Introduction: This question set on Cell Cycle Regulation is tailored for M.Pharm students studying Cellular and Molecular Pharmacology. It focuses on molecular mechanisms controlling progression through G1, S, G2 and M phases, the key regulators (cyclins, CDKs, CDK inhibitors, checkpoint kinases, APC/C and SCF ubiquitin ligases), and the DNA damage response pathways that govern cell-cycle checkpoints. Clinical relevance is emphasized by including pharmacological modulators such as microtubule-targeting agents and CDK4/6 inhibitors, and how disruption of these pathways contributes to cancer. These MCQs aim to deepen conceptual understanding and to help prepare for exams and drug-development applications involving cell-cycle targets.
Q1. What is the primary biochemical function of cyclin–CDK complexes in cell-cycle control?
- They phosphorylate specific substrate proteins to drive phase transitions
- They directly ubiquitinate proteins to promote degradation
- They form structural components of the mitotic spindle
- They act as membrane receptors for growth factors
Correct Answer: They phosphorylate specific substrate proteins to drive phase transitions
Q2. The G1/S checkpoint primarily monitors which cellular conditions before DNA replication is initiated?
- DNA integrity and mitotic spindle attachment
- Nutrient status and completion of cytokinesis
- DNA damage and growth factor signaling
- Chromosome segregation and cohesin cleavage
Correct Answer: DNA damage and growth factor signaling
Q3. How does p53 typically induce G1 arrest following DNA damage?
- By acting as a kinase that phosphorylates CDKs
- By transcriptionally activating p21 which inhibits cyclin–CDK activity
- By promoting proteasomal degradation of Rb
- By directly binding DNA replication origins to block firing
Correct Answer: By transcriptionally activating p21 which inhibits cyclin–CDK activity
Q4. What is the principal function of the retinoblastoma protein (Rb) in G1 phase?
- To phosphorylate E2F transcription factors
- To bind and inhibit E2F, preventing S-phase gene transcription
- To ubiquitinate cyclin D for degradation
- To activate the anaphase-promoting complex
Correct Answer: To bind and inhibit E2F, preventing S-phase gene transcription
Q5. The anaphase-promoting complex or cyclosome (APC/C) is best described as which type of regulator?
- A serine/threonine phosphatase activating CDKs
- An E3 ubiquitin ligase that targets securin and cyclin B for degradation
- A DNA helicase required for origin firing
- A mitotic motor protein that separates chromosomes
Correct Answer: An E3 ubiquitin ligase that targets securin and cyclin B for degradation
Q6. The SCF (Skp1–Cullin–F-box) complex primarily contributes to cell-cycle progression by:
- Stabilizing cyclin B during mitosis
- Targeting specific phosphorylated proteins for ubiquitination during G1/S transition
- Directly phosphorylating Rb to release E2F
- Cleaving cohesin to allow sister-chromatid separation
Correct Answer: Targeting specific phosphorylated proteins for ubiquitination during G1/S transition
Q7. Which sensor kinase is predominantly activated by DNA double-strand breaks?
- ATR
- CHK1
- ATM
- mTOR
Correct Answer: ATM
Q8. Activation of checkpoint kinases Chk1 and Chk2 leads to cell-cycle arrest mainly through which mechanism?
- Direct activation of CDK activity
- Phosphorylation and inhibition of Cdc25 phosphatases, preventing CDK activation
- Proteolytic cleavage of cyclins by caspases
- Recruitment of origin recognition complex to DNA
Correct Answer: Phosphorylation and inhibition of Cdc25 phosphatases, preventing CDK activation
Q9. How do Wee1 kinase and Cdc25 phosphatase reciprocally regulate CDK1 activity at the G2/M transition?
