What function does DNA polymerase alpha-primase perform in initiating DNA synthesis?

• A. It provides proofreading for synthesized DNA.
• B. It supplies RNA-DNA primers to initiate DNA synthesis.
• C. It seals Okazaki fragments.
• D. It unwinds DNA.

Answer: (B)

Initiating DNA synthesis requires a starting point with a free 3' end, which DNA polymerases cannot create on their own. DNA polymerase alpha-primase arrives at the replication fork to supply exactly that starting point: its primase activity makes a short RNA primer, and then its polymerase activity extends that primer with a short stretch of DNA. This creates an RNA–DNA primer that provides a ready 3' hydroxyl for the main, high-fidelity polymerases (like delta and epsilon) to take over and elongate the new strand. That priming and brief DNA extension is why this function is the best description.

The other roles described correspond to different enzymes: proofreading is handled by polymerases with 3'→5' exonuclease activity, not by this primase–polymerase mix; sealing Okazaki fragments is accomplished by ligase; unwinding DNA is performed by helicase.