Mitosis is the process by which a eukaryotic cellseparates the chromosomes in its cell nucleus into twoidentical sets, in two separate nuclei. It is generallyfollowed immediately by cytokines is, which divides thenuclei, cytoplasm, organelles and cell membrane intotwo cells containing roughly equal shares of thesecellular components. Karyokinesis and cytokinesistogether define the mitotic (M) phase of the cell cycle—the division of the mother cell into two daughtercells, genetically identical to each other and to theirparent cell. This accounts for approximately 10% of thecell cycle.
During this first mitotic stage, the nucleolus fades andchromatin (replicated DNA and associated proteins)condenses into chromosomes. Each replicatedchromosome comprises two chromatids, both with thesame genetic information. Microtubules of thecytoskeleton, responsible for cell shape, motility andattachment to other cells duringinterphase, disassemble. And the building blocks ofthese microtubules are used to grow the mitotic spindlefrom the region of the centrosomes.In short the chromatin condenses into chromosomes;centrioles move toward opposite poles of the cell.
The nuclear membrane disappears completely.In animal cells, the two pair of centrioles align atopposite poles of the cell.Polar fibers (microtubules that make up the spindlefibers) continue to extend from the poles to the center ofthe cell.Chromosomes move randomly until they attach (attheir kinetochores) to polar fibers from both sides oftheir centromeres.Chromosomes align at the metaphase plate at rightangles to the spindle poles.Chromosomes are held at the metaphase plate by theequal forces of the polar fibers pushing on thecentromeres of the chromosomes.
When every kinetochores is attached to a cluster ofmicrotubules and the chromosomes have lined up along themetaphase plate, the cell proceeds to anaphase then proteinsthat bind sister chromatids together are cleaved. These sisterchromatids now become separate daughterchromosomes, and are pulled apart by shorteningkinetochores microtubules and move toward the respectivecentrosomes to which they are attached. Next, the nonkinetochores microtubules elongate, pulling the centrosomes(and the set of chromosomes to which they are attached)apart to opposite ends of the cell. The force that causes thecentrosomes to move towards the ends of the cell is stillunknown, although there is a theory that suggests that therapid assembly and breakdown of microtubules may causemovement .
In Telophase, the chromosomes are cordoned off into distinct new nuclei in the emerging daughter cells The polar fibers continue to lengthen. Nuclei (plural form of nucleus) begin to form at opposite poles. The nuclear envelopes of these nuclei are formed from remnant pieces of the parent cells nuclear envelope and from pieces of the endomembrane system. Nucleoli (plural form of nucleolus) also reappear. Chromatin fibers of chromosomes uncoil.After these changes, Telophase/mitosis is largely complete and the genetic "contents" of one cell have been divided equally into two.
Cytokinesis is the division in two of the cytoplasm. Itoccurs near or after the end of nuclear division.In animals, the cell membrane folds in all around thecell. From the outside, the cell looks like a half-deflatedballoon that has an invisible thread looped around it; ascytokinesis proceeds the thread is increasinglytightened. Really, the tightening is by protein fibers ofthe cytoskeleton, orientated in various directions justbelow the cell membrane. In Plant cells , the Golgi body manufactures a plate(middle lamella) of vesicles in a plane between the twodaughter-nuclei. This plate spreads out from the centerof the cell, eventually abutting against the cellmembrane all round the cell in that plane. The cell thenlays down a cell wall, replacing callose with celluloseand pectin.
Mitosis plays a very vital role in maintenance of chromosomal set and its importance is following :1.Growth. The number of cells within an organism increases bymitosis and this is the basis of growth in multicellular organisms.2.Cell Replacement. Cells are constantly sloughed off, dying andbeing replaced by new ones in the skin and digestive tract. Whendamaged tissues are repaired, the new cells must be exact cofunction of cells.3.Regeneration. Some pies of the cells being replaced so as toretain normal animals can regenerate parts of the body, andproduction of new cells are achieved by mitosis.4.Vegetative Reproduction. Some plants produce offspring whichare genetically similar to themselves. These offspring are calledclones. It is a method of multiplication in unicellular organismsmeans asexual reproduction.
The cell goes through dramatic changes in ultra structure, itsorganelles disintegrate and reform in a matter of hours, andchromosomes are jostled constantly by probing microtubules.Chromosomes may become damaged. An arm of thechromosome may be broken and the fragmentlost, causing deletion. The fragment may incorrectly reattach toanother, non-homologous chromosome, causing translocation.It may reattach back to the original chromosome, but inreverse orientation, causing chromosomal inversion. Or, it maybe treated erroneously as a separatechromosome, causing chromosomal duplication. The effects ofthese genetic abnormalities depend on the specific nature ofthe error and can range from no noticeable effect at all toorganism death. Tumors are caused, They could be benignand malignant. Cancer is essentially a disease related tomitosis, involving a breakdown in the regulation process suchthat there is uncontrolled cell replication.
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