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Theory of Intelligent Design, the best explanation of Origins » Molecular biology of the cell » CYTOKINESIS

CYTOKINESIS

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1 CYTOKINESIS on Wed Jul 29, 2015 12:20 pm

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CYTOKINESIS

The final step in the cell cycle is cytokinesis, the division of the cytoplasm in two. In most cells, cytokinesis follows every mitosis, although some cells, such as early Drosophila embryos and some mammalian hepatocytes and heart muscle cells, undergo mitosis without cytokinesis and thereby acquire multiple nuclei. In most animal cells, cytokinesis begins in anaphase and ends shortly after the completion of mitosis in telophase. The first visible change of cytokinesis in an animal cell is the sudden appearance of a pucker, or cleavage furrow, on the cell surface. The furrow rapidly deepens and spreads around the cell until it completely divides the cell in two. The structure underlying this process is the contractile ring—a dynamic assembly composed of actin filaments, myosin II filaments, and many structural and regulatory proteins. During anaphase, the ring assembles just beneath the plasma membrane



The ring gradually contracts, and, at the same time, fusion of intracellular vesicles with the plasma membrane inserts new membrane adjacent to the ring. This addition of membrane compensates for the increase in surface area that accompanies cytoplasmic division. When ring contraction is completed, membrane insertion and fusion seal the gap between the daughter cells.

Actin and Myosin II in the Contractile Ring Generate the Force for Cytokinesis

In interphase cells, actin and myosin II filaments form a cortical network underlying the plasma membrane. In some cells, they also form large cytoplasmic bundles called stress fibers . As cells enter mitosis, these arrays of actin and myosin disassemble; much of the actin reorganizes, and myosin II filaments are released. As the sister chromatids separate in anaphase, actin and myosin II begin to accumulate in the rapidly assembling contractile ring which also contains numerous other proteins that provide structural support or assist in ring assembly. Assembly of the contractile ring results in part from the local formation of new actin filaments, which depends on formin proteins that nucleate the assembly of parallel arrays of linear, unbranched actin filaments.



The contractile ring. (A) A drawing of the cleavage furrow in a dividing cell. (B) An electron micrograph of the ingrowing edge of a cleavage furrow of a dividing animal cell. (C) Fluorescence micrographs of a dividing slime mold amoeba stained for actin (red) and myosin II (green). Whereas all of the visible myosin II has redistributed to the contractile ring, only some of the actin has done so; the rest remains in the cortex of the nascent daughter cells.

After anaphase, the overlapping arrays of actin and myosin II filaments contract to generate the force that divides the cytoplasm in two. Once contraction begins, the ring exerts a force large enough to bend a fine glass needle that is inserted in its path. As the ring constricts, it maintains the same thickness, suggesting that its total volume and the number of filaments it contains decrease steadily. Moreover, unlike actin in muscle, the actin filaments in the ring are highly dynamic, and their arrangement changes continually during cytokinesis. The contractile ring is finally dispensed with altogether when cleavage ends, as the plasma membrane of the cleavage furrow narrows to form the midbody.
The midbody persists as a tether between the two daughter cells and contains the remains of the central spindle, a large protein structure derived from the antiparallel
interpolar microtubules of the spindle midzone, packed tightly together within a dense matrix material



After the daughter cells separate completely, some of the components of the residual midbody often remain on the inside of the plasma membrane of each cell, where they may serve as a mark on the cortex that helps to orient the spindle in the subsequent cell division.

Local Activation of RhoA Triggers Assembly and Contraction of the Contractile Ring

RhoA, a small GTPase of the Ras superfamily, controls the assembly and function of the contractile ring at the site of cleavage. RhoA is activated at the cell cortex at the future division site, where it promotes actin filament formation, myosin II assembly, and ring contraction. It stimulates actin filament formation by activating formins, and it promotes myosin II assembly and contractions by activating multiple protein kinases, including the Rho-activated kinase Rock



Picture above: Regulation of the contractile ring by the GTPase RhoA. Like other Rho family GTPases, RhoA is activated by a RhoGEF protein and inactivated by a Rho GTPase-activating protein (RhoGAP). The active GTPbound form of RhoA is focused at the future cleavage site. By binding formins, activated RhoA promotes the assembly of actin filaments in the contractile ring. By activating Rho-activated protein kinases, such as Rock, it stimulates myosin II filament formation and activity, thereby promoting contraction of the ring.

These kinases phosphorylate the regulatory myosin light chain, a subunit of myosin II, thereby stimulating bipolar myosin II filament formation and motor activity. RhoA is thought to be activated by a guanine nucleotide exchange factor (Rho-GEF), which is found at the cell cortex at the future division site and stimulates the release of GDP and binding of GTP to RhoA ( see picture above )We know little about how the RhoGEF is localized or activated at the division site, although the microtubules of the anaphase spindle seem to be involved.

