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You are watching: The nucleolus begins to fade from view

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

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A unique feature of the nucleus is the it disassembles and re-forms every time many cells divide. At the beginning of mitosis, the chromosomes condense, the core source disappears, and the atom envelope division down, resulting in the release of many of the components of the nucleus right into the cytoplasm. In ~ the finish of mitosis, the process is reversed: The chromosomes decondense, and nuclear envelopes re-form around the separated to adjust of daughter chromosomes. Thing 14 presents a substantial discussion of mitosis; in this ar we will consider the mechanisms associated in the disassembly and also re-formation of the nucleus. The process is controlled largely by reversible phosphorylation and also dephosphorylation of atom proteins result from the activity of the Cdc2 protein kinase, i beg your pardon is a an essential regulator of mitosis in all eukaryotic cells.

Dissolution the the nuclear Envelope

In most cells, the disassembly that the nuclear envelope point out the finish of the prophase that mitosis (Figure 8.29). However, this disassembly that the cell nucleus is no a universal function of mitosis and also does not happen in every cells. Some unicellular standard scale (e.g., yeasts) undergo so-called closed mitosis, in which the nuclear envelope remains undamaged (Figure 8.30). In closed mitosis, the daughter chromosomes move to the contrary poles the the nucleus, which climate divides in two. The cell of greater eukaryotes, however, usually undergo open up mitosis, which is characterized by failure of the atom envelope. The daughter chromosomes then move to opposite poles of the mitotic spindle, and brand-new nuclei reassemble about them.


Figure 8.29

The nucleus during mitosis. Micrographs illustrating the gradual stages of mitosis in a tree cell. During prophase, the chromosomes condense, the core source disappears, and also the atom envelope breaks down. At metaphase, the condensed chromosomes (more...)


Figure 8.30

Closed and also open mitosis. In close up door mitosis, the atom envelope stays intact and also chromosomes migrate to the contrary poles the a spindle within the nucleus. In open mitosis, the nuclear envelope breaks down and also then re-forms around the 2 sets of be separated (more...)

Disassembly that the nuclear envelope, which parallels a similar malfunction of the endoplasmic reticulum, involves alters in all 3 of that components: The nuclear membranes are fragmentized into vesicles, the nuclear pore complexes dissociate, and the nuclear lamina depolymerizes. The finest understood that these occasions is depolymerization the the atom lamina—the meshwork the filaments underlying the nuclear membrane. The atom lamina is created of fibrous proteins, lamins, which associate v each other to kind filaments. Disassembly the the atom lamina outcomes from phosphorylation of the lamins, which causes 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 presented in chapter 7 (see figure 7.40) and will be debated in detail in chapter 14 together a main regulator that mitosis. Cdc2 (as well as other protein kinases caused in mitotic cells) phosphorylates all the different species of lamins, and treatment of isolated nuclei v Cdc2 has actually been shown to be adequate to induce depolymerization the the nuclear lamina. Moreover, the necessity for lamin phosphorylation in the malfunction of the nuclear lamina has actually been demonstrated straight by the construction of mutant lamins that can no longer be phosphorylated. Once genes encoding these mutant lamins were introduced right into cells, your expression was discovered to block normal failure of the nuclear lamina as the cells entered mitosis.


Figure 8.31

Dissolution that the nuclear lamina. The atom lamina is composed of a meshwork the lamin filaments. At mitosis, Cdc2 and also other protein kinases phosphorylate the lamins, leading to the filaments come dissociate into complimentary lamin dimers.

In concert through dissolution the the nuclear lamina, the atom membrane pieces into vesicles (Figure 8.32). The B-type lamins remain linked with this vesicles, yet lamins A and C dissociate native the atom membrane and are exit as free dimers in the cytosol. This difference arises since the B-type lamins are permanently modified by the enhancement of lipid (prenyl groups), whereas the C-terminal prenyl groups of A- and also C-type lamins are removed by proteolysis adhering to their incorporation right into the lamina. The atom pore complexes also dissociate right into subunits together a result of phosphorylation of several nuclear pore proteins. Integral nuclear membrane protein are likewise phosphorylated at mitosis, and phosphorylation of these proteins may be important in vesicle formation and also in dissociation of the atom membrane native both chromosomes and the atom lamina.


