All animal cells have a centrosome, located near the nucleus in a nondividing cell. At the center of the centrosome are two cen-trioles, which are positioned at right angles to each other. Each centriole is composed of nine evenly spaced bundles of micro-tubules, with three microtubules per bundle (fig. 3.30). Surrounding the two centrioles is an amorphous mass of material called the pericentriolar material. Microtubules grow out of the pericentriolar material, which is believed to function as the center for the organization of microtubules in the cytoskeleton.
Through a mechanism that is still incompletely understood, the centrosome replicates itself during interphase if a cell is going to divide. The two identical centrosomes then move away from each other during prophase of mitosis and take up positions at opposite poles of the cell by metaphase. At this time, the centrosomes produce new microtubules. These new microtubules are very dynamic, rapidly growing and shrinking as if they were "feeling out" randomly for chromosomes. A micro-tubule becomes stabilized when it finally binds to the proper region of a chromosome. In this way, the microtubules from both
Chapter Three centrosomes form the spindle fibers that are attached to each of the replicated chromosomes at metaphase (fig 3.31).
The spindle fibers pull the chromosomes to opposite poles of the cell during anaphase, so that at telophase, when the cell pinches inward, two identical daughter cells will be produced. This also requires the centrosomes, which somehow organize a ring of contractile filaments halfway between the two poles. These filaments are attached to the cell membrane, and when they contract, the cell is pinched in two. The filaments consist of actin and myosin proteins, the same contractile proteins present in muscle.
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