​In yeast, temperature-sensitive mutants of cell cycle regulators, cdc2 (the key cyclin-dependent kinase), cdc13 (required for telomeric DNA replication), and cdc13 rad9 (which carries an additional mutation in the DNA damage sensor) were isolated. When grown in non-permissive temperatures for a few hours, different phenotypes were observed in these mutants. Choose the option that correctly describes the most likely phenotype for all of these mutants.

Correct option is C

1. cdc2 Mutants:
• Cdc2 (Cyclin-dependent kinase) is essential for cell cycle progression.
• It regulates G1/S transition, G2/M transition, and mitosis.
• When mutated, cells fail to progress past these checkpoints, leading to arrest at G1/S, G2, and M phases.
2. cdc13 Mutants:
• Cdc13 is required for telomere maintenance and DNA replication.
• Its loss causes telomere damage, triggering a G2 DNA damage checkpoint arrest.
3. cdc13 rad9 Mutants:
• Rad9 is a DNA damage checkpoint protein required for stopping the cell cycle upon DNA damage.
• If rad9 is mutated in a cdc13 background, the telomere defects fail to trigger a checkpoint arrest, allowing the cells to continue dividing for a few generations until lethal damage accumulates.

Thus, the most accurate description of the phenotypes of these mutants is option (c).

Information Booster:

1. Cdc2 (CDK1 in higher eukaryotes) is a master regulator of the cell cycle.
2. Cdc13 is a telomere-binding protein essential for protecting telomeres from degradation.
3. Rad9 is a checkpoint protein that signals DNA damage and induces cell cycle arrest.
4. Loss of Rad9 in cdc13 mutants leads to unchecked telomere loss, delaying cell cycle arrest.
5. Yeast temperature-sensitive mutants are commonly used to study essential genes.
6. CDK1 (human homolog of cdc2) is a key target in cancer therapy, as its dysregulation leads to uncontrolled cell proliferation.