In which stage the cell begins a stage by which the cytoplasm divides resulting in two cells with identical nuclei?

The cell cycle culminates in the division of the cytoplasm by cytokinesis. In a typical cell, cytokinesis accompanies every mitosis, although some cells, such as Drosophila embryos (discussed later) and vertebrate osteoclasts (discussed in Chapter 22), undergo mitosis without cytokinesis and become multinucleate. Cytokinesis begins in anaphase and ends in telophase, reaching completion as the next interphase begins.

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. In animal cells and many unicellular eucaryotes, the structure that accomplishes cytokinesis is the contractile ring—a dynamic assembly composed of actin filaments, myosin II filaments, and many structural and regulatory proteins. The ring assembles just beneath the plasma membrane and contracts to constrict the cell into two (see Figure 18-4). At the same time, new membrane is inserted into the plasma membrane adjacent to the contractile ring by the fusion of intracellular vesicles. This addition of membrane is required to compensate for the increase in surface area that accompanies cytoplasmic division. Thus, cytokinesis can be considered to occur in four stages—initiation, contraction, membrane insertion, and completion.

The central problem for a cell undergoing cytokinesis is to ensure that it occurs at the right time and in the right place. Cytokinesis must not occur too early in M-phase, or it will disrupt the path of the separating chromosomes. It must also occur at the right place to separate the two segregating sets of chromosomes properly so that each daughter cell receives a complete set.

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

The mitotic spindle in animal cells not only separates the daughter chromosomes, it also specifies the location of the contractile ring, and thereby the plane of cell division. The contractile ring invariably forms in the plane of the metaphase plate, at right angles to the long axis of the mitotic spindle, thereby ensuring that division occurs between the two sets of separated chromosomes. The part of the spindle that specifies the division plane varies depending on the cell type: in some cells, it is the astral microtubules; in others, it is the overlapping antiparallel microtubules in the central spindle.

The relationship between the spindle microtubules and the placement of the contractile ring has been studied by manipulating fertilized eggs of marine invertebrates. After fertilization, these embryos undergo a series of rapid cleavage divisions, without intervening periods of growth. In this way, the original egg is progressively divided up into smaller and smaller cells. During cytokinesis, the cleavage furrow appears suddenly on the surface of the cell and deepens rapidly (Figure 18-30). Because the cytoplasm is clear, the spindle can be observed in real time through 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.

Figure 18-30

Cleavage in a fertilized frog egg. In these scanning electron micrographs, the cleavage furrow is especially obvious and well defined, as the cell is unusually large. The furrowing of the cell membrane is caused by the activity of the contractile ring (more...)

How does the mitotic spindle control the plane of division? Ingenious experiments in large embryonic cells demonstrate that a cleavage furrow forms midway between the asters originating from the two centrosomes, even when the two centrosomes are not connected to each other by a mitotic spindle (Figure 18-31). Thus, in these cells, the microtubule asters—not the chromosomes or other parts of the spindle—signal to the cell cortex to specify where the contractile ring should assemble. In other cells, the central spindle, rather than the astral microtubules, is apparently responsible for this specification. In either case, it has been speculated that the overlapping microtubules may provide tracks for motor proteins to deliver contractile ring regulators, and perhaps new membrane, to the appropriate region of the dividing cell. But, in fact, the molecular mechanism by which the spindle positions the cleavage furrow remains a mystery.

Figure 18-31

An experiment demonstrating the influence of the position of microtubule asters on the subsequent plane of cleavage in a large egg cell. If the mitotic spindle is mechanically pushed to one side of the cell with a glass bead, the membrane furrowing is (more...)

In some cells, the site of ring assembly is chosen before mitosis, according to a landmark placed in the cortex during a previous cell cycle. In budding yeasts, for example, a ring of proteins called septins assembles before mitosis, adjacent to a bud scar left on the cell surface as the mother and daughter cells separated in the previous division. The septins are thought to form a scaffold onto which other components of the contractile ring, including myosin II, assemble. As we discuss later, in plant cells, an organized band of microtubules and actin filaments assembles just before mitosis and marks the site where the cell wall will assemble and divide the cell in two.

