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chap 12-13 - 1 Describe the necessary preparatory steps for...

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1. Describe the necessary preparatory steps for mitosis to occur. Interphase is the first main event that occurs as a preparatory step before mitosis. This process is divided into three categories: the first being G1, in which the cell expands, the S phase in which DNA is synthesized, and G2 with the continuation of cell growth and production of division related materials. In G2 the nucleus is still defined and bounded by the nuclear envelope. The chromosomes have already been duplicated during interphase, but still cannot be distinguished individually because of the loosely packed form of the chromatin fibers. In prophase the chromatin fibers now become more tightly coiled. The chromatids duplicated during the S phase appear as two identical sister chromatids joined together. The centrosomes begin to move to polar ends of the cell propelled by the addition of tubulin to the microtubules between them. In prometaphase the nuclear envelope breaks down, allowing the spindles to interact with the chromosomes. This interaction enables the microtubules to attach to the kinetochore of each of the two chromatids. The chromosomes convene on the metaphase plate during metaphase as the centrosomes are now at opposite ends of the cell. Anaphase occurs as the centromers of each chromosome separate. The liberated sister chromatids move to opposite ends of the cell as their kinetochore microtubules shorten. By the end of anaphase, the amount of chromosomes on each pole of the cell is equivalent. During telophase the nonkinetochore microtubules, attached to the membrane, elongate the cell. Nuclear envelopes form from the remnants of the parent cells envelope. The chromatin becomes less coiled in its new domain. Cytokinesis is occurring, separating the cytoplasm by creating a cleavage in the parent cell. 2. explain the models of poleward chromosomal movement. One main type of poleward chromosomal movement is microtubule motor proteins are part of the kinetochore and use the energy of ATP hydrolysis to pull the chromosomes along its bound microtubules. The other main kind of chromosomal movement is driven by the depolymerization as subunits of tubulin disassociate, and the kinetochore is obliged to slide poleward to maintain its binding to the microtubule. The movement of chromosomes throughout mitosis is attributed to microtubules. Tubulin is constantly added to the microtubules on their plus end closest to the chromosomes. Tubulin is subtracted from microtubules at the minus end closest to the centrosomes. During metaphase the tension is strongest near the chromosomes because the mictrobules from both poles are grabbing the chromatids on the metaphase plate. During anaphase, as the parent cells is splitting into two separate cells, the microtubules shorten in order to bring the new chromosomes into their respective, new cells. This shortening occurs at the kinetochore as the subunits of tubulin are subtracted from the microtubules.
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