Prophase

Prophase is the opening stage of karyokinesis, and it accomplishes four things at once: the chromatin material condenses into visibly distinct chromosomes, the centrioles migrate toward opposite poles of the cell, the mitotic apparatus (spindle) begins to take shape, and the nuclear membrane starts to break down. As condensation proceeds, each chromosome becomes clearly visible under the light microscope, and its two sister chromatids — held together at a single centromere — can be distinguished. At the same time the nucleolus starts to shrink and disappear, and the nuclear membrane disintegrates gradually rather than all at once. The centrosome, which had already duplicated back in interphase, now …