Practice question

Linkage between seven methylguanosine cap and first transcribed nucleotide involves unusual five prime to five prime triphosphate bridge, chemically distinct from standard three prime to five prime phosphodiester backbone connecting nucleotides within chain. Reaction catalyzed by guanylyltransferase creates G-five prime ppp five prime-N arrangement where three phosphate groups link inverted guanosine to RNA, blocking free five prime phosphate. Such inverted connectivity makes RNA resistant to five prime to three prime exonucleases Xrn1 and Xrn2 because exonucleases require free monophosphate entry site, thus protecting messenger RNA stability in both nucleus and cytoplasm.