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You are watching: Why does the nuclear membrane disappear during prophase

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

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A distinct feature of the cell nucleus is the it disassembles and also re-forms every time many cells divide. In ~ the start of mitosis, the chromosomes condense, the nuclear source disappears, and also the nuclear envelope breaks down, bring about the relax of many of the materials of the nucleus right into the cytoplasm. In ~ the finish of mitosis, the procedure is reversed: The chromosomes decondense, and also nuclear envelopes re-form roughly the separated to adjust of daughter chromosomes. Thing 14 gift a considerable discussion of mitosis; in this ar we will think about the mechanisms connected in the disassembly and also re-formation of the nucleus. The process is controlled largely by reversible phosphorylation and dephosphorylation of atom proteins resulting from the action of the Cdc2 protein kinase, which is a an important regulator that mitosis in all eukaryotic cells.

Dissolution that the nuclear Envelope

In many cells, the disassembly that the nuclear envelope marks the finish of the prophase that mitosis (Figure 8.29). However, this disassembly the the nucleus is not a universal function of mitosis and also does not take place in all cells. Some unicellular eukaryotes (e.g., yeasts) undergo so-called close up door mitosis, in i m sorry the atom envelope remains intact (Figure 8.30). In close up door mitosis, the daughter chromosomes move to the opposite poles the the nucleus, which then divides in two. The cells of greater eukaryotes, however, commonly undergo open mitosis, i beg your pardon is defined by malfunction of the atom envelope. The daughter chromosomes then migrate to the contrary poles that the mitotic spindle, and brand-new nuclei reassemble roughly them.


Figure 8.29

The nucleus during mitosis. Micrographs portraying the gradual stages that mitosis in a plant cell. Throughout prophase, the chromosomes condense, the nucleolus disappears, and also the atom envelope division down. In ~ metaphase, the condensed chromosomes (more...)


Figure 8.30

Closed and open mitosis. In close up door mitosis, the atom envelope continues to be intact and also chromosomes move to opposite poles that a spindle within the nucleus. In open up mitosis, the nuclear envelope breaks down and then re-forms roughly the 2 sets of be separate (more...)

Disassembly of the nuclear envelope, which parallels a similar breakdown of the endoplasmic reticulum, involves alters in all three of its components: The atom membranes are broke up into vesicles, the atom pore complexes dissociate, and also the nuclear lamina depolymerizes. The best understood that these occasions is depolymerization the the atom lamina—the meshwork of filaments basic the atom membrane. The nuclear lamina is created of fibrous proteins, lamins, i beg your pardon associate with each other to type filaments. Disassembly the the atom lamina results from phosphorylation of the lamins, which causes the filaments to failure into separation, personal, instance lamin dimers (Figure 8.31). Phosphorylation the the lamins is catalytic analysis by the Cdc2 protein kinase, i beg your pardon was introduced in chapter 7 (see number 7.40) and also will be discussed in information in chapter 14 together a main regulator that mitosis. Cdc2 (as fine as various other protein kinases set off in mitotic cells) phosphorylates all the different varieties of lamins, and treatment of isolated nuclei with Cdc2 has been displayed to be enough to induce depolymerization the the atom lamina. Moreover, the requirement for lamin phosphorylation in the malfunction of the nuclear lamina has actually been demonstrated directly by the building of mutant lamins that deserve to no longer be phosphorylated. As soon as genes encoding this mutant lamins were introduced into cells, their expression was discovered to block normal failure of the nuclear lamina together the cells gone into mitosis.


Figure 8.31

Dissolution of the nuclear lamina. The nuclear lamina consists of a meshwork that lamin filaments. In ~ mitosis, Cdc2 and other protein kinases phosphorylate the lamins, leading to the filaments to dissociate into complimentary lamin dimers.

In concert with dissolution of the atom lamina, the nuclear membrane pieces into engine (Figure 8.32). The B-type lamins remain connected with these vesicles, however lamins A and also C dissociate indigenous the nuclear membrane and are exit as cost-free dimers in the cytosol. This distinction arises because the B-type lamins room permanently modification by the addition of lipid (prenyl groups), whereas the C-terminal prenyl groups of A- and also C-type lamins are eliminated by proteolysis following their incorporation right into the lamina. The atom pore complexes likewise dissociate right into subunits together a result of phosphorylation of number of nuclear sharp proteins. Integral atom membrane proteins are likewise phosphorylated in ~ mitosis, and phosphorylation of this proteins may be crucial in vesicle formation and in dissociation of the nuclear membrane indigenous both chromosomes and also the atom lamina.


Figure 8.32

Breakdown the the nuclear membrane. Together the atom lamina dissociates, the atom membrane pieces into vesicles. The B-type lamins remain bound to these vesicles, while lamins A and C space released as complimentary dimers.

