Explain why it is necessary that the nuclear membrane disintegrates during mitosis.

A unique feature of the nucleus is that it disassembles and re-forms each time most cells divide. At the beginning of mitosis, the chromosomes condense, the nucleolus disappears, and the nuclear envelope breaks down, resulting in the release of most of the contents of the nucleus into the cytoplasm. At the end of mitosis, the process is reversed: The chromosomes decondense, and nuclear envelopes re-form around the separated sets of daughter chromosomes. Chapter 14 presents a comprehensive discussion of mitosis; in this section we will consider the mechanisms involved in the disassembly and re-formation of the nucleus. The process is controlled largely by reversible phosphorylation and dephosphorylation of nuclear proteins resulting from the action of the Cdc2 protein kinase, which is a critical regulator of mitosis in all eukaryotic cells.

Dissolution of the Nuclear Envelope

In most cells, the disassembly of the nuclear envelope marks the end of the prophase of mitosis (Figure 8.29). However, this disassembly of the nucleus is not a universal feature of mitosis and does not occur in all cells. Some unicellular eukaryotes (e.g., yeasts) undergo so-called closed 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 higher eukaryotes, however, usually undergo open mitosis, which is characterized by breakdown of the nuclear envelope. The daughter chromosomes then migrate to opposite poles of the mitotic spindle, and new nuclei reassemble around them.

Figure 8.29

The nucleus during mitosis. Micrographs illustrating the progressive stages of mitosis in a plant cell. During prophase, the chromosomes condense, the nucleolus disappears, and the nuclear envelope breaks down. At metaphase, the condensed chromosomes (more...)

Figure 8.30

Closed and open mitosis. In closed mitosis, the nuclear envelope remains intact and chromosomes migrate to opposite poles of a spindle within the nucleus. In open mitosis, the nuclear envelope breaks down and then re-forms around the two sets of separated (more...)

Disassembly of the nuclear envelope, which parallels a similar breakdown of the endoplasmic reticulum, involves changes in all three of its components: The nuclear membranes are fragmented into vesicles, the nuclear pore complexes dissociate, and the nuclear lamina depolymerizes. The best understood of these events is depolymerization of the nuclear lamina—the meshwork of filaments underlying the nuclear membrane. The nuclear lamina is composed of fibrous proteins, lamins, which associate with each other to form filaments. Disassembly of the nuclear lamina results from phosphorylation of the lamins, which causes 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 (see Figure 7.40) and will be discussed in detail in Chapter 14 as a central regulator of mitosis. Cdc2 (as well as other protein kinases activated in mitotic cells) phosphorylates all the different types of lamins, and treatment of isolated nuclei with Cdc2 has been shown to be sufficient to induce depolymerization of the nuclear lamina. Moreover, the requirement for lamin phosphorylation in the breakdown of the nuclear lamina has been demonstrated directly by the construction of mutant lamins that can no longer be phosphorylated. When genes encoding these mutant lamins were introduced into cells, their expression was found to block normal breakdown of the nuclear lamina as the cells entered mitosis.

Figure 8.31

Dissolution of the nuclear lamina. The nuclear lamina consists of a meshwork of lamin filaments. At mitosis, Cdc2 and other protein kinases phosphorylate the lamins, causing the filaments to dissociate into free lamin dimers.

In concert with dissolution of the nuclear lamina, the nuclear membrane fragments into vesicles (Figure 8.32). The B-type lamins remain associated with these vesicles, but lamins A and C dissociate from the nuclear membrane and are released as free dimers in the cytosol. This difference arises because the B-type 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 following their incorporation into the lamina. The nuclear pore complexes also dissociate into subunits as a result of phosphorylation of several nuclear pore proteins. Integral nuclear membrane proteins are also phosphorylated at mitosis, and phosphorylation of these proteins may be important in vesicle formation as well as in dissociation of the nuclear membrane from both chromosomes and the nuclear lamina.

Figure 8.32

Breakdown of the nuclear membrane. As the nuclear lamina dissociates, the nuclear 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 major change in nuclear structure during mitosis is chromosome condensation. The interphase chromatin, which is already packaged into nucleosomes, condenses approximately a thousandfold further to form the compact chromosomes seen in mitotic cells (Figure 8.33). This condensation is needed to allow 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 can no longer be transcribed, so all RNA synthesis stops during mitosis. As the chromosomes condense and transcription ceases, the nucleolus also disappears.

Figure 8.33

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

The condensed DNA in metaphase chromosomes appears to be organized into large loops, each encompassing about a hundred kilobases of DNA, which are attached to a protein scaffold (see Figure 4.13). Despite its fundamental importance, the mechanism of chromosome condensation during mitosis is not understood. The basic unit of chromatin structure is the nucleosome, which consists of 146 base pairs of DNA wrapped around a histone core containing two molecules each of histones H2A, H2B, H3, and H4 (see Figure 4.8). One molecule of histone H1 is bound to the DNA as it enters each nucleosome core particle, and interactions between these H1 molecules are involved in the folding of chromatin into higher-order, more compact structures. Histone H1 is a substrate for the Cdc2 protein kinase and is phosphorylated during mitosis of most cells, consistent with its phosphorylation playing a role in mitotic chromosome condensation. However, recent experiments have shown that phosphorylation of histone H1 is not required for chromosome condensation, so the potential role of H1 phosphorylation is unclear. In contrast, phosphorylation of histone H3 has been found to be required for condensation of mitotic chromosomes, although the mechanism by which H3 phosphorylation affects chromosome condensation remains to be elucidated.

Recent studies have also identified protein complexes called condensins that play a major role in chromosome condensation. Condensins are required for chromosome condensation in extracts of mitotic cells and appear to function by wrapping DNA around itself, thereby compacting chromosomes into the condensed mitotic structure. Condensins are phosphorylated and activated by the Cdc2 protein kinase, providing a direct link between activation of Cdc2 and mitotic chromosome condensation.

Re-formation of the Interphase Nucleus

During the completion of mitosis (telophase), two new nuclei form around the separated sets of daughter chromosomes (see Figure 8.29). Chromosome decondensation and reassembly of the nuclear envelope appear to be signaled by inactivation of Cdc2, which was responsible for initiating mitosis by phosphorylating cellular target proteins, including the lamins, histone H3, and condensins. The progression from metaphase to anaphase involves the activation of a ubiquitin-mediated proteolysis system 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, resulting in exit from mitosis and the re-formation of interphase nuclei.

The initial step in re-formation of the nuclear envelope is the binding of the vesicles formed during nuclear membrane breakdown to the surface of chromosomes (Figure 8.34). This interaction of membrane vesicles with chromosomes may be mediated by both lamins and integral membrane proteins of the inner nuclear membrane. The vesicles then fuse 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 chromosome decondensation. The vesicles first fuse to form membranes around individual chromosomes, which then fuse with each other to form a complete single nucleus.

Figure 8.34

Re-formation 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-type lamins. The vesicles then fuse, the nuclear (more...)

The initial re-formation of the nuclear envelope around condensed chromosomes excludes cytoplasmic molecules from the newly assembled nucleus. The new nucleus is then able to expand via the selective import of nuclear proteins from the cytoplasm. Because nuclear localization signals are not cleaved from proteins that are imported to the nucleus, the same nuclear proteins that were released into the cytoplasm following disassembly of the nuclear envelope at the beginning of mitosis can be reimported into the new nuclei formed after mitosis. The nucleolus, too, re-forms as the chromosomes decondense and transcription of the rRNA genes begins, completing the return from mitosis to an interphase nucleus.