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Cytokinesis () is the part of the
cell division Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ...
process during which the
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
of a single
eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of nuclear division in mitosis and
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...
. During cytokinesis the spindle apparatus partitions and transports duplicated
chromatid A chromatid (Greek ''khrōmat-'' 'color' + ''-id'') is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chr ...
s into the cytoplasm of the separating daughter
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
. It thereby ensures that
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...
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 oogenesis in animals the ovum takes almost all the cytoplasm and
organelles In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...
. This leaves very little for the resulting polar bodies, which in most species die without function, though they do take on various special functions in other species. Another form of mitosis occurs in tissues such as
liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...
and skeletal muscle; it omits cytokinesis, thereby yielding multinucleate cells. 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 image:Phragmoplast.png, 300px, Phragmoplast and cell plate formation in a plant cell during cytokinesis. Left side: Phragmoplast forms and cell plate starts to assemble in the center of the cell. Towards the right: Phragmoplast enlarges in a donut ...
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 prokaryotic process of 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
chromatid A chromatid (Greek ''khrōmat-'' 'color' + ''-id'') is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chr ...
s do not separate completely before the anaphase.


Etymology and pronunciation

The word "cytokinesis" () uses combining forms of '' cyto-'' + '' kine-'' + '' -sis'', New Latin from Classical Latin and
Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic pe ...
, reflecting " cell" and '' kinesis'' ("motion, movement"). It was coined by
Charles Otis Whitman Charles Otis Whitman (December 6, 1842 – December 14, 1910) was an American zoologist, who was influential to the founding of classical ethology (study of animal behavior). A dedicated educator who preferred to teach a few research students at ...
in 1887. Origin of this term is from Greek (', a hollow), Latin derivative ' (cellular), Greek (', movement).


Animal cell

Animal cell cytokinesis begins shortly after the onset of sister
chromatid A chromatid (Greek ''khrōmat-'' 'color' + ''-id'') is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chr ...
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. 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 central spindle (or ''spindle midzone'') forms when non-kinetochore microtubule fibers are bundled between the spindle poles. A number of different species including ''
H. sapiens Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, an ...
'', '' D. melanogaster'' and ''
C. elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (r ...
'' require the central spindle in order to efficiently undergo cytokinesis, although the specific
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (biology), morphology or physical form and structure, its Developmental biology, developmental proc ...
associated with its absence varies from one species to the next (for example, certain Drosophila cell types are incapable of forming a cleavage furrow without the central spindle, whereas in both ''C. elegans''
embryos An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
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. 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.


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. 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.


Actin-myosin ring assembly and contraction

At the cytokinesis furrow, it is the actin-myosin contractile ring that drives the cleavage process, during which cell membrane and wall grow inward, which eventually pinches the mother cell in two. The key components of this ring are the filamentous protein actin and the motor protein myosin II. The contractile ring assembles equatorially (in the middle of the cell) at the cell cortex (adjacent to the cell membrane). Rho protein family (RhoA protein in mammalian cells) is a key regulator of contractile ring formation and contraction in animal cells. The RhoA pathway promotes assembly of the actin-myosin ring by two main effectors. First, RhoA stimulates nucleation of unbranched actin filaments by activation of Diaphanous-related formins. This local generation of new actin filaments is important for the contractile ring formation. This actin filament formation process also requires a protein called profilin, which binds to actin monomers and helps load them onto the filament end. Second, RhoA promotes myosin II activation by the kinase ROCK, which activates myosin II directly by phosphorylation of the myosin light chain and also inhibits myosin phosphatase by phosphorylation of the phosphatase-targeting subunit MYPT. Besides actin and myosin II, the contractile ring contains the scaffolding protein anillin. Anillin binds to actin, myosin, RhoA, and CYK-4, and thereby links the equatorial cortex with the signals from the central spindle. It also contributes to the linkage of the actin-myosin ring to the plasma membrane. Additionally, anillin generates contractile forces by rectifying thermal fluctuations. Another protein, septin, has also been speculated to serve as a structural scaffold on which the cytokinesis apparatus is organized. Following its assembly, contraction of the actin-myosin ring leads to ingression of the attached plasma membrane, which partitions the cytoplasm into two domains of emerging sister cells. The force for the contractile processes is generated by movements along actin by the motor protein myosin II. Myosin II uses the free energy released when ATP is hydrolyzed to move along these actin filaments, constricting the cell membrane to form a cleavage furrow. Continued
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysi ...
causes this cleavage furrow to ingress (move inwards), a striking process that is clearly visible through a light microscope.


