Practice question

Five prime cap shields messenger RNA from rapid degradation mediated by highly processive five prime to three prime exonucleases. Without cap, uncapped pre-mRNAs bearing five prime monophosphate are substrates for nuclear Xrn2 Rat1 and cytoplasmic Xrn1 surveillance pathways that degrade aberrant or decapped transcripts as part of quality control. Cap physically blocks entry into exonuclease active site channel and recruits cap binding complex CBP20-CBP80 that sterically occludes end. It also masks RNA from innate immune sensors RIG-I and IFIT that detect exposed five prime triphosphates of viral RNAs.