Practice question

DNA polymerases universally catalyze nucleotide addition exclusively in 5' to 3' direction. The enzyme recognizes free 3' hydroxyl group of primer and forms phosphodiester bond with incoming deoxynucleoside triphosphate, releasing pyrophosphate. Even though forward and reverse primers anneal to opposite strands in antiparallel orientation, each extends chemically from its own 5' end toward 3', proceeding toward each other across target region. No natural polymerase synthesizes DNA 3' to 5'. Bidirectional synthesis would violate mechanistic requirement for 3' OH nucleophile in polymerase active site.