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Mitosis
Mitosis is a type of cell division that occurs in somatic cells (non-reproductive cells) and is responsible for growth, development, tissue repair, and asexual reproduction in some organisms. It results in the production of two daughter cells that are genetically identical to the parent cell.
The process of mitosis can be divided into several distinct stages:
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Prophase: In prophase, chromatin (the loosely packed form of DNA) condenses into visible chromosomes. The nuclear envelope breaks down, and the nucleolus disappears. The mitotic spindle, composed of microtubules, begins to form and extends between two structures called centrosomes, which move to opposite ends of the cell.
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Prometaphase: During prometaphase, the nuclear envelope completely disintegrates, allowing the spindle fibers to interact with the condensed chromosomes. Each chromosome has a protein structure called a kinetochore that forms at the centromere region. The kinetochore attaches to the spindle fibers, which will later facilitate the movement of chromosomes.
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Metaphase: During metaphase, the chromosomes align at the equatorial plane or the metaphase plate, which is located at the center of the cell. The chromosomes are attached to the spindle fibers via their kinetochores. This alignment ensures that each daughter cell will receive an equal and complete set of chromosomes.
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Anaphase: In anaphase, the sister chromatids, which are the replicated copies of each chromosome held together at the centromere, separate and move towards opposite poles of the cell. The spindle fibers shorten and pull the sister chromatids apart. Once separated, each chromatid is considered an individual chromosome.
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Telophase: Telophase marks the final stage of mitosis. The separated chromosomes reach the poles of the cell and begin to decondense, reverting to their chromatin form. The nuclear envelope reforms around the chromosomes at each pole, and the nucleolus reappears. Meanwhile, the mitotic spindle disassembles, and cytokinesis, the division of the cytoplasm, initiates.
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Cytokinesis: Cytokinesis is the process of dividing the cytoplasm to form two daughter cells. In animal cells, a contractile ring composed of actin and myosin filaments forms at the equator of the cell. It contracts, creating a cleavage furrow that pinches the cell in two. In plant cells, a cell plate forms at the equator, gradually expanding until it fuses with the cell walls, resulting in the formation of two daughter cells.
The end result of mitosis is the production of two genetically identical daughter cells, each with the same number of chromosomes as the parent cell. This ensures the distribution of genetic material and allows for growth, tissue repair, and the maintenance of a constant chromosome number in somatic cells.
Mitosis occurs in somatic cells, while meiosis is the type of cell division that occurs in germ cells to produce gametes (sperm and eggs).