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Mitotic Spindle Function and Assemby The mitotic spindle is a fascinating and complex molecular machine that performs the crucial task of distributing an exact complement of chromosomes to each daughter cell during mitosis (Heald, 2000). The segregation of chromosomes is complete by the spindle. It is a bipolar system (plus (+) and minus (-)) and is made of microtubules. This organelle undergoes a programmed series of motility events that ultimately leads to the segregation of the replicated chromosomes. The force-producing mechanisms that drive spindle movement have become clear over years of study. Mitotic roles for five motors of the kinesin related family and a dynein family motor have been defined. The proper assembly and function of the mitotic spindle would not be possible without the presence of motor proteins, which couple the energy of ATP hydrolysis to force production. AIR-1 and AIR-2 are two highly related proteins that are uniquely localized to the mitotic spindle and are required at different points in the mitotic cycle. The AIR-1 protein is associated with mitotic centromeres and is required early in mitosis for the proper assembly and function of the mitotic spindle. For chromosome segregation and cell c