Figure 1. Human kinetochore ultrastructure. The box on the left shows proteins whose locations in the ultrastructure have not been determined by immunoelectron microscopy. Note that MCAK may be located in the inner plate and/or the interzone, as well as in the heterochromatin domain. In addition, three‐dimensional deconvolution studies have shown that CENPA and CENPC are juxtaposed and that CENPA is predominantly located beneath the kinetochore.

Figure 2. Solenoid model of the centromeric chromatin domain. Blocks of CENPA (open spheres) and H3 nucleosomes (filled spheres) are arranged in a solenoid. During metaphase, CENPA nucleosomes are present on the poleward side of the chromosome, and H3 nucleosomes are located between the paired sister kinetochores. Alternatively, in the looping model (not shown), centromeric DNA might loop, rather than spiral, through the elliptical structure.

Figure 3. Role of the kinetochore in progression of anaphase. The spindle checkpoint delays mitosis (open arrow) in response to kinetochores that are improperly attached to microtubules (spheres). This delay occurs when components of the spindle checkpoint are recruited to the kinetochore. These components inhibit the anaphase‐promoting complex (APC) and thus delay mitosis. CDC20 is a specificity factor that activates the APC; the activated complex then binds to securin and allows the ubiquitination of the substrate. Securin inhibits the protease activity of separase. The APC targets securin and destroys it, thereby allowing separase to cleave the cohesin (boxes) that holds the sister chromatids together. This cleavage triggers anaphase.