The spindle apparatus (also called the mitotic spindle or simply the spindle) is a dynamic microtubule structure that moves sister chromatids apart during mitosis. Before the spindle can form, its orientation is determined by the centrosome (“central body”), an organelle in the cytoplasm near the nucleus. In many organisms the centrosome consists of a pair of centrioles, each one a hollow tube formed by nine *microtubule triplets. During S phase the centrosome doubles, and at the beginning of prophase the two centrosomes separate from one another, moving to opposite ends of the nuclear envelope. These identify the “poles” toward which chromosomes move during anaphase. The cells of plants and fungi lack centrosomes, but distinct microtubule organizing centers at each end of the cell play the same role.

The positions of the centrosomes determine the plane at which the animal cell will divide; therefore they determine the spatial relationship between the two new cells. This relationship may be of little consequence to single free-living cells such as yeasts, but it is important for cells in a multicellular organism. For example, during development from a fertilized egg to an embryo, the daughter cells from some divisions must be positioned correctly to receive signals to form new tissues.