Final answer:
The correct option is A. Cyclins D, E, A, and B are active at different stages of the cell cycle, with Cyclin D in the G1 phase, Cyclin E in the S phase, Cyclin A in the G2 phase, and Cyclin B in the M phase. CDKs, which partner with cyclins to regulate the cycle, are primarily found in the cytoplasm and can also be present in the nucleus.
Step-by-step explanation:
Specific cyclins and cyclin-dependent kinases (CDKs) are active at different stages of the cell cycle. Here's where they are primarily located within the cell cycle:
- Cyclin D is involved in the transition from G0 to G1 phase and throughout the G1 phase.
- Cyclin E accumulates during the G1 phase and its levels peak in the early S phase, where it is involved in the initiation of DNA replication.
- Cyclin A associates with CDKs during S phase and is involved in DNA synthesis. Its levels continue to increase into the G2 phase.
- Cyclin B associates with CDKs during the G2 phase and peaks in the M phase, playing a crucial role in the process of mitosis.
CDKs, being enzymes that phosphorylate other proteins, are mostly located in the cytoplasm of the cell, though they can also be present in the nucleus where they associate with their cyclin partners to regulate the cell cycle.