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Fig. 8. Model of APCcdc20 and APCcdh1 activation during meiosis II. Unfertilized MII eggs, which have high CDK1 levels, have low APCcdc20 and APCcdh1 activity. A sperm-derived Ca2+ signal initiates activation of APCcdc20 at gamete fusion, which is responsible for cyclin B1/securin degradation and an associated loss in CDK1 activity. Degradation of these APCcdc20 substrates permits second polar body formation and at this time APCcdh1 activity appears. APCcdh1 is responsible for degradation of KEN-box substrates such as cdc20. APCcdh1 activity drops when pronuclei form and the zygote enters S phase of the first embryonic cell cycle.





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