Advanced Placement (AP) Biology Practice Exam

Which of the following best describes cyclin?

1. A regulatory protein with a constant level

2. A protein involved in DNA replication

3. A regulatory protein whose concentration fluctuates cyclically

4. A structure involved in cell division

The correct answer is: A regulatory protein whose concentration fluctuates cyclically

Cyclin is a type of regulatory protein that plays a critical role in the cell cycle by activating cyclin-dependent kinases (CDKs). The concentration of cyclins varies throughout the cell cycle, increasing and decreasing at specific stages. This fluctuation is essential for the precise regulation of cell cycle progression, ensuring that the cell moves carefully through different phases, such as G1, S, G2, and M. As cyclins bind to CDKs, they trigger various cellular processes like DNA replication, mitosis, and cell division. Each specific cyclin is synthesized and degraded at different times, which is key to coordinating the complex events of the cell cycle. This dynamic nature of cyclin concentration is what distinguishes it from other proteins that may have a more stable or constant level within the cell. Thus, the best description of cyclin is that it is a regulatory protein whose concentration fluctuates cyclically as the cell progresses through the different stages of the cell cycle.