Practice question

Polymerase must discriminate sugar identity to produce RNA chemically distinct from DNA. Active site steric gate comprising conserved tyrosine and hydrogen bonding pocket sensing presence of 2' hydroxyl ensures selection of ribonucleoside triphosphates over deoxyribonucleotides. Presence of bulky side chain obstructs deoxyribose positioning, while ribose 2' OH forms favorable contacts. Though also distinguishing purines versus pyrimidines by template pairing, most critical selective aspect for transcription definition is use of ribonucleotides conferring alkali lability and secondary structure capability. Specificity maintains functional separation between transient RNA messages and permanent genomic DNA storage.