Some Cells Reposition Their Spindle to Divide Asymmetrically

Most cells divide symmetrically. In most animal cells, for example, the contractile ring forms around the equator of the parent cell, so that the two daughter cells produced are of equal size and have similar properties. This symmetry results from the placement of the mitotic spindle, which in most cases tends to center itself in the cytoplasm. The centering process depends both on astral microtubules and on motor proteins that either push or pull on the astral microtubules to center the spindle.

There are many instances in development, however, when cells divide asymmetrically to produce two cells that differ in size, in the cytoplasmic contents they inherit, or in both. Usually, the two daughter cells are destined to develop along different pathways. To create daughter cells with different fates, the mother cell must first segregate some components (called fate determinants) to one side of the cell and then position the plane of division so that the appropriate daughter cell inherits these components (Figure 18-32). To position the plane of division asymmetrically, the spindle has to be moved in a controlled manner within the dividing cell. It seems likely that such spindle movements are directed by changes in local regions of the cell cortex and that motor proteins localiazed there pull one of the spindle poles, via its astral microtubules, to the appropriate region (Figure 18-33). Some of the proteins required for such asymmetrical divisions have been identified through genetic analyses in C. elegans and Drosophila (discussed in Chapter 21), and some of these seem to have a similar role in vertebrates.

Figure 18-32

An asymmetric cell division segregating cytoplasmic components to only one daughter cell. These light micrographs illustrate the controlled asymmetric segregation of specific cytoplasmic components to one daughter cell during the first division of a fertilized (more...)

Figure 18-33

Spindle rotation. (A) A possible mechanism underlying the controlled rotation of a mitotic spindle. The red bar represents a specialized region of cell cortex toward which one spindle pole is pulled by its astral microtubules. (B) Fluorescence micrographs (more...)

Asymmetric division is particularly important in plant cells. As these cells cannot move after division, the selection of division planes is crucial for controlling tissue morphology. We discuss later how the plane of division is determined in these cells.

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

As the astral microtubules in anaphase become longer lived and less dynamic in response to the loss of M-Cdk activity, the contractile ring begins to assemble beneath the plasma membrane. Much of the preparation for cytokinesis, however, happens earlier in mitosis, before the division of the cytoplasm actually begins. In interphase cells, actin and myosin filaments are assembled into a cortical network and, in some cells, also into large cytoplasmic bundles called stress fibers (discussed in Chapter 16). As cells enter mitosis, these arrays disassemble; much of the actin is reorganized, and myosin II filaments are released. As the chromatids separate in anaphase, myosin II begins to accumulate in the rapidly assembling contractile ring (Figure 18-34).

Figure 18-34

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 (more...)

In many cells, cytokinesis requires the activation of one or more members of the polo-like family of protein kinases. These kinases regulate the assembly of both the mitotic spindle and the contractile ring and are therefore thought to help coordinate mitosis and cytokinesis, but it is uncertain how they do so. The fully assembled contractile ring contains many proteins in addition to actin and myosin II. The overlapping arrays of actin filaments and bipolar myosin II filaments, however, generate the force that divides the cytoplasm in two. They are thought to contract by a mechanism that is biochemically similar to that used by smooth muscle cells; in both cases, for example, the contraction begins when Ca2+-calmodulin activates myosin light-chain kinase to phosphorylate myosin II. Once contraction has been stimulated, the ring develops a force large enough to bend a fine glass needle that is inserted in the path of the constricting ring.

How the contractile ring constricts is still a mystery. It seems not to operate by a simple “purse-string” mechanism, with actin and myosin II filaments sliding past each other as in skeletal muscle (see Figure 16-71). As the ring constricts, the ring maintains the same thickness in cross-section, suggesting that its total volume and the number of filaments it contains decrease steadily. Moreover, unlike in muscle, the actin filaments in the ring are highly dynamic, and their arrangement changes extensively during cytokinesis.

