Spindle fibers are made up of microtubules. These clusters of microtubules are responsible for moving chromosomes during cell division. Spindle fibers are created at the opposite poles of the cells during the prophase stage of mitosis.
During the metaphase of mitosis, the spindle fibers extend toward the middle, called the metaphase plate. Because of the equal forces exerted by the spindle fibers on the centromeres of the chromosomes, the chromosomes are held in place at the metaphase plate. Furthermore, during anaphase, sister chromatids are pulled by spindle fibers toward the spindle poles. The spindle fibers, which are not connected to the chromatids, increase the length of the cell.