Fig. 8. Model of APC^cdc20 and APC^cdh1 activation during meiosis II. Unfertilized MII eggs, which have high CDK1 levels, have low APC^cdc20 and APC^cdh1 activity. A sperm-derived Ca²⁺ signal initiates activation of APC^cdc20 at gamete fusion, which is responsible for cyclin B1/securin degradation and an associated loss in CDK1 activity. Degradation of these APC^cdc20 substrates permits second polar body formation and at this time APC^cdh1 activity appears. APC^cdh1 is responsible for degradation of KEN-box substrates such as cdc20. APC^cdh1 activity drops when pronuclei form and the zygote enters S phase of the first embryonic cell cycle.