A temperature-sensitive S. pombe mutant exhibits cell cycle arrest both at the G1 to S and G2 to M transition phases. This is possibly a mutant of:

Correct option is C

Explanation:

• Schizosaccharomyces pombe (S. pombe) is a model organism for studying the cell cycle.
• Cdc2 (Cyclin-dependent kinase 1, CDK1 in higher eukaryotes) is a key regulator of the cell cycle in S. pombe.
• Cdc2 is required for both the G1 to S phase transition and the G2 to M phase transition.
• A temperature-sensitive mutant of Cdc2 would fail to progress through these checkpoints, leading to cell cycle arrest at both transitions (G1/S and G2/M).
• Since the phenotype described includes arrest at both G1/S and G2/M, this suggests a defect in Cdc2 function.

Thus, the correct answer is Cdc2 only (option 3).

Information Booster

1. Cdc2 is the homolog of CDK1 in eukaryotic cells, and it is essential for cell cycle progression.
2. At the G1/S checkpoint, Cdc2 interacts with Cyclin C to initiate DNA replication.
3. At the G2/M checkpoint, Cdc2 interacts with Cyclin B to promote mitotic entry.
4. Temperature-sensitive mutations in Cdc2 cause cells to arrest at G1/S and G2/M due to failure in kinase activity.
5. Loss-of-function mutations in Cdc2 result in cell cycle arrest, whereas hyperactive mutations cause premature mitosis.
6. S. pombe was instrumental in discovering CDKs, which earned Leland Hartwell, Paul Nurse, and Tim Hunt the 2001 Nobel Prize in Physiology or Medicine.
7. Cdc2 phosphorylation status (by Wee1 and Cdc25) regulates mitotic entry.

Additional Information

• Option (1) Clb1 → Incorrect, as Clb1 is a cyclin involved in mitosis in Saccharomyces cerevisiae (not S. pombe).
• Option (2) Cyclin B → Incorrect, as Cyclin B alone does not regulate the G1/S transition; it primarily regulates the G2/M transition.
• Option (4) Cdc2 and Clb3 → Incorrect, because Clb3 is a cyclin from S. cerevisiae, not S. pombe.