As mitosis proceeds through its four continuous stages, the replicated chromosomes are manipulated so that sister chromatids are equally split among the two daughter cells. At the heart of this manipulation are the spindle fibers, which are cylindrical, hollow cytoskeletal elements composed of the protein tubulin (microtubules). These spindle fibers with the help of accessory proteins called motor proteins produce chromosome movements. Some fibers actually attach to a region of the centromere and pull the chromatids apart in opposite directions, others lengthen to elongate the cell and increase the distance between the newly separated sister chromatids.