Mitosis.Major Events Occurring During Mitosis And Cytokinesis.Include Different Phases Of Cell Duplication

620 words - 3 pages

IntroductionCell division has fascinated cell biologists for more than a hundred years. Among many good reasons for this fascination is that mitosis and cytokinesis are beautiful to watch: M phase undoubtedly represents the most dramatic phase of the entire cell cycle, and most cellular structures undergo profound reorganizations. In all eukaryotic organisms, including fungi, plants and animals, a microtubule (MT)-based spindle apparatus is assembled and used for chromosome segregation. Similarly, cytokinesis results from a cooperation between two major cytoskeletal systems, one based on MTs, the other on actin filaments. However, a detailed comparison of mitosis in evolutionarily distant organisms, or even the study of different cell types within one and the same organism, reveals many variations on these common themes. This article will focus primarily on mitosis and cytokinesis in animal cells, such as they ...view middle of the document...

It begins during anaphase of mitosis and its completion marks the end of M phase (These subdivisions are useful but should not distract from the fact that cell division is a continuous process and that individual events are highly coordinated in both time and space.During prophase, interphase chromatin condenses into well-defined chromosomes (each of these chromosomes has undergone replication during the preceding S phase, and therefore consists of two sister chromatids that are held together along their lengths). The assembly of a bipolar MT spindle begins when the unique interphasic MT-organizing centre (MTOC) splits; these separate MTOCs then migrate apart and thereby define the poles of the future spindle apparatus. (In animal cells, the principal MTOCs are termed centrosomes, whereas functionally analogous but morphologically distinct structures in fungi are referred to as spindle pole bodies). Animal cell centrosomes duplicate during S phase but they form distinct MTOCs only after separating at the onset of prophase. Concomitant with the disassembly of the interphase MT network, centrosomes begin nucleating highly dynamic MTs which probe space in all directions. Subsequent to nuclear envelope breakdown, which begins during prophase and ends during prometaphase, some MTs will be captured by specialized proteinaceous structures on mitotic chromosomes, known as kinetochores (Figure 2b). Monopolar attachments of chromosomes are unstable, but the back-to-back arrangement of kinetochores on sister chromatids favours their eventual attachment to MTs emanating from opposite poles, resulting in a stable, bipolar attachment. Chromosomes will then congress to an equatorial plane, the metaphase plate. Throughout metaphase, chromosomes oscillate about a central position in the equatorial plane, indicating that they are under tension due to a balance of forces. Anaphase onset occurs in response to a global, as yet poorly understood signal. It is marked by the simultaneous separation of all sister chromatids and thought to result from a loss of sister chromatid cohesion, rather than an increase in poleward moving forces.

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