Metaphase mitotic apparatus

Cells with abnormal numbers of chromosomes form when certain cell-cycle checkpoints are nonfunctional. As discussed in Chapter 21, the unreplicated-DNA checkpoint normally prevents entry into mitosis unless all chromosomes have completely replicated their DNA the spindle-assembly checkpoint prevents entry into anaphase unless all the replicated chromosomes attach properly to the metaphase mitotic apparatus and the chromosome-segregation checkpoint prevents exit from mitosis and cytokinesis if the chromosomes segregate improperly (see Figure 21-32, steps [H- 3D. As advances are made in Identifying the proteins that detect these abnormalities and mediate cell-cycle arrest, the molecular basis for the functional defects leading to aneuploidy in tumor cells will become clearer. 

Figure 34.1 Motion within cells. This high-voltage electron micrograph shows the mitotic apparatus in a metaphase mammalian cell. The targe cylindrical objects are chromosomes, and the threadlike structures stretched across the center are microtubules—tracks for the molecular motors that move chromosomes. Many processes, including chromosome segregation in mitosis, depend on the action of molecular-motor proteins.. [Courtesy of Dr. J. R. McIntosh.]...

At metaphase, the mitotic apparatus is organized into two parts a central mitotic spindle and a pair of asters (Figure 20-31a see also Figure 20-2c). The spindle is a bilaterally symmetric bundle of microtubules and associated proteins with the overall shape of a football it is divided into opposing halves at the equator of the cell by the metaphase chromosomes. An aster is a radial array of microtubules at each pole of the spindle. 

Fig. 2. Mitotic apparatus isolated by dispersion of cytoplasm with digitonin. Dispersed cytoplasm in background. At lower left, an unbroken egg is seen, with mitotic apparatus inside. (Low power, phase contrast.) Fig. 3. Isolated mitotic apparatus concentrated by centrifugation. Dispersed cytoplasm has been removed. (Low power, phase contrast.) Fig. 4. Isolated mitotic apparatus from cell in metaphase. (High power, phase contrast.) Fig. 5. Isolated mitotic apparatus from cell in anaphase. Magnification is same as in Fig. 4, illustrating the increase in size of the mitotic apparatus during Phase II, when the chromosomes separate and the cell divides. (High power, phase contrast.)...

Fig. 7. Low-power electron photomicrograph of isolated mitotic apparatus at metaphase (X3400). After isolation, mitotic apparatus was treated with neutral 0s04, embedded in methacrylate, and sectioned. Chromosomal fibers apparent. Spindle and asters appear to be gels with oriented regions corresponding to fillers seen with light microscope. Electron microscopy carried out by Miss P. F. Harris.)...

Damage to the mitotic spindle (B). The contractile proteins of the spindle apparatus must draw apart the replicated chromosomes before the cell can divide. This process is prevented by the so-called spindle poisons (see also colchicine, p. 326) that arrest mitosis at metaphase by disrupting the assembly into spindle threads of microtubuli. These consist of the proteins a-and p-tubulin. Surplus tubuli are broken down, enabling the tubulin subunits to be recycled. 

Griseofulvin is a mitotic spindle poison. In vitro, ii rapidly arrests cell divirsion in metaphase. It causes a rapid, reversible dissolution of the mitotic spindle apparatus, ably by binding with the tubulin dimer that is required for microtubule assembly. The selective toxicity to fungi o probably due to the propensity of the drug to concentrate in ti.ssues rich in keratin, where dermatophytes typically cstalv li.sh infections. 

A NUMBER of polycyclic products of different chemical constitution, but all of natural origin, have the ability to inhibit growth by arresting cells at mitosis. At low dose levels, all these compounds inhibit cells in metaphase by interfering with the formation of the mitotic spindle apparatus. It is likely that at the biochemical level, they all act by a basically similar mechanism of action. 

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