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During prophase, the nucleus disappears, spindle fibers form, and DNA condenses into chromosomes (sister chromatids). During metaphase, the sister chromatids align along the equator of the cell by attaching their centromeres to the spindle fibers. During anaphase, sister chromatids are separated at the centromere and are pulled towards opposite poles of the cell by the mitotic spindle. During telophase, chromosomes arrive at opposite poles and unwind into thin strands of DNA, the spindle fibers disappear, and the nuclear membrane reappears.
The spindle fibers form at opposites sides, or poles, of the cell and extend towards the middle. During anaphase, spindle fibers pull chromosomes to opposite poles in preparation for the creation of two new cells. Some spindle fibers never attach to chromosomes and, instead, work to lengthen the cell. Centrioles are important in the organization of microtubules, including spindle fibers. Centrioles do not create the fibers themselves, but instead direct the positioning of the spindle. Centrioles also play an important role in cellular organization.