Which of the following properties is associated with a cyclin-dependent kinase (Cdk)?
A) A Cdk is inactive, or "turned off," in the presence of a cyclin.
B) The number of Cdk molecules increases during the S and G₂ phases and decrease during M.
C) A Cdk is an enzyme that catalyzes the attachment of kinetochores to microtubules.
D) A Cdk is an enzyme that attaches phosphate groups to other proteins.

Question

Quizplus · Accepted Answer

Cyclin-dependent kinases (Cdks) are a family of enzymes that add phosphate groups to other proteins, a process called phosphorylation. This modification can cause activation or inactivation of the targeted protein, thereby regulating cell cycle progression. Cdks are active only when bound to cyclins, which undergo fluctuations in concentration throughout the cell cycle. Therefore, A is incorrect. While there is an increase in Cdk activity during S and G₂ phases, this is due to the accumulation of cyclin molecules rather than an increase in Cdk numbers per se, so B is also incorrect. C is incorrect because Cdks do not attach kinetochores to microtubules; rather, they regulate processes such as DNA replication and chromosome segregation.

Which of the Following Properties Is Associated with a Cyclin-Dependent