Advanced Placement (AP) Biology Practice Exam

What is cyclin-dependent kinase (Cdk) activated by?

1. Binding to ATP

2. Binding to a specific cyclin

3. Binding to the cell membrane

4. Binding to chromosomes

The correct answer is: Binding to a specific cyclin

Cyclin-dependent kinases (Cdks) are a group of enzymes that play a crucial role in regulating the cell cycle. Their activation is contingent upon binding to specific proteins known as cyclins. When a cyclin binds to a Cdk, it causes a conformational change that activates the kinase, allowing it to phosphorylate target substrates. This phosphorylation is essential for the progression through different phases of the cell cycle, such as the transition from G1 to S or from G2 to M phase. The binding of cyclins to Cdks is highly regulated and specific; different cyclins associate with different Cdks at various points in the cell cycle, ensuring that cellular processes occur in a timely and orderly fashion. This mechanism underscores the importance of cyclins in cell cycle control and the precise timing required for cell division. While ATP binding is important for the kinase activity of Cdks, it is the cyclin binding that is critical for their activation and regulation. Therefore, the correct choice highlights the essential role of cyclins in the functionality of Cdk, which is pivotal for the progression and regulation of the cell cycle.