What is the role of primase in replication?

• A. Primase binds to RNA primers to initiate DNA synthesis.
• B. Primase synthesizes RNA primers to provide 3'OH ends for DNA polymerases to initiate synthesis.
• C. Primase unwinds the DNA ahead of the fork.
• D. Primase seals nicks after synthesis.

Answer: (B)

Primase supplies the starting point for DNA synthesis by making short RNA primers. These primers provide a 3′-OH end that DNA polymerases can extend from, because polymerases cannot begin DNA synthesis from scratch. On the leading strand, a single primer can start continuous synthesis; on the lagging strand, primase repeatedly lays down primers for each Okazaki fragment. After synthesis, the RNA primers are removed and replaced with DNA, and the backbone is sealed by ligase. Other fork activities are handled by different enzymes—helicase unwinds the DNA, and ligase seals nicks—so primase’s job is specifically to generate the RNA primer that starts DNA growth.