mitosis

mitosis The normal process of nuclear division (occurring at cell division) by which 2 daughter nuclei are produced, each identical to the parent nucleus. Before mitosis begins each chromosome replicates to form 2 sister chromatids; these then separate during mitosis so that one duplicate goes into each daughter nucleus. The result is 2 daughter nuclei, each with an identical complement of chromosomes and hence of genes. Mitosis is conventionally divided into 4 phases: prophase, metaphase, anaphase, and telophase. During prophase, chromosomes become visible within the nucleus because they shorten, thicken, and coil up as a spiral. Each chromosome is longitudinally double except in the region of the centromere, and each single strand of the chromosome is called a chromatid. The nucleolus and nuclear membrane disappear. In lower plant cells, in metaphase the chromosomes move within the spindle connecting the 2 centrosomes (formed by division of the centriole into 2 at the start of prophase, and the subsequent moving apart of these 2 daughter centrioles). The chromosomes finally arrange themselves along the equator of the spindle. During anaphase, the 2 chromatids making up each chromosome separate as the centromere becomes functionally double, and are thus converted to independent chromosomes moving to opposite poles. During telophase, the spindle disappears, a nuclear envelope reappears around each of the 2 groups of daughter chromosomes, the chromosomes return to their extended state, and the nucleoli reappear. This may be followed by cytokinesis. The final result is 2 daughter cells with identical nuclear contents. Compare MEIOSIS.