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You are watching: Which of the following events occurs during prophase?

Cooper GM. The Cell: A molecule Approach. Second edition. Sunderland (MA): Sinauer Associates; 2000.


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M phase is the many dramatic duration of the cell cycle, involving a significant reorganization of essentially all cell components. Throughout mitosis (nuclear division), the chromosomes condense, the atom envelope of many cells division down, the cytoskeleton reorganizes to type the mitotic spindle, and also the chromosomes relocate to the opposite poles. Chromosome distinction is climate usually followed by cell department (cytokinesis). Although many of these events have been disputed in ahead chapters with respect come the structure and role of the nucleus and also cytoskeleton, they room reviewed below in the context of a coordinated view of M phase and the activity of MPF.


Stages that Mitosis

Although countless of the details of mitosis vary among different organisms, the an essential processes that ensure the faithful distinction of sister chromatids space conserved in all eukaryotes. These straightforward events that mitosis encompass chromosome condensation, formation of the mitotic spindle, and attachment of chromosomes come the spindle microtubules. Sister chromatids then separate from each other and move to opposite poles of the spindle, adhered to by the formation of daughter nuclei.

Mitosis is conventionally divided into four stages—prophase, metaphase, anaphase, and also telophase—which are depicted for an animal cell in numbers 14.23 and also 14.24. The start of prophase is significant by the figure of condensed chromosomes, each of which is composed of two sister chromatids (the daughter DNA molecules created in S phase). These newly replicated DNA molecule remain linked throughout S and G2, ending up being untangled throughout the procedure of chromatin condensation. The condensed sisters chromatids room then held together at the centromere, i m sorry (as discussed in thing 4) is a DNA sequence to i beg your pardon proteins bind to form the kinetochore—the website of eventual attachment that the spindle microtubules. In addition to chromosome condensation, cytoplasmic changes leading to the development of the mitotic spindle initiate during prophase. The centrosomes (which had duplicated during interphase) separate and move come opposite political parties of the nucleus. There they offer as the two poles of the mitotic spindle, which starts to form during late prophase.


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Figure 14.23

Stages that mitosis in an pet cell. Throughout prophase, the chromosomes condense and also centrosomes relocate to opposite political parties of the nucleus, initiating formation of the mitotic spindle. Breakdown of the nuclear envelope then permits spindle microtubules to attach (more...)


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Figure 14.24

Fluorescence micrographs that chromatin, keratin, and also microtubules throughout mitosis that newt lung cells. Chromatin is stained blue, keratin is stained red, and also microtubules are stained green. (Conly L. Rieder/ biological Photo Service.)


In higher eukaryotes the end of prophase corresponds to the breakdown of the nuclear envelope. As discussed in chapter 8, however, atom envelope break down is no a universal feature of mitosis. In particular, yeasts and many various other unicellular eukaryotes undergo “closed mitosis,” in i m sorry the atom envelope remains intact (see number 8.30). In these cells the spindle pole bodies are installed within the nuclear envelope, and the cell core divides in two following migration of daughter chromosomes come opposite poles the the spindle.

Following perfect of prophase, the cabinet enters prometaphase—a transition period between prophase and metaphase. During prometaphase the microtubules that the mitotic spindle affix to the kinetochores that condensed chromosomes. The kinetochores the sister chromatids are oriented ~ above opposite political parties of the chromosome, so they attach to microtubules create from the contrary poles the the spindle. The chromosomes shuffle ago and forth till they at some point align top top the metaphase plate in the facility of the spindle. At this stage, the cell has actually reached metaphase.

Most cells remain just briefly at metaphase before proceeding to anaphase. The shift from metaphase to anaphase is triggered by not correct of the link between sister chromatids, which climate separate and move come opposite poles that the spindle. Mitosis ends through telophase, during which nuclei re-form and also the chromosomes decondense. Cytokinesis usually begins during late anaphase and is virtually complete by the finish of telophase, bring about the formation of 2 interphase daughter cells.


MPF and Progression come Metaphase

Mitosis involves dramatic alters in multiple cellular components, leading to a major reorganization that the whole structure that the cell. As discussed earlier in this chapter, these occasions are initiated by activation of the MPF protein kinase (Cdc2/cyclin B). It appears that MPF not only acts together a master regulator that the M phase transition, phosphorylating and activating other downstream protein kinases, but likewise acts directly by phosphorylating few of the structure proteins involved in this to move reorganization (Figure 14.25).


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Figure 14.25

Targets of MPF. MPF cause multiple nuclear and cytoplasmic transforms at the beginning of M phase, both by activating other protein kinases and by phosphorylating proteins such as condensins and the nuclear lamins.


