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You are watching: Why is it important for the nuclear membrane to disintegrate during mitosis

Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland also (MA): Sinauer Associates; 2000.

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A distinct attribute of the nucleus is that it disassembles and re-forms each time the majority of cells divide. At the beginning of mitosis, the chromosomes condense, the nucleolus disshows up, and the nuclear envelope breaks dvery own, leading to the release of many of the contents of the nucleus into the cytoplasm. At the finish of mitosis, the procedure is reversed: The chromosomes deconthick, and also nuclear envelopes re-create about the separated sets of daughter chromosomes. Chapter 14 presents an extensive conversation of mitosis; in this area we will certainly take into consideration the mechanisms affiliated in the disassembly and re-formation of the nucleus. The process is managed mainly by reversible phosphorylation and dephosphorylation of nuclear proteins resulting from the activity of the Cdc2 protein kinase, which is a crucial regulator of mitosis in all eukaryotic cells.

Dissolution of the Nuclear Envelope

In the majority of cells, the disassembly of the nuclear envelope marks the finish of the prophase of mitosis (Figure 8.29). However before, this disassembly of the nucleus is not a global attribute of mitosis and also does not occur in all cells. Some unicellular eukaryotes (e.g., yeasts) undergo so-called closed mitosis, in which the nuclear envelope continues to be intact (Figure 8.30). In closed mitosis, the daughter chromosomes migrate to oppowebsite poles of the nucleus, which then divides in two. The cells of higher eukaryotes, however, normally undergo open up mitosis, which is characterized by breakdown of the nuclear envelope. The daughter chromosomes then move to oppowebsite poles of the mitotic spindle, and brand-new nuclei reassemble around them.


Figure 8.29

The nucleus in the time of mitosis. Micrographs showing the gradual stperiods of mitosis in a plant cell. During prophase, the chromosomes conthick, the nucleolus disshows up, and also the nuclear envelope breaks dvery own. At metaphase, the condensed chromosomes (even more...)


Figure 8.30

Closed and open up mitosis. In closed mitosis, the nuclear envelope stays intact and also chromosomes migrate to opposite poles of a spindle within the nucleus. In open up mitosis, the nuclear envelope breaks dvery own and then re-creates approximately the 2 sets of separated (more...)

Disassembly of the nuclear envelope, which parallels a comparable breakdvery own of the endoplasmic reticulum, involves changes in all three of its components: The nuclear membranes are fragmentized into vesicles, the nuclear pore complexes dissociate, and also the nuclear lamina depolymerizes. The best understood of these occasions is depolymerization of the nuclear lamina—the meshwork-related of filaments underlying the nuclear membrane. The nuclear lamina is written of fibrous proteins, lamins, which associate with each various other to develop filaments. Disassembly of the nuclear lamina results from phosphorylation of the lamins, which reasons the filaments to break down into individual lamin dimers (Figure 8.31). Phosphorylation of the lamins is catalyzed by the Cdc2 protein kinase, which was introduced in Chapter 7 (watch Figure 7.40) and will be debated in information in Chapter 14 as a central regulator of mitosis. Cdc2 (and also various other protein kinases activated in mitotic cells) phosphorylates all the different kinds of lamins, and also therapy of isolated nuclei with Cdc2 has actually been presented to be sufficient to induce depolymerization of the nuclear lamina. In addition, the requirement for lamin phosphorylation in the breakdown of the nuclear lamina has been demonstrated straight by the building and construction of mutant lamins that have the right to no longer be phosphorylated. When genes encoding these mutant lamins were introduced right into cells, their expression was uncovered to block normal breakdvery own of the nuclear lamina as the cells entered mitosis.


Figure 8.31

Dissolution of the nuclear lamina. The nuclear lamina consists of a meshwork-related of lamin filaments. At mitosis, Cdc2 and other protein kinases phosphorylate the lamins, resulting in the filaments to dissociate into complimentary lamin dimers.

In concert via dissolution of the nuclear lamina, the nuclear membrane fragments right into vesicles (Figure 8.32). The B-form lamins reprimary connected through these vesicles, but lamins A and also C dissociate from the nuclear membrane and are released as free dimers in the cytosol. This distinction arises because the B-kind lamins are permanently modified by the addition of lipid (prenyl groups), whereas the C-terminal prenyl groups of A- and C-type lamins are removed by proteolysis adhering to their incorporation right into the lamina. The nuclear pore complexes also dissociate right into subsystems as a result of phosphorylation of numerous nuclear pore proteins. Integral nuclear membrane proteins are also phosphorylated at mitosis, and also phosphorylation of these proteins may be necessary in vesicle formation and also in dissociation of the nuclear membrane from both chromosomes and also the nuclear lamina.


