In the next stage of mitosis, known as prometaphase, the nuclear envelope breaks down and the microtubules of the mitotic spindle attach to chromosomes (Fig. 11.5). The microtubules radiating from the centrosomes grow and shrink as they explore the region of the cell where the nucleus once was. This process of growing and shrinking depends on the dynamic instability of microtubules, discussed in Chapter 10.

As the ends of the microtubules encounter chromosomes, they attach to the chromosomes at their centromeres. Associated with the centromere of each chromosome are two protein complexes called kinetochores, one located on each side of the constriction (Fig. 11.6). Each kinetochore is associated with one of the two sister chromatids and forms the site of attachment for a single spindle microtubule. This arrangement ensures that each sister chromatid is attached to a spindle microtubule radiating from one of the poles of the cell. The symmetrical tethering of each chromosome to the two poles of the cell is essential for proper chromosome segregation.