Figure 8.32

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

Chromosome Condensation

The other significant change in atom structure during mitosis is chromosome condensation. The interphase chromatin, i beg your pardon is already packaged into nucleosomes, condenses roughly a thousandfold additional to form the compact chromosomes checked out in mitotic cells (Figure 8.33). This condensation is needed to enable the chromosomes to move along the mitotic spindle without coming to be tangled or damaged during their distribution to daughter cells. DNA in this very condensed state can no longer be transcribed, so every RNA synthesis stops throughout mitosis. As the chromosomes condense and transcription ceases, the nucleolus additionally disappears.

Figure 8.33

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

The condensed DNA in metaphase chromosomes appears to be arranged into big loops, each encompassing around a hundred kilobases of DNA, which room attached to a protein frame (see number 4.13). In spite of its fundamental importance, the mechanism of chromosome condensation throughout mitosis is not understood. The simple unit that chromatin framework is the nucleosome, which is composed of 146 base pairs the DNA wrapped approximately a histone main point containing two molecules each of histones H2A, H2B, H3, and also H4 (see figure 4.8). One molecule of histone H1 is bound come the DNA as it enters every nucleosome core particle, and interactions in between these H1 molecule are associated in the urgent of chromatin right into higher-order, much more compact structures. Histone H1 is a substrate because that the Cdc2 protein kinase and is phosphorylated throughout mitosis of many cells, consistent with that is phosphorylation play a duty in mitotic chromosome condensation. However, current experiments have shown 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 the histone H3 has been uncovered to be compelled for condensation that mitotic chromosomes, back the mechanism whereby H3 phosphorylation influence chromosome condensation remains to it is in elucidated.

Recent researches have also identified protein complexes dubbed condensins the play a significant role in chromosome condensation. Condensins are forced for chromosome condensation in extracts that mitotic cells and also appear to duty by wrapping DNA around itself, in order to compacting chromosomes right into the condensed mitotic structure. Condensins are phosphorylated and also activated by the Cdc2 protein kinase, giving a direct link between activation of Cdc2 and mitotic chromosome condensation.

Re-formation the the Interphase Nucleus

During the perfect of mitosis (telophase), two new nuclei type around the separated to adjust of daughter chromosomes (see figure 8.29). Chromosome decondensation and reassembly of the nuclear envelope show up to be signaled by inactivation the Cdc2, which to be responsible for initiating mitosis by phosphorylating cellular target proteins, consisting of the lamins, histone H3, and also condensins. The progression from metaphase to anaphase entails the activation the a ubiquitin-mediated proteolysis device that inactivates Cdc2 by degrading its regulatory subunit, cyclin B (see figure 7.40). Inactivation of Cdc2 leader to the dephosphorylation of the proteins that were phosphorylated in ~ the initiation of mitosis, resulting in leave from mitosis and also the re-formation the interphase nuclei.

The initial step in re-formation that the atom envelope is the binding of the motor formed throughout nuclear membrane break down to the surface of chromosomes (Figure 8.34). This interaction of membrane vesicles v chromosomes might be mediated by both lamins and integral membrane protein of the inner nuclear membrane. The vesicles then fuse to kind a double membrane around the chromosomes. This is adhered to by reassembly of the nuclear pore complexes, re-formation the the nuclear lamina, and chromosome decondensation. The vesicles very first fuse to form membranes approximately individual chromosomes, which then fuse through each various other to type a complete solitary nucleus.

Figure 8.34

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

The early stage re-formation that the nuclear envelope around condensed chromosomes excludes cytoplasmic molecule from the freshly assembled nucleus. The brand-new nucleus is climate able to expand via the selective import of atom proteins indigenous the cytoplasm. Due to the fact that nuclear localization signals are not cleaved from protein that room imported come the nucleus, the exact same nuclear proteins that were released right into the cytoplasm adhering to disassembly that the atom envelope in ~ the start of mitosis can be reimported right into the new nuclei formed after mitosis. The nucleolus, too, re-forms together the chromosomes decondense and transcription of the rRNA gene begins, perfect the return indigenous mitosis come an interphase nucleus.

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