The Microtubules of the Mitotic Spindle Determine the Plane of Animal Cell Division

The central problem in cytokinesis is how to ensure that division occurs at the right time and in the right place. Cytokinesis must occur only after the two sets of chromosomes are fully segregated from each other, and the site of division must be placed between the two sets of daughter chromosomes, thereby ensuring that each daughter cell receives a complete set. The correct timing and positioning of cytokinesis in animal cells are achieved by mechanisms that depend on the mitotic spindle. During anaphase, the spindle generates signals that initiate furrow formation at a position midway between the spindle poles, thereby ensuring that division occurs between the two sets of separated chromosomes. Because these signals originate in the anaphase spindle, this mechanism also contributes to the correct timing of cytokinesis in late mitosis. Cytokinesis also occurs
at the correct time because dephosphorylation of Cdk substrates, which depends on cyclin destruction in metaphase and anaphase, initiates cytokinesis. We now describe these regulatory mechanisms in more detail, with an emphasis on cytokinesis in animal cells. Studies of the fertilized eggs of marine invertebrates first revealed the importance
of spindle microtubules in determining the placement of the contractile ring. After fertilization, these embryos cleave rapidly without intervening periods of growth. In this way, the original egg is progressively divided into smaller and smaller cells. Because the cytoplasm is clear, the spindle can be observed in real time with a microscope. If the spindle is tugged into a new position with a fine glass needle in early anaphase, the incipient cleavage furrow disappears, and a new one develops in accord with the new spindle site—supporting the idea that signals generated by the spindle induce local furrow formation. How does the mitotic spindle specify the site of division? Three general mechanisms
have been proposed, and most cells appear to employ a combination of these



Three current models of how the microtubules of the anaphase spindle generate signals that influence the positioning of the contractile ring. No single model explains all the
observations, and furrow positioning is probably determined by a combination of these mechanisms, with the importance of the different mechanisms varying in different organisms. See text for details.

The first is termed the astral stimulation model, in which the astral microtubules carry furrow-inducing signals, which are somehow focused in a ring on the cell cortex, halfway between the spindle poles. Evidence for this model comes from ingenious experiments in large embryonic cells, which demonstrate that a cleavage furrow forms midway between two asters, even when the two centrosomes nucleating the asters are not connected to each other by a mitotic spindle

A second possibility, called the central spindle stimulation model, is that the spindle midzone, or central spindle, generates a furrow-inducing signal that specifies the site of furrow formation at the cell cortex

A third model proposes that, in some cell types, the astral microtubules promote the local relaxation of actin–myosin bundles at the cell cortex. According to this astral relaxation model, the cortical relaxation is minimal at the spindle equator, thus promoting cortical contraction at that site

In some cell types, the site of ring assembly is chosen before mitosis. In budding yeasts, for example, a ring of proteins called septins assembles in late G1 at the future division site. The septins are thought to form a scaffold onto which other components of the contractile ring, including myosin II, assemble. In plant cells, an organized band of microtubules and actin filaments, called the preprophase band, assembles just before mitosis and marks the site where the cell wall will assemble and divide the cell in two, as we now discuss.

The Phragmoplast Guides Cytokinesis in Higher Plants

In most animal cells, the inward movement of the cleavage furrow depends on an increase in the surface area of the plasma membrane. New membrane is added at the inner edge of the cleavage furrow and is generally provided by small membrane vesicles that are transported on microtubules from the Golgi apparatus to the furrow. Membrane deposition is particularly important for cytokinesis in higher-plant cells. These cells are enclosed by a semirigid cell wall. Rather than a contractile ring dividing the cytoplasm from the outside in, the cytoplasm of the plant cell is partitioned from the inside out by the construction of a new cell wall, called the cell plate, between the two daughter nuclei



The assembly of the cell plate begins in late anaphase and is guided by a structure called the phragmoplast, which contains microtubules derived from the mitotic spindle. Motor
proteins transport small vesicles along these microtubules from the Golgi apparatus to the cell center. These vesicles, filled with polysaccharide and glycoproteins required for the synthesis of the new cell wall, fuse to form a disc-like, membraneenclosed structure called the early cell plate. The plate expands outward by further vesicle fusion until it reaches the plasma membrane and the original cell wall and divides the cell in two. Later, cellulose microfibrils are laid down within the matrix of the cell plate to complete the construction of the new cell wall



The special features of cytokinesis in a higher-plant cell. The division plane is established before M phase by a band of microtubules and actin filaments (the preprophase band)
at the cell cortex. At the beginning of telophase, after the chromosomes have segregated, a new cell wall starts to assemble inside the cell at the equator of the old spindle. The interpolar microtubules of the mitotic spindle remaining at telophase form the phragmoplast. The plus ends of these microtubules no longer overlap but end at the cell equator.
Golgi-derived vesicles, filled with cell-wall material, are transported along these microtubules and fuse to form the new cell wall, which grows outward to reach the plasma membrane and original cell wall. The plasma membrane and the membrane surrounding the new cell wall fuse, separating the two daughter cells.

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