Dna are on opposite ends of the cell nuclei reform. the cell begins to split into two.

Anaphase and telophase I

Anaphase I of meiosis begins with the release of cohesion between the arms of sister chromatids, much as it does during mitosis. As positioning of bivalent pairs is random, assortment of maternal and paternal chromosomes in each telophase nucleus is also random. Critically, sister centromeres, and thus chromatids, do not separate during anaphase I.

During meiosis I, cytoplasmic division occurs by specialized mechanisms. In females, the division is highly asymmetric, producing one egg and one tiny cell known as a polar body. In males, the process results in production of spermatocytes that remain joined by small cytoplasmic bridges.

2 Connecting Cell Cortex to Nuclei

At telophase of divisions when two daughter nuclei are formed, WRM-1 localized preferentially to the posterior than anterior nuclei (Fig. 3.1C) (Takeshita & Sawa, 2005; Nakamura et al., 2005). This is in good contrast to its anterior cortical localization that is still observed during telophase. Photobleaching experiments revealed that WRM-1 in the anterior cytoplasm and nucleus as well as that in the posterior side accumulates in the posterior nucleus and that the nuclear export rates of WRM-1 are higher in the anterior nucleus. This nuclear asymmetry of WRM-1 is regulated by WRM-1 itself on the anterior cortex, as expression of WRM-1::CAAX that uniformly localized to the cortex inhibits WRM-1 localization in both nuclei (Mizumoto & Sawa, 2007a). Cortical WRM-1 recruits APR-1 to the anterior cortex. In apr-1 mutants, WRM-1 nuclear export is inhibited, resulting in its localization in both nuclei. Thus, APR-1 on the cortex mediates the effects of WRM-1 in the inhibition of WRM-1 nuclear localization.

In other organisms, it is well known that APC functions in the degradation of β-catenin (Cadigan & Peifer, 2009; MacDonald, Tamai, & He, 2009). However, in asymmetric cell division in C. elegans, levels of WRM-1/β-catenin are not affected in apr-1 mutants. APC is also known to stabilize microtubules by binding to their plus ends in mammalian cells (Dikovskaya, Zumbrunn, Penman, & Näthke, 2001). Although this function of APC has not been shown to regulate β-catenin, we have recently showed that APC regulates β-catenin nuclear localization through microtubules in the EMS blastomere (Sugioka et al., 2011). APR-1 on the anterior cortex stabilizes astral microtubules, creating asymmetry of spindle (more astral microtubules from the anterior spindle pole than from the posterior one) (Fig. 3.1C). Disruption of this spindle asymmetry by laser irradiation of the anterior spindle pole disrupted nuclear asymmetry of WRM-1, while the enhancement of the spindle asymmetry by the posterior irradiation caused concomitant increase of WRM-1 nuclear asymmetry. Further, the posterior irradiation in mom-2/Wnt mutants in which asymmetry of spindle and nuclear WRM-1 is disrupted rescued asymmetric POP-1/TCF localization (see below) regulated by nuclear WRM-1. These results showed that spindle microtubules stabilized by APR-1 enhance export of WRM-1 from the anterior nucleus, creating its nuclear asymmetry. How spindle regulates WRM-1 nuclear export is not known. The requirements of kinesin for WRM-1 localization raised the model that microtubule-dependent transport of WRM-1 toward the cell cortex removes it from the perinuclear region, enhancing its nuclear export (Sugioka et al., 2011).

Telophase

In the telophase stage (seeFigure 3-14G andH), the two sets of daughter chromosomes are pushed completely apart. Then, the mitotic apparatus dissipates, and a new nuclear membrane develops around each set of chromosomes. This membrane is formed from portions of the endoplasmic reticulum that are already present in the cytoplasm. Shortly thereafter, the cell pinches in two, midway between the two nuclei. This pinching is caused by the formation of a contractile ring ofmicrofilaments composed ofactin and probablymyosin (the two contractile proteins of muscle) at the juncture of the newly developing cells that pinches them off from each other.

