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

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A unique feature that the cell nucleus is that it disassembles and re-forms every time most cells divide. In ~ the beginning of mitosis, the chromosomes condense, the nuclear source disappears, and also the atom envelope breaks down, resulting in the release of many of the materials of the nucleus right into the cytoplasm. At the end of mitosis, the process is reversed: The chromosomes decondense, and also nuclear envelopes re-form around the separated sets of daughter chromosomes. Thing 14 presents a considerable discussion the mitosis; in this ar we will think about the mechanisms affiliated in the disassembly and also re-formation the the nucleus. The process is regulated largely through reversible phosphorylation and dephosphorylation of nuclear proteins result from the action of the Cdc2 protein kinase, i beg your pardon is a crucial regulator of mitosis in all eukaryotic cells.

Dissolution the the atom Envelope

In most cells, the disassembly the the atom envelope clues the finish of the prophase that mitosis (Figure 8.29). However, this disassembly the the nucleus is not a universal feature of mitosis and also does not occur in all cells. Some unicellular standard scale (e.g., yeasts) experience so-called close up door mitosis, in which the nuclear envelope remains intact (Figure 8.30). In closed mitosis, the daughter chromosomes migrate to opposite poles of the nucleus, which then divides in two. The cells of greater eukaryotes, however, commonly undergo open up mitosis, i m sorry is characterized by failure of the nuclear envelope. The daughter chromosomes then migrate to opposite poles that the mitotic spindle, and new nuclei reassemble around them.


Figure 8.29

The nucleus throughout mitosis. Micrographs portraying the steady stages of mitosis in a tree cell. Throughout prophase, the chromosomes condense, the core source 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 remains intact and chromosomes move to the opposite poles the a spindle in ~ the nucleus. In open up mitosis, the nuclear envelope division down and then re-forms roughly the two sets of be separate (more...)

Disassembly that the nuclear envelope, i m sorry parallels a similar break down of the absorbent reticulum, involves changes in all three of that components: The atom membranes are broke up into vesicles, the atom pore complexes dissociate, and also the atom lamina depolymerizes. The best understood of these events is depolymerization the the nuclear lamina—the meshwork of filaments underlying the atom membrane. The nuclear lamina is composed of fibrous proteins, lamins, i m sorry associate through each other to form filaments. Disassembly the the atom lamina results from phosphorylation the the lamins, which reasons the filaments to break down into individual lamin dimers (Figure 8.31). Phosphorylation the the lamins is catalyzed by the Cdc2 protein kinase, i beg your pardon was introduced in chapter 7 (see figure 7.40) and also will be discussed in detail in chapter 14 together a main regulator of mitosis. Cdc2 (as fine as other protein kinases caused in mitotic cells) phosphorylates every the different species of lamins, and treatment of isolated nuclei through Cdc2 has been shown to be adequate to induce depolymerization that the atom lamina. Moreover, the need for lamin phosphorylation in the breakdown of the nuclear lamina has actually been demonstrated directly by the building of mutant lamins that have the right to no much longer be phosphorylated. As soon as genes encoding these mutant lamins to be introduced into cells, their expression was found to block normal break down of the nuclear lamina as the cells gotten in mitosis.


Figure 8.31

Dissolution the the nuclear lamina. The atom lamina is composed of a meshwork the lamin filaments. In ~ mitosis, Cdc2 and other protein kinases phosphorylate the lamins, causing the filaments to dissociate into free lamin dimers.

In concert v dissolution the the nuclear lamina, the nuclear membrane fragments into vesicles (Figure 8.32). The B-type lamins remain linked with this vesicles, but lamins A and also C dissociate from the nuclear membrane and also are exit as cost-free dimers in the cytosol. This difference arises due to the fact that the B-type lamins room permanently amendment by the enhancement of lipid (prenyl groups), vice versa, the C-terminal prenyl groups of A- and also C-type lamins are gotten rid of by proteolysis complying with their incorporation into the lamina. The nuclear pore complexes likewise dissociate into subunits together a an outcome of phosphorylation of several nuclear pore proteins. Integral atom membrane protein are likewise phosphorylated in ~ mitosis, and also phosphorylation of this proteins might be necessary in vesicle formation and in dissociation that the atom membrane indigenous both chromosomes and the nuclear lamina.


