Which of the following events do not occur in prophase of mitosis?

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You watching: Which of the following events do not occur in prophase of mitosis?

Cooper GM. The Cell: A Molecular Approach. second edition. Sunderland also (MA): Sinauer Associates; 2000.


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M phase is the the majority of dramatic duration of the cell cycle, involving a major reorganization of virtually all cell components. Throughout mitosis (nuclear division), the chromosomes conthick, the nuclear envelope of the majority of cells breaks down, the cytoskeleton reorganizes to form the mitotic spindle, and also the chromosomes relocate to opposite poles. Chromosome segregation is then usually adhered to by cell division (cytokinesis). Although many kind of of these occasions have actually been debated in previous chapters with respect to the structure and also function of the nucleus and also cytoskeleton, they are reregarded below in the context of a coordinated check out of M phase and also the action of MPF.


Steras of Mitosis

Although many type of of the details of mitosis vary among different organisms, the basic procedures that encertain the faithful segregation of sister chromatids are conoffered in all eukaryotes. These standard occasions of mitosis encompass chromosome condensation, formation of the mitotic spindle, and attachment of chromosomes to the spindle microtubules. Sister chromatids then separate from each various other and also relocate to oppowebsite poles of the spindle, followed by the formation of daughter nuclei.

Mitosis is conventionally split right into four stages—prophase, metaphase, anaphase, and telophase—which are illustrated for an animal cell in Figures 14.23 and 14.24. The start of prophase is marked by the appearance of condensed chromosomes, each of which consists of two sister chromatids (the daughter DNA molecules developed in S phase). These freshly replicated DNA molecules remajor intertwined throughout S and G2, coming to be untangled in the time of the procedure of chromatin condensation. The condensed sister chromatids are then organized together at the centromere, which (as discussed in Chapter 4) is a DNA sequence to which proteins bind to create the kinetochore—the site of ultimate attachment of the spindle microtubules. In addition to chromosome condensation, cytoplasmic changes bring about the breakthrough of the mitotic spindle initiate throughout prophase. The centrosomes (which had actually duplicated throughout interphase) separate and move to oppowebsite sides of the nucleus. There they serve as the two poles of the mitotic spindle, which begins to create during late prophase.


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Figure 14.23

Stperiods of mitosis in an pet cell. During prophase, the chromosomes condense and also centrosomes move to opposite sides of the nucleus, initiating development of the mitotic spindle. Breakdown of the nuclear envelope then permits spindle microtubules to connect (even more...)


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Figure 14.24

Fluorescence micrographs of chromatin, keratin, and microtubules in the time of mitosis of newt lung cells. Chromatin is stained blue, keratin is stained red, and also microtubules are stained green. (Conly L. Rieder/ Biological Photograph Service.)


In higher eukaryotes the end of prophase corresponds to the breakdown of the nuclear envelope. As disputed in Chapter 8, yet, nuclear envelope breakdown is not a global attribute of mitosis. In particular, yeasts and many type of other unicellular eukaryotes undergo “closed mitosis,” in which the nuclear envelope continues to be undamaged (check out Figure 8.30). In these cells the spindle pole bodies are embedded within the nuclear envelope, and the nucleus divides in 2 following migration of daughter chromosomes to oppowebsite poles of the spindle.

Following completion of prophase, the cell enters prometaphase—a shift period between prophase and metaphase. Throughout prometaphase the microtubules of the mitotic spindle attach to the kinetochores of condensed chromosomes. The kinetochores of sister chromatids are oriented on opposite sides of the chromosome, so they attach to microtubules emanating from oppowebsite poles of the spindle. The chromosomes shuffle back and forth until they inevitably align on the metaphase plate in the center of the spindle. At this phase, the cell has actually reached metaphase.

Most cells remajor only briefly at metaphase prior to proceeding to anaphase. The shift from metaphase to anaphase is motivated by breakage of the link between sister chromatids, which then separate and move to oppowebsite poles of the spindle. Mitosis ends via telophase, throughout which nuclei re-form and the chromosomes deconthick. Cytokinesis usually starts in the time of late anaphase and also is almost complete by the finish of telophase, resulting in the development of two interphase daughter cells.


MPF and Progression to Metaphase

Mitosis involves dramatic alters in multiple cellular components, leading to a major reorganization of the entire structure of the cell. As discussed previously in this chapter, these events are initiated by activation of the MPF protein kinase (Cdc2/cyclin B). It appears that MPF not only acts as a master regulator of the M phase shift, phosphorylating and activating various other downstream protein kinases, however likewise acts straight by phosphorylating some of the structural proteins associated in this cellular recompany (Figure 14.25).