In addition to specifying the site of contractile ring assembly in early anaphase, in many cells, microtubules also work continuously during anaphase and telophase to stabilize the advancing cleavage furrow. Drugs that depolymerize microtubules, for example, cause the actin filaments in the contractile ring to become less organized. Moreover, if a needle is used to tear microtubules away from the cell cortex, the contractile ring disassembles and the cleavage furrow regresses. It is not known how the microtubules stabilize the ring, although it has been shown that growing microtubules can activate some members of the Rho family of small GTPases, which in turn stimulate actin polymerization (discussed in Chapter 16). One member of this family, Rho A, is required for 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, which now consists of the two sets of antiparallel overlap microtubules packed tightly together within a dense matrix material (Figure 18-35). Remarkably, in some cells, before cytokinesis has been completed, the mother centriole from one or both daughter cells separates from its daughter centriole (see Figure 18-5c) and migrates into the midbody, where it lingers for minutes, before returning to its daughter cell. Only then do the two daughter cells separate to complete cytokinesis. What the centriole might do in the midbody to trigger the final steps of cytokinesis is not known. 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.

Figure 18-35

The midbody. (A) A scanning electron micrograph of an animal cell in culture in the process of dividing; the midbody still joins the two daughter cells. (B) A conventional electron micrograph of the midbody of a dividing animal cell. Cleavage is almost (more...)

Membrane-enclosed Organelles Must Be Distributed to Daughter Cells During Cytokinesis

The process of mitosis ensures that each daughter cell receives a full complement of chromosomes. But when a eucaryotic cell divides, each daughter cell must also inherit all of the other essential cell components, including the membrane-enclosed organelles. As discussed in Chapter 12, organelles like mitochondria and chloroplasts cannot assemble spontaneously from their individual components; they can arise only from the growth and division of the preexisting organelles. Similarly, cells cannot make a new endoplasmic reticulum (ER) unless some part of it is already present.

How, then, are the various membrane-enclosed organelles segregated when a cell divides? Organelles such as mitochondria and chloroplasts are usually present in large enough numbers to be safely inherited if, on average, their numbers roughly double once each cycle. The ER in interphase cells is continuous with the nuclear membrane and is organized by the microtubule cytoskeleton. Upon entry into M phase, the reorganization of the microtubules releases the ER, which fragments as the nuclear envelope breaks down. The Golgi apparatus probably fragments as well, although in some cells it seems to redistribute transiently into the ER, only to re-emerge at telophase. Some of the organelle fragments associate with the spindle microtubules via motor proteins, thereby hitching a ride into the daughter cells as the spindle elongates in anaphase.

Mitosis Can Occur Without Cytokinesis

Although nuclear division is usually followed by cytoplasmic division, there are exceptions. Some cells undergo multiple rounds of nuclear division without intervening cytoplasmic division. In the early Drosophila embryo, for example, the first 13 rounds of nuclear division occur without cytoplasmic division, resulting in the formation of a single large cell containing 6000 nuclei, arranged in a monolayer near the surface (Figure 18-36). This arrangement greatly speeds up early development, as the cells do not have to take the time to go through all the steps of cytokinesis for each division. After these rapid nuclear divisions, cells are created around each nucleus in one round of coordinated cytokinesis called cellularization. Contractile rings form at the cell surface, and the plasma membrane extends inward and pinches off to enclose each nucleus.

Figure 18-36

Mitosis without cytokinesis in the Drosophila embryo. (A) The first 13 nuclear divisions occur synchronously and without cytoplasmic division to create a large syncytium. Most of the nuclei then migrate to the cortex, and the plasma membrane extends (more...)

Nuclear division without cytokinesis also occurs in some types of mammalian cells. Osteoclasts, trophoblasts, and some hepatocytes and heart muscle cells, for example, become multinucleated in this way.

The Phragmoplast Guides Cytokinesis in Higher Plants

Most higher-plant cells are enclosed by a semirigid cell wall, and their mechanism of cytokinesis is different from that just described for animal cells. 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 (Figure 18-37). The orientation of the cell plate determines the positions of the two daughter cells relative to neighboring cells. It follows that altering the planes of cell division, together with enlargement of the cells by expansion or growth, leads to different cell and tissue shapes that help determine the form of the plant.