Figure 8.32

Breakdown the the nuclear membrane. As the atom lamina dissociates, the nuclear membrane pieces into vesicles. The B-type lamins remain bound to these vesicles, if lamins A and C are released as totally free dimers.

Chromosome Condensation

The other significant change in atom structure during mitosis is chromosome condensation. The interphase chromatin, i m sorry is already packaged into nucleosomes, condenses approximately a thousandfold additional to kind the compact chromosomes seen in mitotic cells (Figure 8.33). This condensation is needed to permit the chromosomes to move along the mitotic spindle without becoming tangled or damaged during their distribution to daughter cells. DNA in this extremely condensed state deserve to no much longer be transcribed, so every RNA synthetic stops throughout mitosis. As the chromosomes condense and also transcription ceases, the nucleolus additionally disappears.

Figure 8.33

Chromosome condensation. Electron micrograph reflecting the condensation of separation, personal, instance chromosomes throughout the prophase that mitosis. (K. G. Murti/Visuals Unlimited.)

The condensed DNA in metaphase chromosomes appears to be arranged into large loops, every encompassing around a hundred kilobases of DNA, which are attached come a protein scaffold (see number 4.13). In spite of its an essential importance, the mechanism of chromosome condensation throughout mitosis is not understood. The an easy unit of chromatin framework is the nucleosome, which consists of 146 base pairs of DNA wrapped roughly a histone core containing 2 molecules each of histones H2A, H2B, H3, and also H4 (see figure 4.8). One molecule of histone H1 is bound to the DNA together it enters each nucleosome main point particle, and interactions in between these H1 molecule are connected in the urgently of chromatin into higher-order, much more compact structures. Histone H1 is a substrate for the Cdc2 protein kinase and is phosphorylated throughout mitosis of many cells, continual with that is phosphorylation play a function in mitotic chromosome condensation. However, recent experiments have displayed that phosphorylation the histone H1 is not compelled for chromosome condensation, for this reason the potential duty of H1 phosphorylation is unclear. In contrast, phosphorylation that histone H3 has actually been found to be compelled for condensation that mitotic chromosomes, although the mechanism whereby H3 phosphorylation affects chromosome condensation remains to be 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 appear to duty by pack DNA about itself, in order to compacting chromosomes right into the condensed mitotic structure. Condensins are phosphorylated and activated by the Cdc2 protein kinase, giving a straight link in between activation the Cdc2 and also mitotic chromosome condensation.

Re-formation that the Interphase Nucleus

During the perfect of mitosis (telophase), two new nuclei form around the separated sets of daughter chromosomes (see figure 8.29). Chromosome decondensation and also reassembly the the atom envelope show up to it is in signaled through inactivation of Cdc2, which to be responsible because that initiating mitosis by phosphorylating to move target proteins, including the lamins, histone H3, and also condensins. The development from metaphase come anaphase requires the activation that a ubiquitin-mediated proteolysis mechanism that inactivates Cdc2 through degrading its regulatory subunit, cyclin B (see number 7.40). Inactivation of Cdc2 leader to the dephosphorylation of the proteins the were phosphorylated at the initiation the mitosis, result in departure from mitosis and the re-formation of interphase nuclei.

The initial step in re-formation the the nuclear envelope is the binding of the motor formed during nuclear membrane failure to the surface of chromosomes (Figure 8.34). This communication of membrane vesicles with chromosomes may be mediated through both lamins and integral membrane proteins of the inner nuclear membrane. The vesicles climate fuse to type a double membrane approximately the chromosomes. This is followed by reassembly of the atom pore complexes, re-formation of the atom lamina, and chromosome decondensation. The vesicles an initial fuse to form membranes roughly individual chromosomes, which climate fuse v each other to type a complete single nucleus.

Figure 8.34

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

The early re-formation the the atom envelope about condensed chromosomes excludes cytoplasmic molecules from the recently assembled nucleus. The new nucleus is climate able to expand via the selective import of atom proteins native the cytoplasm. Since nuclear localization signals room not cleaved from protein that are imported to the nucleus, the exact same nuclear proteins the were released right into the cytoplasm following disassembly the the atom envelope in ~ the beginning of mitosis deserve to be reimported into the new nuclei developed after mitosis. The nucleolus, too, re-forms together the chromosomes decondense and transcription of the rRNA gene begins, completing the return from mitosis to an interphase nucleus.

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