Metaphase to anaphase transition

FIG. 2. Pathway of CSF activation. The figure denotes p90Rsk as the target of the MAPK pathway required for generation of CSF activity, which inhibits the metaphase to anaphase transition in MIL... 

Loss of sister chromatid cohesion would therefore be sufficient for the sudden movement of chromatids to opposite poles at the metaphase to anaphase transition. According to this hypothesis, a specific apparatus binds chromatids together during replication, holds them in an orientation that facilitates the attachment of sister kinetochores to spindles extending to opposite poles, and resists the splitting force that results from this bipolar attachment to the spindle. Destruction of this specialized cohesive structure triggers movement of chromatids to opposite poles at the onset of anaphase. 

The pall of fog that has shrouded the sister separation process for over a century is starting to lift and this long mysterious process has started to reveal its secrets. There is now convincing evidence that the sudden movement of chromosomes to the poles at the onset of anaphase is triggered by cleavage of specific sister chromatid cohesion proteins. Future research must address the structural basis of cohesion and how it is established only at replication forks. It must also address the generality of mechanisms that dismantle cohesion at the metaphase to anaphase transition and how mistakes in this process contribute to human disease. 

Ciosk R, Zachariae W, Michaelis C, Shevchenko A, Mann M, Nasmyth K 1998 An ESP1 /PDS1 complex regulates loss of sister chromatid cohesion at the metaphase to anaphase transition in... 

Surana U, Amon A, Dowzer C, McGrew J, Byers B, Nasmyth K 1993 Destruction of the CDC28/CLB mitotic kinase is not required for the metaphase to anaphase transition in budding yeast. EMBO J 12 1969-1978... 

Cell cycle is one of the physiological processes in which the role of uhiquitin—proteasome-mediated proteolysis is well established. With the advent of yeast mutants that interfered with various phases of the cell cycle, cyclin-dependent kinases (Cdks) were found to have a critical role in regulating the cell cycle. Typically, Cdks activated hy regulatory proteins are known as cyclins. Different Cdk—cyclin complexes are formed at specific stages of the cell cycle such as the S-phase (in which DNA synthesis occurs) and the metaphase. The transition from metaphase to anaphase depends on degradation of cyclins. " " Systematic biochemical studies showed that cyclins were substrates for the uhiquitin—proteasome pathway (Table 5). 

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.

Important decisions for the course of mitosis are the G2/M transition and within mitosis, the transition from metaphase to anaphase (review Draetta and Eckstein, 1997). 

Taxol acts by preventing the transition from metaphase to anaphase during mitosis through the polymerisation of tubulin, this including stable non-functional microtubules. This is a different mechanism of action from those of other plant antitumour substances. Taxol is used in case of ovarian cancer which has spread to other organs (metastasised) and... 

The Xenopus system has proven instrumental in determining the mechanism controlling exit from mitosis at the metaphase/anaphase transition. Studies in this area have relied heavily on extracts prepared from fully mature oocytes/ unfertilized eggs that are arrested at metaphase of the second meiotic division. Upon Ca2+ addition, anaphase is initiated and the extract enters the first embryonic cell cycle to replicate DNA. The activity responsible for metaphase arrest was discovered by Masui at the same time as MPF (Masui Markert 1971), and given the name cytostatic factor (CSF). CSF has never been purified... 

Stratmann R, Lehner CF 1996 Separation of sister chromatids in mitosis requires the Drosophila pimples product, a protein degraded after the metaphase/anaphase transition. Cell 84 25-35 Sumner AT 1991 Scanning electron microscopy of mammalian chromosomes from prophase to telophase. Chromosoma 100 410-418... 

Proteolysis of mitotic cyclins is activated at the metaphase/anaphase transition and is only switched off at the start of S phase. The mitotic cychn A is degraded before cyc-hn B. Lasting activity of the cyclosome during G1 phase is thought to be responsible for the lack of detection of mitotic cyclins in G1 phase. Renewed accumulation of mitotic cyclins is only possible again when APC activity is switched off at the start of S phase. 

Mouse bone marrow mioronuoleus test Looks for small nucleus-like structures in erythrocytes from bone marrow of animals exposed to the test substance these micronuclei are due to material from damaged chromosomes not included in the daughter nuclei during the metaphase-anaphase transition of mitosis, and left behind in the erythrocytes after expulsion of the nucleus from the erythroblasts Drosophila recessive lethal mutation test Looks for increased embryonic lethality in Drosophila melanogaster, a fly species, due to recessive mutations induced by the test substance... 

Passage through three critical cell-cycle transitions— Gi —> S phase, metaphase —> anaphase, and anaphase telophase and cytokinesis—Is Irreversible because these transitions are triggered by the regulated degradation of proteins, an Irreversible process. As a consequence, cells are forced to traverse the cell cycle In one direction only. 

The transition from prometaphase to metaphase is indistinct, but the termination of metaphase is readily recognized by the sudden separation of chromosomes and their anaphase motion toward opposite poles. Anaphase motion, like prometaphase, is parallel to the interpolar spindle axis and similar speeds are observed (reviewed by Mazia, 1961, p. 274). Anaphase differs in that daughter units separate and motion is more uniform. All the chromosomes in any one cell begin motion at about the same time and move at about... 

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