Figure 8.32

Breakdown of the nuclear membrane. As the nuclear lamina dissociates, the nuclear membrane pieces right into vesicles. The B-kind lamins remain bound to these vesicles, while lamins A and also C are released as cost-free dimers.

Chromosome Condensation

The various other major change in nuclear framework during mitosis is chromosome condensation. The interphase chromatin, which is already packaged right into nucleosomes, condenses roughly a thousandfold further to form the compact chromosomes seen in mitotic cells (Figure 8.33). This condensation is essential to permit the chromosomes to relocate along the mitotic spindle without ending up being tangled or broken during their distribution to daughter cells. DNA in this very condensed state can no much longer be transcribed, so all RNA synthesis stops in the time of mitosis. As the chromosomes condense and transcription ceases, the nucleolus also disappears.

Figure 8.33

Chromosome condensation. Electron micrograph reflecting the condensation of individual chromosomes throughout the prophase of mitosis. (K. G. Murti/Visuals Endless.)

The condensed DNA in metaphase chromosomes appears to be organized right into big loops, each encompassing about a hundred kilobases of DNA, which are attached to a protein scaffold (see Figure 4.13). Despite its basic prominence, the device of chromosome condensation during mitosis is not understood. The basic unit of chromatin framework is the nucleosome, which is composed of 146 base pairs of DNA wrapped approximately a histone core containing 2 molecules each of histones H2A, H2B, H3, and also H4 (check out Figure 4.8). One molecule of histone H1 is bound to the DNA as it enters each nucleosome core pshort article, and interactions in between these H1 molecules are connected in the folding of chromatin into higher-order, more compact frameworks. Hirock H1 is a substrate for the Cdc2 protein kinase and also is phosphorylated in the time of mitosis of many cells, constant through its phosphorylation playing a duty in mitotic chromosome condensation. However, current experiments have displayed that phosphorylation of histone H1 is not required for chromosome condensation, so the potential function of H1 phosphorylation is unclear. In comparison, phosphorylation of hirock H3 has been uncovered to be compelled for condensation of mitotic chromosomes, although the system through which H3 phosphorylation affects chromosome condensation stays to be elucidated.

Recent research studies have actually additionally determined protein complexes dubbed condensins that play a significant role in chromosome condensation. Condensins are forced for chromosome condensation in extracts of mitotic cells and also show up to feature by wrapping DNA roughly itself, thereby compacting chromosomes into the condensed mitotic structure. Condensins are phosphorylated and also activated by the Cdc2 protein kinase, offering a straight link between activation of Cdc2 and mitotic chromosome condensation.

Re-development of the Interphase Nucleus

Throughout the completion of mitosis (telophase), 2 brand-new nuclei develop about the separated sets of daughter chromosomes (watch Figure 8.29). Chromosome decondensation and reassembly of the nuclear envelope show up to be signaled by inactivation of Cdc2, which was responsible for initiating mitosis by phosphorylating cellular tarobtain proteins, including the lamins, hirock H3, and also condensins. The progression from metaphase to anaphase involves the activation of a ubiquitin-mediated proteolysis mechanism that inactivates Cdc2 by degrading its regulatory subunit, cyclin B (see Figure 7.40). Inactivation of Cdc2 leads to the dephosphorylation of the proteins that were phosphorylated at the initiation of mitosis, leading to exit from mitosis and the re-formation of interphase nuclei.

The initial action in re-formation of the nuclear envelope is the binding of the vesicles created during nuclear membrane breakdown to the surface of chromosomes (Figure 8.34). This interaction of membrane vesicles via chromosomes might be mediated by both lamins and also integral membrane proteins of the inner nuclear membrane. The vesicles then fusage to form a double membrane around the chromosomes. This is followed by reassembly of the nuclear pore complexes, re-formation of the nuclear lamina, and also chromosome decondensation. The vesicles first fuse to create membranes around individual chromosomes, which then fuse via each various other to form a finish single nucleus.

Figure 8.34

Re-development of the nuclear envelope. The first step in reassembly of the nuclear envelope is the binding of membrane vesicles to chromosomes, which may be mediated by both integral membrane proteins and B-kind lamins. The vesicles then fuse, the nuclear (more...)

The initial re-formation of the nuclear envelope approximately condensed chromosomes excludes cytoplasmic molecules from the freshly assembled nucleus. The brand-new nucleus is then able to expand also via the selective import of nuclear proteins from the cytoplasm. Since nuclear localization signals are not cleaved from proteins that are imported to the nucleus, the very same nuclear proteins that were released into the cytoplasm complying with disassembly of the nuclear envelope at the start of mitosis can be reimported into the new nuclei formed after mitosis. The nucleolus, also, re-develops as the chromosomes deconthick and also transcription of the rRNA genes begins, completing the rerotate from mitosis to an interphase nucleus.

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