Question

why length of each phases are different in mitotic division? tell about each phases prophase, prometaphase, metaphase, anaphase and telophase.

Answer 1

Answer:

Mitosis occurs in four phases. The first phase is prophase. Cells in prophase have an enlarged nucleus, the nucleolus is no longer visible, and the chromosomes appear as short jumbled strands within the nucleus. Cells spend about 14 percent of the cell cycle in prophase. This is the next largest amount of time spent in a phase after interphase. This phase takes longer than the others because the nuclear envelope fragments and the microtubules have to attach to the chromosomes.

Metaphase follows prophase. Cells in metaphase have the chromosomes, which appear as long thin strands under the microscope, lined up along the center of the cell. Metaphase takes about 4 percent of the time required for the completion of a cell cycle. This is one of the quickest phases because all that occurs is the lining up of the chromosomes at the center of the cell.

The third phase is anaphase. This phase only takes about 0.8 percent of the cell cycle to complete, which is the shortest time required out of all the phases. During anaphase the sister chromatids, or the two identical parts of a chromosome, are pulled apart to opposite ends of the cell by the spindle fibers. This allows each daughter cell to have an identical copy of each of the original cell’s chromosomes.

The last stage of mitosis is telophase. Telophase takes up about 3 percent of the cell cycle. This is also a short phase. During telophase the chromosomes appear at the opposite ends of the cell and a new nuclear membrane begins to form around the chromosomes in each half of the cell. Cytokinesis occurs at the end of mitosis as the cytoplasm divides and two distinct cells are formed. In plant cells, a cell plate forms halfway between the divided nuclei, and then a cell wall appears in the cell plate. In animal cells, the cell membrane moves inward until the cytoplasm in pinched in half. Each half has its own nucleus and cytoplasmic organelles.

Mitosis, during which the cell makes preparations for and completes cell division only takes about 2 hours.

Prophase

Mitosis begins with prophase, during which chromosomes recruit condensing and begin to undergo a condensation process that will continue until metaphase. In most species, cohesin is largely removed from the arms of the sister chromatids during prophase, allowing the individual sister chromatids to be resolved. Cohesin is retained, however, at the most constricted part of the chromosome, the centromere. During prophase, the spindle also begins to form as the two pairs of centrioles move to opposite poles and microtubules begin to polymerize from the duplicated centrosomes.

Prometaphase

Prometaphase begins with the abrupt fragmentation of the nuclear envelope into many small vesicles that will eventually be divided between the future daughter cells. The breakdown of the nuclear membrane is an essential step for spindle assembly. Because the centrosomes are located outside the nucleus in animal cells, the microtubules of the developing spindle do not have access to the chromosomes until the nuclear membrane breaks apart.

Prometaphase is an extremely dynamic part of the cell cycle. Microtubules rapidly assemble and disassemble as they grow out of the centrosomes, seeking out attachment sites at chromosome kinetochores, which are complex platelike structures that assemble during prometaphase on one face of each sister chromatid at its centromere. As prometaphase ensues, chromosomes are pulled and tugged in opposite directions by microtubules growing out from both poles of the spindle, until the pole-directed forces are finally balanced. Sister chromatids do not break apart during this tug-of-war because they are firmly attached to each other by the cohesin remaining at their centromeres. At the end of prometaphase, chromosomes have a bi-orientation, meaning that the kinetochores on sister chromatids are connected by microtubules to opposite poles of the spindle.

Metaphase

Next, chromosomes assume their most compacted state during metaphase, when the centromeres of all the cell's chromosomes line up at the equator of the spindle. Metaphase is particularly useful in cytogenetics, because chromosomes can be most easily visualized at this stage. Furthermore, cells can be experimentally arrested at metaphase with mitotic poisons such as colchicine. Video microscopy shows that chromosomes temporarily stop moving during metaphase. A complex checkpoint mechanism determines whether the spindle is properly assembled, and for the most part, only cells with correctly assembled spindles enter anaphase.

Anaphase

The progression of cells from metaphase into anaphase is marked by the abrupt separation of sister chromatids. A major reason for chromatid separation is the precipitous degradation of the cohesion molecules joining the sister chromatids by the protease separase.

Two separate classes of movements occur during anaphase. During the first part of anaphase, the kinetochore microtubules shorten, and the chromosomes move toward the spindle poles. During the second part of anaphase, the spindle poles separate as the non-kinetochore microtubules move past each other. These latter movements are currently thought to be catalyzed by motor proteins that connect microtubules with opposite polarity and then "walk" toward the end of the microtubules.

Telophase

Mitosis ends with telophase, or the stage at which the chromosomes reach the poles. The nuclear membrane then reforms, and the chromosomes begin to decondense into their interphase conformations. Telophase is followed by cytokinesis, or the division of the cytoplasm into two daughter cells. The daughter cells that result from this process have identical genetic compositions.

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