When cyclins are synthesized, they act as an activating protein and bind to. This complex then acts as a signal to the cell to pass to the next cell cycle phase. Eventually, the cyclin degrades, deactivating the cdk, thus signaling exit from a particular phase. There are two classes of cyclins: mitotic cyclins and g1 cyclins: a cdks is an enzyme that adds negatively charged phosphate groups to other molecules in a process called phosphorylation. Through phosphorylation, cdks signal the cell that it is ready to pass into the next stage of the cell cycle. Dependent protein kinases are dependent on cyclins, another class of regulatory proteins. The peaks of mpf activity correspond to the peaks of cyclin concentration. The cyclin level rises during the s and g2 phases and then falls abruptly during m phase: on the cell cycle figure that you drew for monday"s class, indicate how and where cyclins and.