Practice question

DNA polymerases initiate synthesis only from provided primer supplying 3' hydroxyl because they cannot stabilize initial dinucleotide. RNA polymerase overcomes this through additional contacts: sigma factor, downstream DNA binding, and ability to hold two initiating ribonucleoside triphosphates base-paired to template at transcription start site via base stacking and hydrogen bonds. First phosphodiester bond forms de novo without prior polymer. This primer independent initiation permits transcription to start at defined +1 sites throughout genome. Consequently DNA template, Mg2+, and rNTPs suffice, while exogenous primer unnecessary.