- Wee1 removes inhibitory phosphates; Cdc25 adds inhibitory phosphates
- Wee1 adds inhibitory phosphate on CDK1; Cdc25 removes inhibitory phosphate to activate CDK1
- Both Wee1 and Cdc25 ubiquitinate CDK1 to regulate its levels
- Both are transcription factors that repress cyclin B expression
Correct Answer: Wee1 adds inhibitory phosphate on CDK1; Cdc25 removes inhibitory phosphate to activate CDK1
Q10. The spindle assembly checkpoint proteins MAD and BUB delay anaphase onset by:
- Activating separase to cleave cohesin
- Inhibiting APC/C activator Cdc20 until all kinetochores are properly attached
- Dephosphorylating histones to condense chromatin
- Recruiting microtubule severing enzymes
Correct Answer: Inhibiting APC/C activator Cdc20 until all kinetochores are properly attached
Q11. Cyclin D–CDK4/6 complexes promote cell-cycle progression primarily by:
- Phosphorylating Rb to release E2F and permit S-phase gene expression
- Directly initiating DNA replication at origins
- Cleaving cohesin complexes to enable chromosome segregation
- Activating APC/C to degrade S-phase cyclins
Correct Answer: Phosphorylating Rb to release E2F and permit S-phase gene expression
Q12. In flow cytometry DNA content analysis, which DNA content values correspond to G1 and G2/M phases?
- G1 = 4N; G2/M = 2N
- G1 = 1N; G2/M = 2N
- G1 = 2N; G2/M = 4N
- G1 = S-phase; G2/M = sub-G1
Correct Answer: G1 = 2N; G2/M = 4N
Q13. Which statement correctly contrasts the mechanisms of taxanes and vinca alkaloids?
- Taxanes destabilize microtubules whereas vinca alkaloids stabilize them
- Taxanes inhibit DNA polymerase while vinca alkaloids inhibit topoisomerase
- Taxanes stabilize microtubules and inhibit depolymerization; vinca alkaloids prevent microtubule polymerization
- Both drug classes act primarily by inhibiting cyclin-dependent kinases
Correct Answer: Taxanes stabilize microtubules and inhibit depolymerization; vinca alkaloids prevent microtubule polymerization
Q14. What is the principal pharmacological effect of selective CDK4/6 inhibitors in hormone receptor–positive breast cancer?
- Induction of mitotic catastrophe by microtubule stabilization
- Inhibition of Rb phosphorylation resulting in G1 cell-cycle arrest
- Activation of APC/C to promote anaphase onset
- Direct DNA intercalation causing double-strand breaks
Correct Answer: Inhibition of Rb phosphorylation resulting in G1 cell-cycle arrest
Q15. p21 (CIP1/WAF1) regulates the cell cycle by which mechanism?
- Serving as an activating subunit for CDK2
- Acting as a CDK inhibitor that binds cyclin–CDK complexes
- Phosphorylating Rb to promote S phase
- Ubiquitinating cyclins for degradation
Correct Answer: Acting as a CDK inhibitor that binds cyclin–CDK complexes
Q16. Which of the following is a widely used biochemical marker for cellular senescence?
- High Ki-67 proliferation index
- Strong telomerase (hTERT) expression
- Senescence-associated β-galactosidase activity (SA-β-gal)
- Low p16INK4a expression
Correct Answer: Senescence-associated β-galactosidase activity (SA-β-gal)
Q17. Ubiquitin–proteasome–mediated degradation is essential during the cell cycle primarily because it:
- Destroys DNA to allow replication
- Removes specific regulatory proteins like cyclins to permit orderly phase transitions
- Forms the mitotic spindle from ubiquitin-coated tubulin
- Serves as the main energy source in S phase
Correct Answer: Removes specific regulatory proteins like cyclins to permit orderly phase transitions
Q18. Which enzyme directly cleaves cohesin complexes to allow sister-chromatid separation at anaphase?
- Securin
- Separase
- Topoisomerase II
- Proteasome subunit beta
Correct Answer: Separase
Q19. What is “origin licensing” in DNA replication and when does it occur?
- Loading of MCM helicase complexes onto replication origins during G1 to permit later S-phase firing
- Activation of replication origins by CDK phosphorylation during M phase
- Cleavage of origins by nucleases to start replication in G2
- Disassembly of replication forks after completion in G0
Correct Answer: Loading of MCM helicase complexes onto replication origins during G1 to permit later S-phase firing
Q20. Which of the following combination strategies exemplifies rational targeting of the cell cycle in cancer therapy?
- Combining microtubule stabilizers with telomerase activators
- Using CDK4/6 inhibitors together with endocrine therapy (e.g., palbociclib plus letrozole) in ER-positive breast cancer
- Pairing ATM inhibitors with antibiotics to boost immune function
- Administering proteasome inhibitors with growth factor to promote cell proliferation
Correct Answer: Using CDK4/6 inhibitors together with endocrine therapy (e.g., palbociclib plus letrozole) in ER-positive breast cancer
