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The mitosis is terminated by activated DBRP (destruction box recognition proteins) which can degrade Cyclin B and Cyclin A. DBRP is activated by phosphorylation catalyzed by MPF (the Cyclin B/Cdk1 complex). During mitosis, the activity of MPF keeps rising until it phosphorylates DBRP to degrade Cyclin B, thereby inhibiting MPF. As explained previously, the morphological changes in early mitosis are mainly due to the phosphorylation of related proteins by MPF. After MPF is inhibited in late mitosis, those morphological changes are reversed: chromatin less condensed, nuclear envelope reformed and spindle disappears.
Figure 8-B-3. Destruction of Cyclin A and Cyclin B. (a) Both Cyclin A and Cyclin B contain a special sequence called destruction box. Here we use the sequence of frog (Xenopus) as an example. (b) Phosphorylated DBRP may cause the break down of Cyclin A and Cyclin B through the ubiquitin system.
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