F Telophase

The main events of telophase include a reappearance and enlargement of the nucleolus, enlargement of the daughter nuclei to their interphase size, decondensation of the chromatin resulting in a brighter appearance of the nuclei with phase-contrast optics, and a period of rapid, postmitotic nuclear migration during which the daughter nuclei become positioned prior to septum formation (Aist, 1969, 1995). Although the natural breaking of the spindle is used to define the onset of telophase (Bayles et al., 1993), telophase events involving the nucleolus, the chromatin, and nuclear size frequently begin moments before the spindle breaks. Thus, there is sometimes overlap between the anaphase B and telophase stages regarding the behavior of the various nuclear components. This is one reason why it is helpful to use only one of several available criteria, (i.e., spindle breakdown) to define the starting point for telophase. The other reason is that the daughter nuclei are not truly independent of each other until spindle breakdown; therefore, technically, the nucleus is still dividing.

Telophase

Telophase, the terminal phase of mitosis, is characterized by cytokinesis, reconstitution of the nucleus and nuclear envelope, disappearance of the mitotic spindle apparatus, and unwinding of the chromosomes into chromatin.

Attelophase, each set of chromosomes has reached its respective pole, the nuclear lamins are dephosphorylated, and the nuclear envelope is reconstituted. The chromosomes uncoil and become organized into heterochromatin and euchromatin of the interphase cell. The nucleolus is developed from thenucleolar organizing regions on each of five pairs of chromosomes.

CPC in Mitosis

From prophase to telophase, localization of the CPC is dynamic and is an indication of the multiple roles the CPC plays in regulating mitotic progression and cell division. During early mitosis, the CPC is found at centromeres and diffusely localized along chromosome arms. Another key mitotic role of Aurora B is carried out during prophase. Along with Cdk1, Aurora B contributes to sister chromatid resolution by phosphorylating the cohesion-stabilizing protein Sororin (Losada, 2014; Dreier et al., 2011). Phosphorylated Sororin dissociates from the cohesion subunit Pds5, which ultimately results in WapL-mediated release of cohesion from chromosome arms. Polo-like kinase is also essential for prophase removal of cohesin, and works by phosphorylating cohesin subunits (Losada, 2014). Cells in which the prophase removal pathway is inhibited show an increase in chromosome loss upon completion of mitosis, indicating the importance of this pathway in maintaining genomic stability (Haarhuis et al., 2013). As mitosis progresses, the CPC concentrates at inner centromeres where it participates in an essential, evolutionarily conserved surveillance mechanism required for high-fidelity chromosome segregation. The spindle assembly checkpoint (also known as the mitotic checkpoint) blocks entry into anaphase until all chromosomes attain a bipolar attachment to the spindle. The best recognized role of the CPC in this process is in ‘error correction’ where inappropriate spindle–kinetochore attachments are converted to unattached kinetochores that trigger the SAC.

Telophase and Cytokinesis

The processes of prophase occur in reverse to telophase. The chromosomes again become diffuse and the spiral threads partly unwind. The nucleolus and the nuclear membrane reappear and the nucleus revert to the interphasic condition with the chromatin appearing as delicate threads. The nuclear membrane reforms from fragments of the parent cell’s nuclear envelope as well as other portions of the inracellular membranes. The cleavage furrow across the equator is complete and the future cell membrane appears across the cell through the center of the spindle. Two daughter cells similar to the original one have been produced. The mitochondria and the Golgi complex are distributed to the daughter cells in approximately equal amounts. The centriole–aster–spindle complex ceases to function.

Reassembly of the Nuclear Envelope

Nuclear envelope reassembly begins during anaphase and is completed during telophase (Fig. 44.19). As in spindle assembly, Ran-GTP promotes early steps of nuclear envelope assembly at the surface of the chromosomes by releasing key components sequestered by importin β. These include several nuclear pore com­ponents, and one of the earliest events in nuclear envelope reassembly involves binding of the nuclear pore scaffold protein ELYS to chromatin. ELYS can recognize DNA regions rich in A : T base pairs, so it is likely to bind directly to the DNA. ELYS then recruits other components of the nuclear pore scaffold and nuclear pore trans-membrane proteins. The pore subsequently matures as various peripheral components and elements of the permeability barrier are added.

The mechanism of nuclear membrane reassembly is debated. In cells where nuclear membranes fragments into vesicles during mitosis, a Ran-GTP–dependent pathway directs at least two discrete populations of vesicles to chromatin where they fuse to reform the nuclear envelope. In cells where the nuclear membrane is absorbed into the endoplasmic reticulum during mitosis, reassembly involves lateral movements of membrane components within the membrane network and their stabilization at preferred binding sites at the periphery of the chromosomes.