Figure 18-37

Cytokinesis in a plant cell in telophase. In this light micrograph, the early cell plate (between the two arrowheads) is forming in a plane perpendicular to the plane of the page. The microtubules of the spindle are stained with gold-labeled antibodies (more...)

The mitotic spindle by itself is not sufficient to determine the exact position and orientation of the cell plate. The first visible sign that a higher-plant cell has become committed to divide in a particular plane is seen in G2, when the cortical array of microtubules disappears in preparation for mitosis. At this time, a circumferential band of microtubules and actin filaments forms a ring around the entire cell just beneath the plasma membrane. Because this cytoskeletal array appears before prophase begins, it is called the preprophase band. The band becomes thinner as the cell progresses to prophase, and it disappears completely before metaphase is reached. Yet, the division plane has somehow been established: when the new cell plate forms later during cytokinesis, it grows outward to fuse with the parental wall precisely at the zone formerly occupied by the preprophase band. Even if the cell contents are displaced by centrifugation after the preprophase band has disappeared, the growing cell plate tends to find its way back to the plane defined by the former preprophase band.

The assembly of the cell plate begins in late anaphase and is guided by a structure called the phragmoplast, which contains the remaining overlap microtubules of the mitotic spindle that interdigitate at their growing plus ends. This region of overlap is similar in structure to the central spindle in animal cells in late anaphase. Small vesicles, largely derived from the Golgi apparatus and filled with polysaccharide and glycoproteins required for the synthesis of the new cell-wall matrix, are transported along the microtubules to the equator of the phragmoplast, apparently by the action of microtubule-dependent motor proteins. Here, the vesicles fuse to form a disclike, membrane-enclosed structure called the early cell plate (see Figure 18-9G). 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 (Figure 18-38).

Figure 18-38

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 (more...)

The Elaborate M Phase of Higher Organisms Evolved Gradually from Procaryotic Fission Mechanisms

Procaryotic cells divide by a process called binary fission. The single, circular DNA molecule replicates and division occurs by the invagination of the plasma membrane and the laying down of new cell wall between the two chromosomes to produce two separate daughter cells. In E. coli, before the chromosome replicates, the single origin of replication (oriC) is located at one pole of the rod-shaped bacterium. As soon as oriC is replicated, one copy of the sequence is immediately translocated to the opposite pole of the cell, after which the rest of the chromosome is replicated. Like the two spindle-pole asters in an animal cell, the bacterial daughter chromosomes at the cell poles somehow determine the location of the plane of cell division, ensuring that fission takes place at the cell equator, so that each daughter cell inherits one chromosome (Figure 18-39). Although a number of genes and proteins involved have been identified, the mechanisms responsible for the active translocation of oriC and the inhibition of fission everywhere but at the equator remain unknown.

Figure 18-39

Cell division in the bacterium E. coli. The single, circular chromosome contains an origin of replication called oriC. Before division, the chromosome is polarized, so that oriC is at one pole of the bacterium. As soon as the oriC sequence is copied, (more...)

Binary fission in procaryotes depends on filaments made of the FtsZ protein. FtsZ is a cytoskeletal GTPase that is structurally related to tubulin and assembles into a ring at the equator of the cell (Figure 18-40A, and see Figure 16-17). The FtsZ filaments are essential for the recruitment of all the other cell division proteins to the division site. Together, these proteins guide the inward growth of the cell wall and membrane, leading to the formation of a septum that divides the cell into two. Bacteria in which the ftsZ gene is inactivated by mutation cannot divide. A FtsZ-based mechanism is also used in the division of chloroplasts in plant cells (Figure 18-40B) and mitochondria in protists. In fungi and animal cells, another self-assembling GTPase called dynamin (discussed in Chapter 13) has apparently taken over the function of FtsZ in mitochondrial division.

Figure 18-40

The FtsZ protein. (A) Fluorescence micrographs showing the location of the FtsZ protein during binary fission in E. coli. The protein assembles into a ring at the center of the cell, where it helps orchestrate cell division. The bacteria here have been (more...)

