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.