Mitosis possible mechanism

Jeppesen, P. (1997). Histone acetylation A possible mechanism for the inheritance of cell memory at mitosis. BioEssays 19 67-74. 

Any model for the biochemical oscillator controlling the onset of mitosis should account for the fact that, in some cells or at certain stages of development, mitosis can be prevented and cells cease to divide. In dynamic terms, such a transient or permanent suppression of mitotic activity can be viewed as a switch of the mitotic control system from an oscillatory into a nonoscillatory regime corresponding to a stable steady state. Models for the mitotic oscillator allow the discussion of possible mechanisms whereby the mitotic clock might be arrested. 

There are three possible mechanisms that describe the fates of the DNA strands that are copied and where they will ultimately end up in the two daughter cells after mitosis. Conservative replication posits that one daughter cell will contain both parent strands and the other daughter cell will contain the two newly synthesized strands of DNA. Semiconservative replication posits that the DNA of both daughter cells contains one strand from the parent and one newly synthesized strand. The dispersive model posits that sections of parental and newly synthesized DNA are scattered throughout both strands of the daughter genomes. Experimental evidence shows that DNA replication is semiconservative. 

Another control mechanism ensures that entry into S phase is only possible if preceded by mitosis. If the cell was able, during G2 phase, to enter a new S phase without mitosis taking place, this would lead to improgrammed multiplication of the chromosome set and thus to polyploidy. For S phase control, see 13.5. 

Mortimer et al., 1999). In contrast, the level of reporter gene expression was enhanced when cells were exposed to lipoplexes during or just before mitosis (Brunner et al., 2000). Finally, the relatively low rate of cell proliferation of primary cultures of ciliated human airway epithelia was found to be one of the determinants for their low transfectability by lipoplex (Fasbender et al., 1997). These results, however, do not preclude the possibility that DNA can be translocated through nuclear pore complex by a NLS-independent mechanism. 

The hormone EPO, 90% of which is produced by the kidneys, initiates and stimulates the production of RBCs. Erythropoiesis is driven by a feedback loop. The main mechanism of action of EPO is to prevent apoptosis, or programmed cell death, of erythroid precursor cells and to allow their proliferation and subsequent maturation. A decrease in tissue oxygen concentration signals the kidneys to increase the production and release of EPO into the plasma, which (1) stimulates stem cells to differentiate into proerythroblasts, (2) increases the rate of mitosis, (3) increases the release of reticulocytes from the marrow, and (4) induces Hgb formation. In normal circumstances, the RBC mass is kept at an almost constant level by EPO matching new erythrocyte production to the namral rate of loss of RBCs. Accelerated Hgb synthesis makes it possible to achieve the critical Hgb concentration necessary for RBCs to mature more rapidly, and a feedback mechanism stops further RBC nucleic acid synthesis, causing an earlier release of reticulocytes. Early appearance of large quantities of reticulocytes in the peripheral circulation (reticulocytosis) is another indication of increased RBC production. 

Yamashiro S, Matsumura F (1991) Mitosis-specific phosphorylation of caldesmon possible molecular mechanism of cell rounding during mitosis. Bioessays 13 563568 Zigmond SH (1996) Signal transduction and actin filament organization. Curr Opin Cell Biol 8 6673... 

How is chromosome motion so directed that controlled distribution to the daughter cells is assured and a chromosome complement perpetuated without alteration This question probably has now been answered at the cellular level. The explanation of distribution will be considered first for bivalents in meiosis since they have furnished the clearest material for analysis. All considerations involving events at the fine structure and molecular level are deferred to a subsequent section. The possible extensions of the explanations, for example, to ordinary mitosis, are also discussed in the subsequent section, along with divergent distribution mechanisms. 

Periwinkle (Mnca) Toxins are vincristine and vinblastine, which inhibit microtubule formation during mitosis and are associated with disrupted axoplasmic transport in neurons Hallucinations can occur, but possibly not as a result of this mechanism... 

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