FIGURE 19-22 Stable and unstable chromosome attachments. When sister kinetochores attach to microtubules emanating from opposite spindle poles, they are stably attached. This configuration is called amphitelic attachment (a). Microtubules (green) pull kinetochores; cohesins resist this pulling force. The resulting tension leads to the outer kinetochore component Ndc80 (yellow) being pulled away from the protein kinase Aurora B (red), which localizes to the inner kinetochore. As a result, Aurora B can no longer phosphorylate Ndc80, and kinetochore-microtubule attachments are stable. When one kinetochore attaches to microtubules emanating from two opposite spindle poles (merotelic attachment, b), or both sister kinetochores attach to microtubules emanating from the same spindle pole (syntelic attachment, c), or only one of the two sister kinetochores attaches to microtubules (monotelic attachment, d), Ndc80 is not pulled away from Aurora B. As a result, Aurora B phosphorylates Ndc80, and Ndc80 can no longer bind to microtubules.