The condensation of interphase chromatin to form the compact chromosomes of mitotic cells is a an essential event in mitosis, vital in permitting the chromosomes to relocate along the mitotic spindle without becoming broken or tangled through one another. As disputed in thing 4, the chromatin in interphase nuclei condenses practically a thousand fold during the formation of metaphase chromosomes. Such highly condensed chromatin cannot be transcribed, so warrior ceases as chromatin condensation takes place. Regardless of the an essential importance of this event, we carry out not totally understand either the framework of metaphase chromosomes or the molecular device of dyed condensation. However, protein complexes called condensins have actually recently been discovered to journey chromosome condensation by wrapping DNA around itself, compacting chromosomes into the condensed mitotic structure. The condensins are phosphorylated directly by the Cdc2 protein kinase, i beg your pardon drives chromatin condensation by activating condensins as cells get in mitosis. One molecular modification that typically accompanies chromosome condensation is phosphorylation that histone H1, so the is noteworthy that histone H1 is likewise a substrate for Cdc2. However, histone H1 phosphorylation is not forced for mitotic chromosome condensation, for this reason the significance of H1 phosphorylation by Cdc2 is unclear. In contrast, chromosome condensation has been shown to require phosphorylation the histone H3. Perhaps surprisingly, however, histone H3 is not phosphorylated by Cdc2 and also the kinase responsible because that H3 phosphorylation in mitotic cells continues to be to be identified.

Breakdown the the atom envelope, which is one of the many dramatic occasions of mitosis, to represent the most plainly defined target because that MPF action. As discussed in chapter 8, Cdc2 phosphorylates the lamins, leading straight to depolymerization the the nuclear lamina (see number 8.31). This is complied with by fragmentation the the nuclear membrane into tiny vesicles, which at some point fuse to kind new daughter nuclei in ~ telophase. The absorbent reticulum and Golgi apparatus likewise fragment into small vesicles, which have the right to then be dispersed to daughter cells at cytokinesis. The break down of these membranes is likewise induced through MPF, and also may in component be mediated through Cdc2 phosphorylation that the Golgi matrix protein GM130, i m sorry is required for the docking the COPI-coated vesicles come the Golgi membrane. Phosphorylation and also inactivation that GM130 by Cdc2 inhibits vesicle docking and also fusion, leading to fragmentation that the Golgi apparatus. However, additional targets the Cdc2 may also be involved, and the mechanisms through which MPF leads to membrane fragmentation stay to be completely elucidated.

The reorganization that the cytoskeleton the culminates in development of the mitotic spindle outcomes from the dynamic instability that microtubules (see thing 11). At the start of prophase, the centrosomes move to opposite sides of the nucleus. The climb in MPF task then induces a dramatic readjust in the dynamic actions of microtubules. First, the rate of microtubule disassembly increases, resulting in depolymerization and shrinkage of the interphase microtubules. This disassembly is believed to an outcome from phosphorylation that microtubule-associated proteins, one of two people by MPF itself or by various other MPF-activated protein kinases. In addition, the number of microtubules emanating from the centrosomes increases, therefore the interphase microtubules are replaced by large numbers of brief microtubules radiating from the centrosomes.

The malfunction of the nuclear envelope then allows some that the spindle microtubules to affix to chromosomes at your kinetochores (Figure 14.26), initiating the procedure of chromosome movement that characterizes prometaphase. The protein assembled at the kinetochore encompass microtubule electric motors that direct the movement of chromosomes toward the minus ends of the spindle microtubules, which space anchored in the centrosome. The action of this proteins, which attract chromosomes toward the centrosome, is opposed by the growth of the spindle microtubules, i m sorry pushes the chromosomes away from the spindle poles. Consequently, the chromosomes in prometaphase shuffle ago and forth in between the centrosomes and the center of the spindle.


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Figure 14.26

Electron micrograph of microtubules attached come the kinetochore of a chromosome. (Conly L. Rieder/ organic Photo Service.)


Microtubules native opposite poles that the spindle eventually affix to the 2 kinetochores the sister chromatids (which are situated on opposite sides of the chromosome), and also the balance of pressures acting ~ above the chromosomes leader to their alignment on the metaphase bowl in the facility of the spindle (Figure 14.27). As debated in thing 11, the spindle consists of both kinetochore microtubules, which are attached to the chromosomes, and polar microtubules, i beg your pardon overlap v one one more in the center of the cell. In addition, short astral microtubules radiate exterior from the centrosomes towards the cabinet periphery.


Figure 14.27

The metaphase spindle. (A) The spindle is composed of three kinds of microtubules. Kinetochore microtubules are attached come chromosomes, polar microtubules overlap in the center of the cell, and also astral microtubules radiate from the centrosome come the cabinet (more...)


