Final answer:
APC complexes are active during the mitotic phase of the cell cycle, particularly during the transition from metaphase to anaphase. They work alongside the regulation of cyclins and Cdk/cyclin complexes to control cell cycle progression.
Step-by-step explanation:
APC complexes, or Anaphase Promoting Complexes, are part of the regulatory system that advances a cell through the cell cycle. Specifically, they are active during the transitions between the phases of the cell cycle. The APC is particularly important at the mitotic phase. It triggers the separation of sister chromatids by tagging certain proteins for degradation, which allows the process of mitosis to proceed from metaphase to anaphase. The role of APC in timing the cell cycle is crucial; it ensures that the cell does not prematurely enter the next phase of division without proper alignment and attachment of chromosomes to the spindle fibers.
The broader regulation of the cell cycle involves fluctuating levels of cyclins and the activation of cyclin-dependent kinases (Cdks). When cyclins bind to Cdks, forming a complex, they must also be phosphorylated in order to be fully active. These Cdk/cyclin complexes then phosphorylate other proteins, which leads to the progression through different stages of the cell cycle. Cyclin levels peak at different checkpoints and then decrease as they are degraded, which makes room for the next set of cyclins specific to the following phase.