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Spindle Checkpoint Model
Cdc20 is continually synthesized in mitosis and acts as an activator of the APC/C. However, in the presence of improperly attached kinetochores, Mad2 binds Cdc20 and this allows the loading of Cdc20 onto BubR1. Once BubR1 is bound to Cdc20, Mad2 is not required to sustain the binding and leaves the complex before or after the complex is presented to the APC/C. The APC/C then promotes ubiquitination and degradation of Cdc20 to sustain the checkpoint.
