Metaphase plate

The phase of the cell cycle where the sister chromatids are separated and distributed onto two daughter nuclei. First, upon entry into mitosis the chromosomes are condensed followed by the breakdown of the nuclear-envelope (prophase). The two centrosomes are separated and induce the formation of the mitotic spindle. Then, the chromosomes are captures by the spindle and aligned on the metaphase plate (metaphase). The sister-chromatids are separated and pulled to the poles of the spindle (anaphase). In telophase, two new nuclei are formed around the separated chromatids. 

A signal transduction pathway required for proper chromosome alignment during mitosis. The spindle assembly checkpoint is activated during mitosis in response to the presence of chromosomes that are not attached to spindle microtubules or that are not properly aligned at the metaphase plate. The spindle checkpoint... 

Several groups of drugs that bind to tubulin at different sites interfere with its polymerization into microtubules. These drugs are of experimental and clinical importance (Bershadsky and Vasiliev, 1988). For example, colchicine, an alkaloid derived from the meadow saffron plant Colchicum autumnale or Colchicum speciosum), is the oldest and most widely studied of these drugs. It forms a molecular complex with tubulin in the cytosol pool and prevents its polymerization into microtubules. Other substances such as colcemid, podophyllotoxin, and noco-dazole bind to the tubulin molecule at the same site as colchicine and produce a similar effect, albeit with some kinetic differences. Mature ciliary microtubules are resistant to colchicine, whereas those of the mitotic spindle are very sensitive. Colchicine and colcemid block cell division in metaphase and are widely used in cytogenetic studies of cultured cells to enhance the yield of metaphase plate chromosomes. 

The duration of the M phase is largely determined by the time necessary for the formation of a functional metaphase spindle and the correct alignment of all chromosomes in the metaphase plate. The spindle assembly checkpoint prevents the exit from the M phase before the proper alignment of all chromosomes into a metaphase plate in many cell types. This kind of control is already operational... 

Homologous chromosomes pair with each other at the metaphase plate. During Aitaphase I, homologs fail te disjoin. At the end of Meiosis I, one daughter ceil has both homologs, and one cell lacks that chromosome. 

During Metaphase II, each chromosome lines up Individually at the metaphase plate. In Anaphase II, sister chromatids are pulled to opposite poles. 

A series of experiments using corn root tips, Zea mays cv Norfolk Market White, was established using the free acid from 10 3 to 10 4 M. Results definitively showed that mitosis was inhibited at the C-metaphase plate at both concentrations... 

The clastogenic effects of male Swiss albino mice fed chromium(VI) trioxide (20 mg/kg body weight) by gavage were studied after 24 hours, bone marrow cells were isolated and 500 metaphase plates were scored for chromosomal aberrations (Sarkar et al. 1993). The treated cells showed a significant increase in aberrations per cell over controls by 4.4-fold. When animals were treated simultaneously with chlorophyllin (1.5 mg/kg), a sodium-copper derivative of chlorophyll and an antioxidant, numbers of aberrations were reduced to nearly background levels. 

Fig. 5-37 Diagram showing (a) the mitotic spindle at metaphase with chromosomes aligned at the metaphase plate and (b) separation of the poles and sister chromatids during anaphase.

Three types of microtubule can readily be defined in the mitotic spindle. Polar microtubules overlap (and probably interact) between the poles and are involved in pushing the poles apart in anaphase. Astral microtubules radiate in all directions and also help separate the poles. Kinetochore microtubules attach themselves to specialized protein structures (kinetochores) located on each side of the centromere of each chromosome. These microtubules are involved in moving the chromosomes to the metaphase plate and in separating sister chromatids at anaphase. The microtubules in the spindle are very dynamic and have a half-life of only a few seconds. This appears to be especially important in the capture of chromosomes by the kinetochore microtubules. Microtubules that miss the target kinetochores are quickly lost because their dynamic instability soon leads to depolymerization. The new microtubules that form may hit the target and be partially stabilized through plus-end capping. 

Cytokinesis begins some time in anaphase. A cleavage furrow begins to form in the plasma membrane in the same plane as the metaphase plate. It is not known how the furrow is positioned. The furrow is created and deepens due to the actions of the contractile ring which is an actomyosin network assembled specifically for this purpose and dispersed afterward. Thus, the stability of this network is quite different from its analog in muscle. The dynamics of these networks are also different (see Prob. 5.16). 

The taxanes originally were extracted from the bark of the Pacific Yew tree and have found widespread use as anticancer agents. The structurally related taxanes, docetaxel and pacUtaxel, bind to and stabilize polymerized microtubules. Cells that enter mitosis in the presence of paclitaxel attempt to assemble a spindle apparatus, however the inhibition of depolymerization renders this structure unable to properly position chromosomes at the metaphase plate. Therefore, although kinetochores bind to microtubules, there is insufficient tension to inactivate the SAC. Paclitaxel-treated cells become blocked in mitosis, and eventually die by the mechanisms just described. Currently, there... 

Metaphase - Each chromosome aligns independently at the metaphase plate. 

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