Final answer:
Cyclin E is present in G1, aiding in the activation of CDK2 at the G1 checkpoint, and CDK2 is present in both G1 and G2, being potentially active in G1. During G2, chromosomes have already replicated, and each has two chromatids, unlike in G1.
Step-by-step explanation:
Cyclin E and CDK2 play critical roles in cell cycle regulation, particularly in the transition through different checkpoints. At the G1 checkpoint, cells prepare for DNA synthesis during the S phase, and it is here that Cyclin E is present and contributes to the activation of CDK2. Moving to the G2 checkpoint, preparation for mitosis is essential, and while CDK2 is present at this point, Cyclin E levels would have fluctuated and decreased as its role is primarily at the G1 checkpoint. Therefore, CDK2 may be active but with different cyclins.
With regards to chromosome structure, in mid G1 (G1), chromosomes typically have not yet replicated, meaning each chromosome generally consists of a single chromatid, whereas in mid G2 (G2), chromosomes have been duplicated during the S phase, so each chromosome will have two sister chromatids. Chromosomes are not usually visible under the microscope during interphase (G1 or G2) since they are in a relaxed or decondensed state and become condensed and visible only as cells enter mitosis.
Combining this information yields the following answers to the student's question:
- Cyclin E is present in G1 (a)
- CDK2 is present in G1 (c)
- CDK2 is active in G1 (e)
- CDK2 is present in G2 (d)
- Chromosomes each have 2 chromatids in G2 (h)