Mitotic M phases

FIG. 1. Timing and morphology of mouse embryos during the first two cleavages. The cortical activity of the one-cell embryo begins during late G2 phase shortly before the entry into the mitotic M phase. Scheme represents shapes of embryos and morphology of their chromatin and microtubule cytoskeleton. 

Regulation of the entry into the early mitotic M phases... 

The entry into the first mitotic M phase at the end of the first embryonic cell cycle requires activation of MPF. In the mouse one-cell embryo this activation is fully autonomous from the nucleus (Ciemerych 1995, Ciemerych et al 1998). It proceeds within the cytoplasts obtained either by enucleation or by bisection of the embryo. Other autonomous phenomena are the cortical activity, or the deformation of the one-cell embryo, directly preceding the entry into first mitosis (Waksmundzka et al 1984) and the cyclic activity of K+ ion channels (Day et al 1998). The role of the cortical activity remains unknown however, the fact that it directly precedes the entry into the first mitotic M phase suggests that it could be linked to the activation... 

When cells reproduce, they do so via a very specific game plan known as the cell cycle. Cell division (mitosis) kicks off the cycle, and after a period of 30 to 60 minutes, the cells go into either a resting phase (called Go) or a presynthetic (gap) phase (called Gi), in which enzyme production occurs in preparation for de novo nucleic acid synthesis. Production of DNAthen occurs in an S phase that can last up to 20 hours. The S phase is followed by a gap phase (G2), in which RNA, critical proteins, and the mitotic spindle apparatus are generated for the next mitotic (M) phase (3,4). 

As in all eukaryotic cells, the cell cycle of Chlamydomonas segnis is divided, into the G, S, G2, and mitotic (M) phases. The S-phase is the period where synthesis of thymidine kinase, histones, and DNA take place and is the most sensitive phase of the cell cycle (Badour et al. 1977). The experimental design of the present study was to stress the cells just prior to their entry into M, G, S or Gj phase and then determine macromolecular changes evidenced at the end of the cell cycle. 

Figure 5.14. Scheme of the microtubule (MT) cycle within the cell cycle. The mitotic (M) phase of the cell cycle is exaggerated with respect to the length of the interphase. The polymerization of tubulin (T) is under the control of specific microtubule organizing centers (MTOCs) at each stage of the MT cycle." ... 

APC is active from mid-M phase (anaphase) to the end of G1 phase and required for disconnecting sister chromatids and exit from M-Phase to Gl. The complex mediates the ubiquitination of Securin and Cyclin B. Degradation of these proteins, which block mitotic progression, promotes anaphase onset and exit from mitosis. 

The very beginning of the first mitotic cell cycle of the mouse embryo seems to be controlled by the mechanisms characteristic for both meiotic and mitotic cell cycles. Active MAP kinase, its substrate p90rsk and the CSF activity itself could influence the cellular processes within the one-cell embryo. Indeed, we have observed that despite the entry into the interphase (as judged by the low activity of MPF) some proteins are actively phosphorylated as during the meiotic M phase (e.g. 35 kDa complex Howlett et al 1986, Szollosi et al 1993), the nuclei and the microtubule interphase network start to form only 1.5 hours after activation (Szollosi et al 1993). This delay in the phenomena characteristic for the interphase could be linked to the mixed meiotic/mitotic character of this early period. This delay probably allows the correct transformation of the sperm nucleus into the male pronucleus. In species like Xenopus or Drosophila the transitional period between the meiotic and the mitotic cell cycle control is probably much shorter since it is proportional to duration of the short first cell cycle of these rapidly cleaving embryos. Mammalian embryos are perhaps the most suitable to study this transition because of the exceptionally long first embryonic cell cycle. 

FIG. 3. HistoneHl kinase activity and schematic representation of the morphology of one-cell mouse embryos (3A) and two-cell stage blastomeres (3B) bisected at the respective G2 phases. N ote that histone H1 kinase activity rises autonomously in anucleate halves of both embryos and blastomeres. However, the degree of the autonomous activation is lower than in theit nucleate counterparts. Activity detected in nucleate halves obtained during respective M phases was taken as 100%. Note that the nucleate halves obtained at theit respective G2 stages do not activate histone HI kinase to the levels observed in the halves obtained in the M phase, and that the mitotic disassembly of microtubules was observed only when the level of histone HI kinase was between 35% and 46% in anucleate halves. 

Boxem M, Srinivasan DG, van den Heuvel S 1999 The Caenorhabditis elegans gene ncc1 encodes a cdc2-related kinase required for M phase in meiotic and mitotic cell divisions, but not for S phase. Development 126 2227—2239... 

Vincristine and vinblastine are generally considered to act specifically on the metaphase portion of the mitotic (M) stage of the cell cycle as a consequence of perturbations of the structure and function of tubulin. A characteristic action of the drugs is production of mitotic arrest in which the tJercentage of cells in mitosis in a given population of cells will rise from a few percent to 50% and more after treatment with a drug such as vinblastine. There are reports, however, that these drugs can interfere with other phases of the cell cycle in ways not clearly related to interference with tubulin function (5). 

Entry of animal cells into mitosis is based on the mitosis-promoting factor (MPF). MPF consists of CDK1 (cdc2) and cyclin B. The intracellular concentration of cyclin B increases constantly until mitosis starts, and then declines again rapidly (top left). MPF is initially inactive, because CDKl is phosphorylated and cyclin B is dephosphorylated (top center). The M phase is triggered when a protein phosphatase [1] dephosphorylates the CDK while cyclin B is phosphorylated by a kinase [2]. in its active form, MPF phosphorylates various proteins that have functions in mitosis—e.g., histone HI (see p. 238), components of the cytoskeleton such as the laminins in the nuclear membrane, transcription factors, mitotic spindle proteins, and various enzymes. 

The loss of p5J-dependcnt G, arrest promotes the progression of cells to G2/M phase where they are the target of mitotic arrest. 

The mitotic index is the fraction or percentage of cells in mitosis within a given cell population. The thymidine labeling index is the fraction of cells incorporating radioactive thymidine. They represent cells in M-phase and S-phase and define the proliferative characteristics of normal and tumor cells. 

Fig. 13.2. Cytologic features of M phase. M phase is divided into the mitotic phases shown, based on characteristic cytologic features. The transition from metaphase to anaphase is an important control point. Cells may stop and pause before this control point. If the control point is crossed, M phase is concluded with cell division.

Morphologically, cell division is only visible in M phase. Under the light microscope, condensation, alignment and segregation of the chromosomes and cell division itself may be observed during M phase. In addition, different mitotic phases can be distinguished, as shown in Fig. 13.2. 

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