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Figure 14.25

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


The condensation of interphase chromatin to create the compact chromosomes of mitotic cells is a crucial event in mitosis, instrumental in permitting the chromosomes to relocate along the mitotic spindle without coming to be broken or tangled through one one more. As discussed in Chapter 4, the chromatin in interphase nuclei condenses nearly a thousand fold throughout the development of metaphase chromosomes. Such extremely condensed chromatin cannot be transcribed, so transcription ceases as chromatin condensation takes area. In spite of the basic prominence of this event, we execute not fully understand also either the framework of metaphase chromosomes or the molecular device of chromatin condensation. However, protein complexes dubbed condensins have actually recently been uncovered to drive chromosome condensation by wrapping DNA around itself, compacting chromosomes right into the condensed mitotic structure. The condensins are phosphorylated straight by the Cdc2 protein kinase, which drives chromatin condensation by activating condensins as cells enter mitosis. One molecular alteration that mostly acsuppliers chromosome condensation is phosphorylation of hirock H1, so it is significant that hirock H1 is likewise a substrate for Cdc2. However, histone H1 phosphorylation is not forced for mitotic chromosome condensation, so the meaning of H1 phosphorylation by Cdc2 is unclear. In contrast, chromosome condensation has actually been shown to require phosphorylation of histone H3. Perhaps surprisingly, yet, histone H3 is not phosphorylated by Cdc2 and the kinase responsible for H3 phosphorylation in mitotic cells stays to be established.

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Breakdvery own of the nuclear envelope, which is among the a lot of dramatic events of mitosis, represents the many plainly identified target for MPF activity. As debated in Chapter 8, Cdc2 phosphorylates the lamins, leading straight to depolymerization of the nuclear lamina (check out Figure 8.31). This is followed by fragmentation of the nuclear membrane into small vesicles, which inevitably fusage to create brand-new daughter nuclei at telophase. The endoplasmic reticulum and Golgi apparatus similarly fragment right into tiny vesicles, which have the right to then be dispersed to daughter cells at cytokinesis. The breakdvery own of these membranes is additionally induced by MPF, and also may in part be mediated by Cdc2 phosphorylation of the Golgi matrix protein GM130, which is forced for the docking of COPI-coated vesicles to the Golgi membrane. Phosphorylation and inactivation of GM130 by Cdc2 inhibits vesicle docking and fusion, resulting in fragmentation of the Golgi apparatus. However before, additional targets of Cdc2 might additionally be involved, and the mechanisms whereby MPF leads to membrane fragmentation remajor to be completely elucidated.

The reorganization of the cytoskeleton that culminates in formation of the mitotic spindle results from the dynamic instcapacity of microtubules (see Chapter 11). At the start of prophase, the centrosomes move to oppowebsite sides of the nucleus. The climb in MPF activity then induces a dramatic adjust in the dynamic actions of microtubules. First, the price of microtubule disassembly increases, leading to depolymerization and also shrinkage of the interphase microtubules. This disassembly is thmust outcome from phosphorylation of microtubule-associated proteins, either by MPF itself or by various other MPF-caused protein kinases. In addition, the variety of microtubules emanating from the centrosomes increases, so the interphase microtubules are reput by large numbers of short microtubules radiating from the centrosomes.

The breakdvery own of the nuclear envelope then enables some of the spindle microtubules to affix to chromosomes at their kinetochores (Figure 14.26), initiating the procedure of chromosome motion that characterizes prometaphase. The proteins assembled at the kinetochore encompass microtubule electric motors that direct the motion of chromosomes towards the minus ends of the spindle microtubules, which are anchored in the centrosome. The activity of these proteins, which draw chromosomes towards the centrosome, is opposed by the growth of the spindle microtubules, which 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 to the kinetochore of a chromosome. (Conly L. Rieder/ Biological Picture Service.)


Microtubules from oppowebsite poles of the spindle inevitably affix to the two kinetochores of sister chromatids (which are situated on opposite sides of the chromosome), and the balance of pressures acting on the chromosomes leads to their alignment on the metaphase plate in the facility of the spindle (Figure 14.27). As questioned in Chapter 11, the spindle is composed of both kinetochore microtubules, which are attached to the chromosomes, and also polar microtubules, which overlap via one one more in the center of the cell. In addition, brief astral microtubules radiate exterior from the centrosomes towards the cell periphery.


