The phosphorylation events are transient and reversible. When cyclin concentrations decrease phosphatases dephosphorylate their target proteins. A key family of target proteins of cyclin-CDK complexes is the Rb protein which normally binds to the Transcription Factor E2F thereby inactivating it. When the E2F is released it turns on genes for the next stage of the cell cycle. Apoptosis is programmed cell death.
Caspases cleave target proteins at specific aspartate sites.
Caspases are classified as either initiators or executioners. Initiators are activated by signals from other classes of proteins and then activate executioners. One of the things that executioners do is to cleave sequestering proteins that normally bind endonucleases. Within a cell an important protein p53 is induced in G1 due to DNA damage. This activates protein p21 that binds to the Cylin-CDK that would normally phosphorylate Rb.