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. 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 The human gene SPAST codes for the microtubule-severing protein of the same name, commonly known as spastin. This gene encodes a member of the AAA (ATPases associated with a variety of cellular activities) protein family. Members of this protei ...
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. Cytokinesis happens only after APC binds with CDC20. This allows for the separation of chromosomes and myosin to work simultaneously. After cytokinesis, non-kinetochore
microtubules Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27  nm and have an inner diameter between 11 a ...
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 cell wall, cytokinesis in plant cells is significantly different from that in animal cells, Rather than forming a contractile ring, plant cells construct a
cell plate image:Phragmoplast.png, 300px, Phragmoplast and cell plate formation in a plant cell during cytokinesis. Left side: Phragmoplast forms and cell plate starts to assemble in the center of the cell. Towards the right: Phragmoplast enlarges in a donut ...
in the middle of the cell. The stages of
cell plate image:Phragmoplast.png, 300px, Phragmoplast and cell plate formation in a plant cell during cytokinesis. Left side: Phragmoplast forms and cell plate starts to assemble in the center of the cell. Towards the right: Phragmoplast enlarges in a donut ...
formation include (1) creation of the
phragmoplast image:Phragmoplast.png, 300px, Phragmoplast and cell plate formation in a plant cell during cytokinesis. Left side: Phragmoplast forms and cell plate starts to assemble in the center of the cell. Towards the right: Phragmoplast enlarges in a donut- ...
, an array of
microtubules Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27  nm and have an inner diameter between 11 a ...
that guides and supports the formation of the
cell plate image:Phragmoplast.png, 300px, Phragmoplast and cell plate formation in a plant cell during cytokinesis. Left side: Phragmoplast forms and cell plate starts to assemble in the center of the cell. Towards the right: Phragmoplast enlarges in a donut ...
; (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 Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...
and other cell wall components; (4) recycling of excess membrane and other material from the
cell plate image:Phragmoplast.png, 300px, Phragmoplast and cell plate formation in a plant cell during cytokinesis. Left side: Phragmoplast forms and cell plate starts to assemble in the center of the cell. Towards the right: Phragmoplast enlarges in a donut ...
; and (5) fusion with the parental cell wall The
phragmoplast image:Phragmoplast.png, 300px, Phragmoplast and cell plate formation in a plant cell during cytokinesis. Left side: Phragmoplast forms and cell plate starts to assemble in the center of the cell. Towards the right: Phragmoplast enlarges in a donut- ...
is assembled from the remnants of the mitotic spindle, and serves as a track for the trafficking of
vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features o ...
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 The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles i ...
as the source of these vesicles, but other studies have suggested that they contain endocytosed material as well. These tubules then widen and fuse laterally with each other, eventually forming a planar, fenestrated sheet /sup>. As the
cell plate image:Phragmoplast.png, 300px, Phragmoplast and cell plate formation in a plant cell during cytokinesis. Left side: Phragmoplast forms and cell plate starts to assemble in the center of the cell. Towards the right: Phragmoplast enlarges in a donut ...
matures, large amounts of membrane material are removed via clathrin-mediated endocytosis /sup> Eventually, the edges of the cell plate fuse with the parental plasma membrane, often in an asymmetrical fashion, thus completing cytokinesis. The remaining fenestrae contain strands of endoplasmic reticulum passing through them, and are thought to be the precursors of plasmodesmata /sup>. The construction of the new cell wall begins within the lumen of the narrow tubules of the young
cell plate image:Phragmoplast.png, 300px, Phragmoplast and cell plate formation in a plant cell during cytokinesis. Left side: Phragmoplast forms and cell plate starts to assemble in the center of the cell. Towards the right: Phragmoplast enlarges in a donut ...
. The order in which different cell wall components are deposited has been determined largely by immuno-electron microscopy. The first components to arrive are pectins, hemicelluloses, and
arabinogalactan protein Arabinogalactan-proteins (AGPs) are highly glycosylated proteins (glycoproteins) found in the cell walls of plants. Each one consists of a protein with sugar molecules attached (which can account for more than 90% of the total mass). They are mem ...
s carried by the secretory vesicles that fuse to form the cell plate. 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 Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...
, the primary component of a mature cell wall /sup>. The
middle lamella The middle lamella is a layer that cements together the primary cell walls of two adjoining plant cells. It is the first formed layer to be deposited at the time of cytokinesis. The cell plate that is formed during cell division itself develops ...
(a glue-like layer containing pectin) develops from the cell plate, serving to bind the cell walls of adjoining cells together.


Forces


Animal cells

Cytokinetic furrow ingression is powered by Type II
Myosin ATPase Myosin ATPase () is an enzyme with systematic name ''ATP phosphohydrolase (actin-translocating)''. This enzyme catalyses the following chemical reaction : ATP + H2O \rightleftharpoons ADP + phosphate ATP hydrolysis provides energy for actomyosi ...
. 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 To the constriction of the cleavage furrow, the total surface area should be increased by supplying the plasma membrane via exocytosis
.


Proteins involved in cytokinesis

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


See also

* * * * * * * *


References


Further reading

*''The Molecular Requirements for Cytokinesis'' by M. Glotzer (2005), Science 307, 1735 *"Animal Cytokinesis: from parts list to mechanism" by Eggert, U.S., Mitchison, T.J., Field, C.M. (2006), Annual Review of Cell Biology 75, 543-66 *''Campbell Biology'' (2010), 580-582
More description and nice images of cell division in plants, with a focus on fluorescence microscopy
* {{Cell cycle Cell cycle Mitosis Meiosis