1. Prophase is the first stage of mitosis. During prophase, the chromosomes, the chromosomal material, or chromatin, gradualling appears as shortened, distinct, rods. This shortening of the chromatin is one of the first observable signs that mitosis has begun. In animal cells the centrioles begin to move to opposite sides of the cell.

Each chromosome is made of two distinct strands that are called chromatids. Each pair of chromatids is held together by a centromere.

As the chromosomes become visible, other events take place within the cell. The nuclear membrane and the nucleolus gradually disintegrate. A new structure, the spindle, appears. The spindle is a three-dimensional structure shaped somewhat like a football. It consists of microtubules that extend across the cell. The fibers of the spindle appear to guide the movements of the chromosomes during mitosis. Most plant cells do not have centrioles, although they have a spindle. Animal cells also have a structure that plant and other cells do not have. This structure is the aster. The aster is made of microtubules that radiate out from the centrioles.