Figure 14.27

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


Proteolysis and the Inactivation of MPF: Anaphase and also Telophase

As questioned previously in this chapter, a vital cell cycle checkallude monitors the alignment of chromosomes on the metaphase spindle. Once this has actually been achieved, the cell proceeds to initiate anaphase and complete mitosis. The progression from metaphase to anaphase outcomes from ubiquitin-mediated proteolysis of essential regulatory proteins, prompted by activation of a ubiquitin ligase (see Figure 7.39) referred to as the anaphase-promoting complex. Activation of the anaphase-fostering complex is induced by MPF at the start of mitosis, so MPF eventually triggers its own destruction. The anaphase-promoting complicated remains inhibited, but, till the cell passes the metaphase checkallude, after which activation of the ubiquitin destruction system brings about the change from metaphase to anaphase and progression through the rest of mitosis.

Activation of the anaphase-fostering complex leads to the deterioration of at leastern 2 crucial regulatory proteins (Figure 14.28). The oncollection of anaphase outcomes from proteolytic destruction of a protein referred to as Scc1, a component of a facility of proteins referred to as cohesins that preserve the connection between sister chromatids while they are aligned on the metaphase plate. Degradation of Scc1 is not catalyzed straight by the anaphase-fostering complicated, which rather degrades a regulatory protein called Pds1. Degradation of Pds1 consequently activates one more protein, called Esp1, which leads to proteolysis of the cohesin Scc1. Cleavage of Scc1 breaks the linkage between sister chromatids, permitting them to segregate by relocating to oppowebsite poles of the spindle (Figure 14.29). The separation of chromosomes throughout anaphase then proceeds as an outcome of the activity of a number of types of motor proteins connected through the spindle microtubules (view Figures 11.48 and 11.49).


Figure 14.28

Targets of the cyclin B proteolysis mechanism. The anaphase-cultivating complex is a ubiquitin ligase that is triggered complying with passage via the metaphase checkpoint. Its activation brings around the shift from metaphase to anaphase by resulting in (more...)


The other key regulatory protein targeted for ubiquiticountry and also degradation by the anaphase-fostering complex is cyclin B. Degradation of cyclin B leads to inactivation of MPF, which is compelled for the cell to exit mitosis and return to interphase. Many type of of the cellular alters affiliated in these transitions are simply the reversal of the events induced by MPF in the time of entry right into mitosis. For instance, reassembly of the nuclear envelope, chromatin decondensation, and the rerotate of microtubules to an interphase state probably result directly from loss of MPF task and also dephosphorylation of proteins that had actually been phosphorylated by MPF at the start of mitosis. As disputed following, inactivation of MPF also triggers cytokinesis.


Cytokinesis

The completion of mitosis is commonly accompanied by cytokinesis, providing climb to 2 daughter cells. Cytokinesis commonly initiates in late anaphase and is motivated by the inactivation of MPF, thereby coordinating nuclear and also cytoplasmic division of the cell. As questioned in Chapter 11, cytokinesis of pet cells is mediated by a contractile ring of actin and also myosin II filaments that creates beneath the plasma membrane (Figure 14.30). The place of this ring is determined by the position of the mitotic spindle, so the cell is eventually cleaved in a airplane that passes via the metaphase plate perpendicular to the spindle. Cleavage proceeds as contractivity of the actin-myosin filaments pulls the plasma membrane inward, ultimately pinching the cell in fifty percent.


Figure 14.30

Cytokinesis of animal cells. (A) Cytokinesis results from contraction of a ring of actin and also myosin filaments, which pinches the cell in 2. (B) Scanning electron micrograph of a frog egg undergoing cytokinesis. (B, David M. Phillips/Visuals Unlimited). (even more...)


The mechanism of cytokinesis is different for greater plant cells, which are surrounded by rigid cell walls. Rather than being pinched in fifty percent by a contractile ring, these cells divide by creating new cell wall surfaces and plasma membranes inside the cell (Figure 14.31). In early on telophase, vesicles transporting cell wall precursors from the Golgi apparatus associate with spindle microtubules and also accumulate at the previous website of the metaphase plate. These vesicles then fusage to develop a huge, membrane-enclosed, discchoose structure, and their polysaccharide contents assemble to form the matrix of a new cell wall (dubbed a cell plate). The cell plate broadens outside, perpendicular to the spindle, until it reaches the plasma membrane. The membrane bordering the cell plate then fuses through the parental plasma membrane, dividing the cell in two.


Figure 14.31

Cytokinesis in greater plants. Golgi vesicles moving cell wall precursors associate through polar microtubules at the previous website of the metaphase plate. Fusion of these vesicles returns a membrane-enclosed, discchoose framework (the beforehand cell plate) that (even more...)


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