Transcription & Translation MCAT Practice Question
A 34-year-old man with a history of recurrent infections and developmental delay undergoes genetic testing. A mutation is identified in the gene encoding guanylyltransferase (capping enzyme), resulting in severely reduced enzyme activity. This enzyme catalyzes the addition of a 7-methylguanosine cap to the 5' triphosphate of nascent mRNA. Cells from this patient are cultured in vitro, and researchers observe a marked decrease in steady-state mRNA levels compared to controls. Which of the following best explains the cellular consequence of deficient mRNA capping in this patient's cells?
Answer choices
- AUncapped mRNA is rapidly degraded by 5' exonucleases, and translation initiation is severely impaired due to loss of ribosome recognitionCorrect answer
- BmRNA remains stable because cap-independent IRES-mediated translation fully compensates for the absence of the 5' cap
- CUncapped mRNA accumulates in the nucleus because exportin-1 cannot recognize transcripts lacking the 7-methylguanosine cap
- DThe 5' triphosphate is instead converted to a monophosphate, allowing normal translation but with slower kinetics
- ETranscription by RNA polymerase II is upregulated as a compensatory mechanism to maintain mRNA abundance
- FUncapped mRNA is protected from degradation but undergoes preferential deadenylation followed by rapid 3' to 5' exosome-mediated decay
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