The structure of cordycepin reveals that it lacks the 3' OH. This functional group is critical for addition of [ribonucleotides] to the growing RNA chain, so after cordycepin is incorporated as a ribonucleoside [monophosphate] into the RNA chain, it prevents the further addition of [ribonucleotides].
Ribonucleotide-based mutagenesis requires two Top1 cleavage events on the same DNA strand and, in the context of transcription, modifications arise mainly when the enzyme cleaves the not-transcribed strand of the reporter.
Thus, 3' OHribonucleotides, monophosphate are the answer.
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