Practice question

DNA polymerases catalyze attack of 3' hydroxyl on alpha-phosphate of incoming dNTP forming phosphodiester linkage. They cannot create that hydroxyl de novo nor initiate phosphodiester bond between two free dNTPs alone. Initiation therefore demands preformed primer-template junction with free 3' OH precisely positioned in active site opposite template. In cells primase, specialized RNA polymerase, synthesizes short RNA primers complementary to template providing required hydroxyl. This obligate primer dependence ensures replication starts only at regulated origins and prevents random DNA synthesis elsewhere.