Mitosis is the process by which a cell ensures each daugher cell will have a complete set of chromosomes. There are five key stages of mitosis: During prophase the chromosomes become condensed and key proteins begin to bind the the kinetochores, preparing for spindle attachment. Upon nuclear envelope breakdown, the cell enters prometaphase, during which the mitotic spindle is formed and the chromosomes attach to microtubules in the spindle via their kinetochores. Once attached, the chromosomes start to allign along the metaphase plate in the center of the spindle. During metaphase, all of the chromosomes are attached to microtubules via their kinetochores, and alligned at the metaphase plate. At anaphase onset, the sister chromatids separate and are moved toward the poles of the spindle. The chromasomal separation and movement toward the poles is called anaphase A. The spindle poles separate as well, which is referred to as anaphase B. Midzone complex formation also occurs in anaphase. During telophase, the mother cell is physically divided into two daughter cells by cytokinesis.