With the evolution of the eucaryotes, the genome increased in complexity, and the chromosomes increased in both number and size. For these organisms, a more elaborate mechanism for dividing the chromosomes between daughter cells was apparently required. Clearly, the mitotic apparatus could not have evolved all at once. In many primitive eucaryotes, such as the dinoflagellate Cryphthecodinium cohnii, mitosis depends on a membrane-attachment mechanism, in which the chromosomes have to bind to the inner nuclear membrane for segregation. The intermediate status of this large, single-celled alga is reflected in the composition of its chromosomes, which, like those of procaryotes, have relatively little associated protein. The nuclear membrane in C. cohnii remains intact throughout mitosis, and the spindle microtubules remain entirely outside the nucleus. Where these spindle microtubules press on the outside of the nuclear envelope, the envelope becomes indented in a series of parallel channels (Figure 18-41). The chromosomes become attached to the inner membrane of the nuclear envelope opposite these channels, and chromosome segregation occurs on the inside of this channeled nuclear membrane. Thus, the extranuclear “spindle” is used to order the nuclear membrane and thereby define the plane of division. Kinetochores in this species seem to be integrated into the nuclear membrane and may therefore have evolved from some membrane component.

Figure 18-41

The use of different chromosome separation mechanisms by different organisms. Some of these may have been intermediate stages in the evolution of the mitotic spindle of higher organisms. For all examples except bacteria, only the central nuclear region (more...)

Eucaryotic tubulin and procaryotic FtsZ clearly have a common evolutionary history. But, microtubules are important for chromosome segregation in even the most primitive eucaryotes, where they are also present in flagellar axonemes (discussed in Chapter 16). Whether the flagellum or the spindle evolved first is unclear.

A somewhat more advanced, although still extranuclear, spindle is seen in hypermastigotes, in which the nuclear envelope again remains intact throughout mitosis. These large protozoa from the guts of insects provide a particularly clear illustration of the independence of spindle elongation and the chromosome movements that separate the chromatids. The sister kinetochores become separated by the growth of the nuclear membrane (to which they are attached) before becoming attached to the spindle. Only when the kinetochores are near the poles of the spindle do they acquire the kinetochore microtubules needed to attach them to the spindle. Because the spindle microtubules remain separated from the chromosomes by the nuclear envelope, the kinetochore microtubules, which are formed outside the nucleus, must somehow attach to the chromosomes through the nuclear membranes. After this attachment has occurred, the kinetochores are drawn poleward in a conventional manner (see Figure 18-41).

Organisms that form spindles inside an intact nucleus may represent a further stage in the evolution of mitotic mechanisms. In both yeasts and diatoms, the spindle is attached to chromosomes by their kinetochores, and the chromosomes are segregated in a way loosely similar to that described for animal cells—except that the entire process generally occurs within the confines of the nuclear envelope (see Figure 18-41). It is thought that the “open” mitosis of higher organisms and the “closed” mitosis of yeasts and diatoms evolved separately from a common ancestor resembling the modern hypermastigote spindle. At present, there is no convincing explanation for why higher plants and animals have evolved a mitotic apparatus that requires the controlled and reversible dissolution of the nuclear envelope.

Summary

Cell division ends as the cytoplasm divides into two by the process of cytokinesis. Except for plants, cytokinesis in eucaryotic cells is mediated by a contractile ring, which is composed of actin and myosin filaments and a variety of other proteins. By an unknown mechanism, the mitotic spindle determines when and where the contractile ring assembles and, thereby, when and where the cell divides. Most cells divide symmetrically to produce two cells of the same content and size. Some cells, however, specifically position their spindle to divide asymmetrically, producing two daughter cells that differ in size, content, or both. Cytokinesis occurs by a special mechanism in higher-plant cells—in which the cytoplasm is partitioned by the construction of a new cell wall, the cell plate, inside the cell. The position of the cell plate is determined by the position of a preprophase band of microtubules and actin filaments. The organization of mitosis in fungi and some protozoa differs from that in animals and plants, suggesting how the complex process of eucaryotic cell division may have evolved.

During which phase does the cytoplasm divide into two identical cells?

What stage is the start of two identical cells?

What stage does the cell divide into 2?