the spindle in the initial stage of anaphase. Mitosis During normal mitosis, the SAC is active for a short duration of minutes. During this period, spindle microtubules attach to chromosomes and rectify any improper attachments. High cyclin B levels are also maintained through inhibition of an E3 ubiquitin ligase that normally seeks out cyclin B for degradation. This particular ligase is referred to as (APC/C) anaphase promoting complex or cyclosome. When the APC/C is inhibited, cyclin B levels are kept high by the SAC and it ultimately protects cyclin-dependent kinase (CDK1). Mitosis is prompted by the activation of (CDK1) by cyclin B. After confirmation of proper attachment of all chromosomes, the SAC is turned off and degradation of cyclin B occurs by way of the (APC/C). Spindle poisons, in contrast, inhibit kinetochores during mitosis and prevent them from forming proper attachments to spindle microtubules. Permanent activation of the SAC ensues along with a mitotic arrest that lasts several hours. These cells will either exit mitosis by a different pathway not normal to mitosis or they will apoptose. Examples Some spindle poisons: Mebendazole Colchicine Griseofulvin Vinca Alkaloids Paclitaxel (Taxol) See also Taxane Mitotic inhibitor References Poisons Mitotic inhibitors