Cell Biology MCAT Practice Question
A 45-year-old woman with a history of recurrent cervical dysplasia undergoes a research study examining cell cycle regulation in her cervical epithelial cells. Synchronization experiments reveal that her cells arrest in G1 phase and fail to progress to S phase. Flow cytometry confirms normal cyclin E expression. Biochemical analysis shows that CDK2 protein is present and binds cyclin E normally, yet CDK2 enzymatic activity is profoundly reduced. Mass spectrometry identifies phosphorylation of CDK2 at threonine 14 and tyrosine 15. Immunoblotting demonstrates that levels of the protein kinase responsible for these inhibitory phosphorylations are markedly elevated. Which of the following best explains the observed G1/S phase arrest?
Answer choices
- AWee1 kinase hyperactivity phosphorylating inhibitory residues on CDK2, with insufficient CAK-mediated activationCorrect answer
- BIncreased expression of p21 or p27 CDK inhibitors blocking CDK2-cyclin E complex activation
- CHypophosphorylated retinoblastoma protein preventing E2F-mediated S phase gene transcription
- DReduced CDC25A phosphatase expression preventing dephosphorylation of inhibitory CDK2 phosphorylation sites
- EProteasomal degradation of cyclin E preventing CDK2 activation and complex formation
- FDefective ATM/ATR checkpoint signaling leading to uncontrolled CDK2 activation
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