Lamin subunits disassembled in prophase are recycled to reassemble at the end of mitosis. Lamina reassembly is triggered by removal of mitosis-specific phosphate groups and methyl-esterification of several COOH side chains on lamin B (Fig. 44.6). Together with ELYS, B-type lamins are among the earliest components of the nuclear envelope to target to the surface of the chromosomes during mid-anaphase. Either at this time or shortly thereafter, other proteins associated with the inner nuclear membrane, including BAF, LAP2, and lamin B receptor (see Fig. 9.10), join the forming envelope. Later during telophase when nuclear import is reestablished, lamin A enters the reforming nucleus and slowly assembles into the peripheral lamina over several hours in the G1 phase. If lamin transport through nuclear pores is prevented, chromosomes remain highly condensed following cytokinesis, and the cells fail to reenter the next S phase.

2.3.2 In vivo reassembly

During mitosis, nucleoli disassemble during prophase and reassemble in telophase (Sirri et al., 2008). The nucleolus has been described as “an organelle formed by the act of building a ribosome” (Mélèse and Xue, 1995) and when transcription is repressed its components in part stay associated to rDNA in the NOR (Roussel et al., 1996) and in part migrate as chromosomal passengers (Hernandez-Verdun and Gautier, 1994).

At the moment of rDNA transcription restart, nucleoli are again formed via PNB formation (Dundr et al., 2000) via a progressive recruitment of proteins involved in early and late processing. PNBs, with their content of nucleolar processing proteins, pre-rRNAs and small nucleolar RNAs (snoRNA), play a role that has not yet been completely clarified. Moreover, it seems clear that proteins with a different functional role leave the PNBs at different moments. Recently, Muro et al. (2010) have demonstrated that fibrillarin passes from one incipient nucleolus to another without stopping in PNBs, while other proteins like B23 shuttle between PNBs and nucleoli. The difference in this traffic would suggest a way of regulating the assembly first of the DFC and then of the GC, and this mechanism would involve the Cajal bodies.

Several factors are probably involved in the rebirth of a nucleolus. Transcription itself is not sufficient to start the event (Section 2.3.1) but nucleolar assembly can start independently of rDNA transcription (Dousset et al., 2000). Apparently a paradox: transcription arrest means disassembly, reassembly does not mean transcription restart. Other factors, such as CDK, may intervene to regulate both transcription and processing (Sirri et al., 2008). The final assembly is rather rapid and very probably a “prenucleolar” interaction of processing proteins is required.

If one considers the incredible amount of proteins that disassemble and reassemble during mitosis, and that most of them redistribute at different locations and then are recruited to PNBs, it is not clear what could be the driving force behind. Diffusion is the easy answer for the movements, and indeed a part of nonribosomal proteins show a nucleolar localization signal (NLS), but not all of them possess this feature (Jacobson and Pederson, 1998).

Diffusion can account for a series of movements, although mediated by signal recognizing sequences, but necessity of order and time might involve other mechanisms. It is known that some proteins are recruited from PNBs in a specific, sequential order (Louvet et al., 2008). It is difficult in this case to imagine diffusion as the only mechanism. As described for other nucleolar functions such as transcription (Dundr et al., 2002) or ribosome subunit movement (Cisterna et al., 2006, 2009) there could be place for motor proteins to give directionality (impossible in diffusion mechanisms), time schedule (also possible only in active mechanisms), and releasing order, if any. The coordination found in the movements of nucleolar proteins suggests that they can maintain their interaction during mitosis; however, the mechanisms behind the interactions are still not clear. The interaction has been clearly shown by FRET analysis (Angelier et al., 2005).

Mitosis

Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase. Interphase is the interval from the end of mitosis until the beginning of the next. Each cell division begins with a phase of DNA replication, referred to as S phase. DNA replication results in two sister chromatids for each chromosome. Prophase is marked by gradual condensation of the chromosomes, disappearance of the nucleolus and nuclear membrane, and the beginning of the formation of the mitotic spindle. At metaphase, the chromosomes become arranged on the equatorial plane, but homologous chromosomes do not pair. In this stage, chromosomes also reach maximum condensation. In anaphase, the chromosomes divide at the centromeric regions, and the two chromatids separate and migrate to opposite poles. Telophase begins with the formation of the nuclear membranes and division of the cytoplasm (Fig. 16.10).