Mitosis, the process in which a cell undergoes nuclear division, is one of the four subdivisions of the cell cycle responsible for cell growth and reproduction. The first step in mitosis is prophase. In prophase the chromatin, diffuse in interphase, condenses into chromosomes. Each chromosome duplicates and has become two sister chromatids. At the end of prophase, the nuclear envelope breaks down into vesicles. The following step in mitosis is metaphase. During metaphase the chromosomes align at the equator of the cell and are held in place by ....