Proteolysis and the Inactivation that MPF: Anaphase and also Telophase

As debated earlier in this chapter, an important cell bike checkpoint monitors the alignment of chromosomes on the metaphase spindle. When this has been accomplished, the cabinet proceeds come initiate anaphase and also complete mitosis. The development from metaphase come anaphase results from ubiquitin-mediated proteolysis of an essential regulatory proteins, triggered by activation the a ubiquitin ligase (see figure 7.39) called the anaphase-promoting complex. Activation that the anaphase-promoting complex is induced through MPF in ~ the beginning of mitosis, for this reason MPF eventually triggers its own destruction. The anaphase-promoting complex remains inhibited, however, until the cell passes the metaphase checkpoint, after i m sorry activation that the ubiquitin deterioration system brings around the transition from metaphase come anaphase and progression through the remainder of mitosis.

Activation the the anaphase-promoting facility leads to the deterioration of at the very least two key regulatory proteins (Figure 14.28). The beginning of anaphase results from proteolytic destruction of a protein dubbed Scc1, a ingredient of a complex of proteins dubbed cohesins that preserve the connection between sister chromatids if they room aligned ~ above the metaphase plate. Destruction of Scc1 is no catalyzed directly by the anaphase-promoting complex, which rather degrades a regulation protein referred to as Pds1. Destruction of Pds1 in turn activates one more protein, dubbed Esp1, which leads to proteolysis of the cohesin Scc1. Cleavage that Scc1 breaks the linkage between sister chromatids, permitting them come segregate by relocating to the contrary poles the the spindle (Figure 14.29). The separation the chromosomes during anaphase climate proceeds as a an outcome of the action of several types of motor proteins linked with the spindle microtubules (see figures 11.48 and also 11.49).


Figure 14.28

Targets the the cyclin B proteolysis system. The anaphase-promoting complex is a ubiquitin ligase the is activated adhering to passage through the metaphase checkpoint. Its activation brings around the shift from metaphase come anaphase by leading to (more...)


The other crucial regulatory protein targeted because that ubiquitination and degradation by the anaphase-promoting facility is cyclin B. Destruction of cyclin B leader to inactivation of MPF, which is forced for the cell to leave mitosis and also return to interphase. Many of the to move changes connected in these transitions are simply the reversal that the occasions induced through MPF throughout entry right into mitosis. For example, reassembly of the atom envelope, chromatin decondensation, and also the return the microtubules come an interphase state probably an outcome directly indigenous loss that MPF activity and dephosphorylation of proteins that had been phosphorylated through MPF in ~ the start of mitosis. As debated next, inactivation the MPF also triggers cytokinesis.


Cytokinesis

The perfect of mitosis is usually accompanied through cytokinesis, providing rise to 2 daughter cells. Cytokinesis normally initiates in so late anaphase and also is triggered by the inactivation that MPF, thereby coordinating nuclear and also cytoplasmic department of the cell. As debated in thing 11, cytokinesis of animal cells is mediated by a contractile ring that actin and myosin II filaments that forms beneath the plasma membrane (Figure 14.30). The ar of this ring is figured out by the place of the mitotic spindle, for this reason the cabinet is eventually cleaved in a aircraft that passes through the metaphase bowl perpendicular to the spindle. Cleavage proceeds together contraction the the actin-myosin filaments traction the plasma membrane inward, eventually pinching the cabinet in half.


Figure 14.30

Cytokinesis of pet cells. (A) Cytokinesis outcomes from convulsion of a ring of actin and myosin filaments, i m sorry pinches the cell in two. (B) Scanning electron micrograph of a frog egg experience cytokinesis. (B, David M. Phillips/Visuals Unlimited). (more...)


The mechanism of cytokinesis is various for higher plant cells, which are surrounded through rigid cell walls. Rather than gift pinched in fifty percent by a contractile ring, this cells division by forming new cell walls and also plasma membranes inside the cell (Figure 14.31). In at an early stage telophase, vesicles transporting cell wall precursors native the Golgi apparatus associate with spindle microtubules and accumulate at the former site the the metaphase plate. These vesicles climate fuse to type a large, membrane-enclosed, disclike structure, and their polysaccharide contents assemble to form the procession of a new cell wall (called a cabinet plate). The cabinet plate broadens outward, perpendicular come the spindle, until it will the plasma membrane. The membrane bordering the cell plate then fuses with the parental plasma membrane, separating the cell in two.

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Figure 14.31

Cytokinesis in greater plants. Golgi vesicles transporting cell wall surface precursors associate through polar microtubules at the previous site the the metaphase plate. Combination of this vesicles yields a membrane-enclosed, disclike structure (the beforehand cell plate) the (more...)


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