Metaphase cell division

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. 

Rime, H., Neant, I., Guerrier, P., and Ozon, R. (1989). 6-Dimethylaminopurine (6-DMAP), a reversible inhibitor of the transition to metaphase during the first meiotic cell division of the mouse oocyte. Dev. Biol. 133 169-179. 

The nucleus of all eucariotic cells contains the carrier of the genetic information in the chromosomes. It is possible to visualize the chromosomes and analyze their number and pattern during a special period of cell division (the metaphase). Alterations from their normal shapes are observed as structural chromosome aberrations. These are chromosome type aberrations (terminal and interstitial deletions, dicentrics and rings), chromatid aberrations (gaps, breaks and exchanges) and sister chromatid exchanges. Spontanous frequencies of such chromosome... 

Phosphorylation also plays an evident part in cell division. Specifically, cytoskeleton-associated protein 2 (CKAP2) has a phosphorylation site at threonine 596. This threonine has been tracked and gets phosphorylated between prophase and metaphase, and dephosphorylated a short time later before anaphase, suggesting that the protein is phosphorylated during the formation of mitotic spindles. ... 

The first cell division of meiosis occurs in the primary oocyte but the process is arrested during prophase and remains so until puberty. Just before ovulation, meiosis, which has been arrested since before birth, resumes. The first division halves the number of chromosomes to produce the haploid secondary oocyte. The process is the same as that in spermatozoa (Chapter 20 see Figure 20.29 ) except that the two resulting haploid daughter cells are unequal in size. One is the large functional secondary oocyte whereas the other is much smaller and is known as the first polar body. The second meiotic division is arrested at metaphase. It is completed only at fertilisation. Once again, the division is unequal. One cell is large, the secondary oocyte. The other is small, a second polar body, which is discarded. 

This reduction is achieved through meiosis, a process in which two successive ceU divisions occur without intervening DNA duplication. As with the cell cycle, DNA duphcation wiU have aheady taken place prior to the cell division, so that during the initial prophase each pair of identical DNA molecules forms a pair of chromatids attached to each other at a centrosome. In contrast to mitosis, however, homologous chromosomes now pair up (metaphase I). Note that in a pair of homologous chromosomes each wiU contain the same genes (one of the pair having come from the mother and one from the father) but many of the alleles wiU be different. 

The bone marrow test is used for the detection of structural chromosome aberrations induced by a test substance in bone marrow cells of animals. A structural chromosome aberration is a change in chromosome structure detectable by microscopic examination of the metaphase stage of cell division, observed as deletions and fragments, intrachanges or interchanges. 

Positive results from the in vitro micronucleus test indicate that the test substance induces chromosome damage and/or damage to the cell division apparams, in cultured mammahan somatic cells. Immunochemical labehng (FISH fluorescence in sim hybridization) of kinetochores, or hybridization with general or chromosome specific centromeric/telomeric probes can provide useful information on the mechanism of micronucleus formation. Use of cytokinesis block facilitates the acquisition of the additional mechanistic information (e.g., chromosome nondisjunction) that can be obtained by FISH techniques. The micronucleus assay has a number of advantages over metaphase analysis performed to measure chromosome aberrations (see OECD TG 487 draft). 

Two copies of each chromosome are stored in the nucleus and take on a special structure during metaphase of mitosis prior to cell division. 

The vinca alkaloids bind avidly to tubulin, a class of proteins that form the mitotic spindle during cell division. The drugs cause cellular arrest in metaphase during mitosis, and cell division cannot be completed. Although the vinca alkaloids usually have been regarded as phase specific in the cell cycle, some mammalian cells are most vulnerable in the late S-phase. 

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.

Vinblastine is an alkaloid derived from the periwinkle plant Vinca rosea. Its mechanism of action involves inhibition of tubulin polymerization, which disrupts assembly of microtubules, an important part of the cytoskeleton and the mitotic spindle. This inhibitory effect results in mitotic arrest in metaphase, bringing cell division to a halt, which then leads to cell death. Vinblastine and other vinca alkaloids are metabolized by the liver P450 system, and the majority of the drug is excreted in feces via the biliary system. As such, dose modification is required in the setting of liver dysfunction. The main adverse effects are outlined in Table 54-4, and they include nausea and vomiting, bone marrow suppression, and alopecia. This agent is also a potent vesicant, and care must be taken in its administration. It has clinical activity in the treatment of Hodgkin s... 

Fig. 3. Schematic illustrations of distinct steps in cell division show the central role of contractile motor action in the process of mitosis. (A) to (C) replication (prophase) (D) formation of the mitotic spindle (metaphase) (E) and (F) chromosome migration (anaphase) and building of the nuclear envelopes, and (G) formation of the contractile ring containing actin and myosin, forming the cleavage furrow and eventually two separate daughter cells. CE, centriole pair A, aster of microtubules N, nucleus M, microtubules C, chromosomes K, kinetochore NR, remnant of nuclear envelope NE, nucelar envelope reforming CR, contractile ring CM, cell membrane]. From Squire (1986).

Colchicine can arrest plant and animal cell division in vitro and in vivo. Mitosis is arrested in metaphase because of failure of spindle formation. Cells with the highest rates of division are affected earliest. High concentrations may completely prevent cells from entering mitosis, and they often die. The action also is characteristic of the vinca alkaloids (vincristine and vinblastine), podophyllotoxin, and griseofulvin. 

Figure 2-4. Chromosomes at metaphase stage of the cell division in the bone marrow cells of albino rats exposed and unexposed to toxic chemicals (in vivo).

G2 ends as the first of the five stages of cell division, prophase, begins. Prophase is defined as the first appearance of condensed chromosomes. This is followed by metaphase, in which spindle fibers appear leading from centromeres to opposite sides, or poles, of the cells. The sister chromosomes (i.e., the pairs formed by replication of single chromosomes in S) become aligned in one plane... 

Figure 19.1 During the metaphase of cell division, a chromosome becomes two sister chromatids attached at the centromere. Chromosome banding exemplified using human chromosome 17.

Figure 19.2 A human somatic cell contains two sets of homologous chromosomes, which may be divided into two types autosomes and sex chromosomes. Autosomes are further divided into seven groups A-G. During the metaphase of cell division, each chromosome has been duplicated. Therefore, this karyotype consists of 92 chromosomes.

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