As the two mitotic asters form, they generate interdigitated microtubules of opposite polarity between them. During prophase, the bipolar kinesin-5 interacts with the antiparallel microtubules and, by moving toward the (+) end of each microtubule, slides them apart and thereby pushes the two asters apart. The (−) end–directed motor, cytoplasmic dynein, can also contribute to aster separation as well as orienting the spindle appropriately in the cell. Dynein does this by associating with the cell cortex and pulling on microtubules nucleated from the mitotic asters. As we discuss shortly, this same mechanism is used to elongate the spindle during anaphase B (see Figure 18-43).