The cell enters prophase as the long tangled DNA molecules, or chromatin, coil into the compact bodies of chromosomes. This coiling process is comparable to taking a thin strand 656 ft (200 m) long and coiling it into a cylinder 1 mm wide by 8 mm long. A structure called the kinetochore is formed on each chromatid at the outer face of the centromere region (see Figure 1). The nuclear membrane breaks down.
During prophase, the cells' cytoskeleton or structural framework made of the protein tubulin breaks down into subunits. From these subunits, a bridge of microtubules called the spindle apparatus forms between the two pairs of centrioles as they move apart. When the centrioles reach opposite ends of the cell, they extend microtubules in all directions. Like a boat moored to a dock with multiple lines, the centriole anchors itself to the cell membrane. This arrangement of microtubules—called an aster because of its star-like pattern—is thought to mechanically strengthen the spindle apparatus. Mitosis does not appear to depend upon the presence of centrioles, however. Destroying centrioles in animal cells with a laser beam does not prevent mitosis. Plant cells equipped with rigid cell walls, but neither centrioles nor asters, also grow by mitotic division.
As prophase continues, a set of microtubules grows from the kinetochore of each sister chromatid. The microtubules extending from each chromatid become attached to opposite poles of the spindle.