that they contain endocytosed material as well.[15][16]

These tubules then widen and fuse laterally with each other, eventually forming a planar, fenestrated sheet [8]. As the

plasmodesmata
[8].

The process of Cytokinesis in a plant cell and an animal cell

The construction of the new

pectins, hemicelluloses, and arabinogalactan proteins carried by the secretory vesicles that fuse to form the cell plate.[18] The next component to be added is callose, which is polymerized directly at the cell plate by callose synthases. As the cell plate continues to mature and fuses with the parental plasma membrane, the callose is slowly replaced with cellulose, the primary component of a mature cell wall [6]. The middle lamella (a glue-like layer containing pectin) develops from the cell plate, serving to bind the cell walls of adjoining cells together.[19][20]

Forces

Animal cells

Cytokinetic furrow ingression is powered by Type II Myosin ATPase. Since Myosins are recruited to the medial region, the contractile forces acting on the cortex resemble a 'purse string' constriction pulling inwards. This leads to the inward constriction. The plasma membrane by virtue of its close association with the cortex via crosslinker proteins [21] To the constriction of the cleavage furrow, the total surface area should be increased by supplying the plasma membrane via exocytosis.[22]

Theoretical models show that symmetric constriction requires both lateral stabilization and constriction forces.[23] Reduction of external pressure and of surface tension (by membrane trafficking) reduce the required stabilization and constriction forces.

Proteins involved in cytokinesis

CEP55 is a mitotic phosphoprotein that plays a key role in cytokinesis, the final stage of cell division.[24][25]

Clinical significance

In some cases, a cell may divide its genetic material and grow in size, but fail to undergo cytokinesis. This results in larger cells with more than one nucleus. Usually this is an unwanted aberration and can be a sign of cancerous cells.[26]

See also

  • Diploid
     – Number of sets of chromosomes in a cell
  • Telophase – Final stage of a cell division for eukaryotic cells both in mitosis and meiosis
  • Prophase – First phase of cell division in both mitosis and meiosis
  • Anaphase – Stage of a cell division
  • Metaphase – Stage of cell division
  • Mitosis – Process in which chromosomes are replicated and separated into two new identical nuclei
  • Cell theory – Biology of cells
  • Cytoskeleton – Network of filamentous proteins that forms the internal framework of cells

References

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  2. ^ "cytokinesis". Lexico UK English Dictionary. Oxford University Press. Archived from the original on 2020-03-22.
  3. ^ "cytokinesis". Merriam-Webster.com Dictionary. Retrieved 2016-01-21.
  4. ^ Battaglia, Emilio (2009). Caryoneme alternative to chromosome and a new caryological nomenclature. Caryologia 62 (4): 1–80. link.
  5. ^
    S2CID 3355851
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  6. ^ a b c d Morgan, David (2007). The Cell Cycle. New Science Press. pp. 157–173.
  7. PMID 34321459
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  8. ^ "Cytokinetic bridge". proteinatlas.org. Archived from the original on 28 August 2019. Retrieved 28 August 2019.
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  10. PMID 17488623
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  11. PMID 11074381
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  12. PMID 7559757
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  13. PMID 11549762
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  14. PMID 15020749
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  15. S2CID 11881080
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  16. PMID 16399085
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  17. PMID 11880633
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  18. ISSN 1040-2519
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  19. S2CID 84936099
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  20. ISBN 978-0-470-04737-8
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  21. ISBN 978-0-8153-3218-3
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  22. PMID 32322581
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  23. PMID 31869912
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  24. PMID 19855176
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  25. PMID 34710087
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  26. ISBN 978-1-947172-04-3
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Further reading
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