Chromosome Condensation

The other significant change in atom structure throughout mitosis is chromosome condensation. The interphase chromatin, which is already packaged right into nucleosomes, condenses roughly a thousandfold more to type the compact chromosomes watched in mitotic cell (Figure 8.33). This condensation is required to permit the chromosomes to move along the mitotic spindle without becoming tangled or broken during their distribution to daughter cells. DNA in this highly condensed state have the right to no longer be transcribed, so all RNA synthesis stops during mitosis. As the chromosomes condense and transcription ceases, the nucleolus likewise disappears.

Figure 8.33

Chromosome condensation. Electron micrograph mirroring the condensation of individual chromosomes throughout the prophase that mitosis. (K. G. Murti/Visuals Unlimited.)

The condensed DNA in metaphase chromosomes shows up to be arranged into huge loops, every encompassing about a hundred kilobases that DNA, which space attached come a protein frame (see number 4.13). Regardless of its an essential importance, the mechanism of chromosome condensation throughout mitosis is not understood. The straightforward unit that chromatin structure is the nucleosome, which is composed of 146 basic pairs that DNA wrapped roughly a histone main point containing two molecules every of histones H2A, H2B, H3, and H4 (see number 4.8). One molecule the histone H1 is bound to the DNA as it enters every nucleosome main point particle, and also interactions between these H1 molecules are affiliated in the folding of chromatin right into higher-order, more compact structures. Histone H1 is a substrate because that the Cdc2 protein kinase and is phosphorylated during mitosis of most cells, continual with the phosphorylation play a duty in mitotic chromosome condensation. However, recent experiments have displayed that phosphorylation of histone H1 is not compelled for chromosome condensation, for this reason the potential role of H1 phosphorylation is unclear. In contrast, phosphorylation that histone H3 has actually been discovered to be compelled for condensation of mitotic chromosomes, return the mechanism whereby H3 phosphorylation affect chromosome condensation stays to it is in elucidated.

Recent research studies have also identified protein complexes referred to as condensins the play a significant role in chromosome condensation. Condensins are forced for chromosome condensation in extracts the mitotic cells and also appear to duty by wrapping DNA roughly itself, thereby compacting chromosomes into the condensed mitotic structure. Condensins space phosphorylated and also activated by the Cdc2 protein kinase, offering a straight link in between activation the Cdc2 and also mitotic chromosome condensation.

Re-formation of the Interphase Nucleus

During the perfect of mitosis (telophase), two new nuclei form around the separated set of daughter chromosomes (see figure 8.29). Chromosome decondensation and reassembly the the atom envelope appear to be signaled by inactivation that Cdc2, which was responsible because that initiating mitosis by phosphorylating cellular target proteins, consisting of the lamins, histone H3, and condensins. The progression from metaphase come anaphase entails the activation that a ubiquitin-mediated proteolysis mechanism that inactivates Cdc2 through degrading its regulation subunit, cyclin B (see number 7.40). Inactivation of Cdc2 leads to the dephosphorylation that the proteins that were phosphorylated in ~ the initiation the mitosis, resulting in exit from mitosis and the re-formation the interphase nuclei.

The initial action in re-formation of the nuclear envelope is the binding of the vesicles formed throughout nuclear membrane break down to the surface ar of chromosomes (Figure 8.34). This interaction of membrane vesicles v chromosomes might be mediated by both lamins and also integral membrane protein of the inner atom membrane. The vesicles climate fuse to type a dual membrane approximately the chromosomes. This is followed by reassembly of the atom pore complexes, re-formation the the atom lamina, and chromosome decondensation. The vesicles an initial fuse to type membranes about individual chromosomes, which then fuse through each other to form a complete solitary nucleus.

Figure 8.34

Re-formation of the atom envelope. The very first step in reassembly of the nuclear envelope is the binding the membrane vesicles to chromosomes, which might be mediated by both integral membrane proteins and B-type lamins. The vesicles climate fuse, the nuclear (more...)

The initial re-formation that the atom envelope roughly condensed chromosomes excludes cytoplasmic molecules from the newly assembled nucleus. The new nucleus is climate able to broaden via the selective income of nuclear proteins from the cytoplasm. Because nuclear localization signals space not cleaved from protein that room imported to the nucleus, the exact same nuclear proteins that were released into the cytoplasm complying with disassembly the the atom envelope in ~ the start of mitosis have the right to be reimported right into the brand-new nuclei developed after mitosis. The nucleolus, too, re-forms as the chromosomes decondense and transcription the the rRNA genes begins, completing the return from mitosis to an interphase nucleus.

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