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Cell Cycle

            A person gets all forty-six of their chromosomes from ones parents in a process called cell division. One very important event related to subsequent cell division does occur during interphase (period between the end of one division and the appearance of the next structural changes that indicate the beginning of the next division), namely, the replication of DNA, which begins about ten h before the first visible signs of division and last about seven h. This period is called the S phase of the cell cycle. Following the end of DNA synthesis, there is a brief interval, G2 phase. The period from the end of the cell division to the beginning of the S phase is the G1 phase of the cell cycle. Cells that leave the cell cycle enter a phase known as the G0 phase in which the process that initiates DNA replication is blocked. A cell in G0 phase, upon receiving an appropriate signal, can reenter the cell cycle, being replicating DN, and proceed to divide. Cell division involves two process: nuclear division, or mitosis, and cytoplasmic division, or cytokinesis. Although mitosis and cytokinesis are separate events, the term mitosis is often used in a broad sense to include the subsequent cytokinesis, and so the two events constitute the M phase (mitosis) of the cell division cycle. Nuclear division that is not followed by cytokinesis produces multinucleated cells found in the liver, placenta, and some embryonic cells and cancer cells. .
             When a DNA molecule replicates, the result is two identical chains termed sister chromatids, which initially are joined together at a single point called the centromere. As a cell begins to divide, each chromatid pair becomes highly coiled and condensed, forming a visible, rod-shaped body, a chromosome. As the duplicated chains condense, the nuclear membrane breaks down, and the chromosomes become linked in the region of their centromeres to spindle fibers.

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