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During nuclear division or mitosis, there is a progres­sive change in the structure and appearance of the chromosomes. Although mitosis is a continuous pro­cess (Figs. 20-20 and 20-21), for convenience it is usu­ally divided into four major stages: prophase, meta- phase, anaphase, and telophase.Prophase, the first stage of mitosis, is characterized by the condensation of the chromosomes, the disap­pearance of nucleoli and the nuclear envelope, and the formation of the microtubules of the spindle. If, prior to prophase, the cell contained a centriole, then a sec­ond centriole is formed; the two centrioles move apart as the spindle formsThe chromosomes become distin­guishable by light microscopy as a result of their pro­gressive shortening and thickening and eventually arc seen to be composed of two sister chromatids held together at the centromere or kinetochore. The sister chromatids are the products of the replication of chro­mosomal DNA during the interphase of the cell cycle.Toward the end of prophase (sometimes called pro­metaphase), the spindle extends between two poles positioned diametrically opposite one another in the cell and the chromosomes migrate toward the center of the spindle. In metaphase, the centromeres of each chromosome are aligned midway across the spindle on a plane called the equatorial plate.At this time the centromeres are linked to the spindle fibers. Some of the spindle fibers do not form associations with any chromosomes and extend directly from one pole to the other. The centromeres are duplicated so that each chromatid becomes an independent chromosome and is attached to a spindle fiber connected to one of the two poles.The onset of anaphase is characterized by the move­ment of the chromosomes toward opposite poles of the spindle. During anaphase, a process called cytokin­esis begins and divides the cell into two halves, thereby physically separating the two complements of chromosomes. Cytokinesis is distinct from but fre­quently synchronized with nuclear division, occurring during the later stages of mitosis. In the final phase of mitosis, called the telophase, the chromosomes reach the poles of the spindle and begin to undergo decon­densation. During the telophase, nucleoli reappear, as does a new nuclear envelope enclosing the chromo­somes
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