Final answer:
The correct event that occurs at specific times in the cell cycle is the ubiquitylation and subsequent degradation of cyclins in proteasomes. This targeted degradation regulates the progression of the cell cycle by ensuring that cyclins are only present when needed.
Step-by-step explanation:
The question concerns the regulation of the cell cycle, specifically actions that occur at specific times during this cycle. Among the options provided, the correct event is that some cyclins are ubiquitylated and degraded in proteasomes. Cyclins regulate the cell cycle by binding to cyclin-dependent kinases (Cdks). The Cdk/cyclin complex controls the progression through different phases of the cell cycle, and to become fully active, it must be phosphorylated. After cyclins have served their purpose in a given phase, they are marked for destruction by ubiquitylation, which targets them for degradation in the proteasomes. This process ensures that cyclins do not accumulate unnecessarily, allowing for the tight regulation required for cell cycle progression.