Advanced Placement (AP) Biology Practice Exam

Which of the following primarily governs the cell cycle?

1. Enzymatic activity only

2. Gene expression only

3. Stop and go-ahead signals

4. Chromatin accessibility

The correct answer is: Stop and go-ahead signals

The correct choice underscores the importance of signaling mechanisms in regulating the cell cycle. The cell cycle is a highly orchestrated series of events that leads to cell growth and division, and it is primarily controlled by various "stop" and "go-ahead" signals. These signals are usually in the form of cyclins and cyclin-dependent kinases (CDKs), which work together to either promote or inhibit progression through the distinct phases of the cycle. The "go-ahead" signals facilitate transitions from one phase to the next, ensuring that processes such as DNA replication and mitosis only occur when conditions are favorable and when previous phases have been successfully completed. Conversely, the "stop" signals can halt the cycle in response to errors, such as DNA damage. This regulatory framework is crucial for maintaining cellular integrity and preventing uncontrolled cell division, which can lead to cancer. In contrast, while enzymatic activity, gene expression, and chromatin accessibility all play roles in various aspects of cellular function and the cell cycle, they do not exclusively govern the process. Enzymatic activity relates more to the biochemical reactions within cells, gene expression pertains to the production of proteins that can influence the cell cycle but is not the primary regulator, and chromatin accessibility affects the ability