Which enzyme lays down the RNA primer during replication?

Which enzyme lays down the RNA primer during replication?

• A. DNA polymerase
• B. Primase
• C. Helicase
• D. Ligase

Answer: (B)

Priming the DNA for synthesis requires a short RNA primer, because DNA polymerases cannot start a new strand on their own—they need a 3' end to extend from. The enzyme that lays down this RNA primer is primase. It synthesizes a brief RNA segment de novo, providing the free 3'-OH that DNA polymerase needs to begin adding DNA nucleotides. Primase often acts together with helicase to coordinate unwinding and primer placement, and on the lagging strand it repeatedly lays down primers for each Okazaki fragment. Eventually the RNA primer is removed and replaced with DNA, and ligase seals the remaining nicks. The other enzymes have different roles: DNA polymerase extends from the primer but cannot start it; helicase unzips the DNA; ligase seals DNA nick after fragments are joined.

Priming the DNA for synthesis requires a short RNA primer, because DNA polymerases cannot start a new strand on their own—they need a 3' end to extend from. The enzyme that lays down this RNA primer is primase. It synthesizes a brief RNA segment de novo, providing the free 3'-OH that DNA polymerase needs to begin adding DNA nucleotides. Primase often acts together with helicase to coordinate unwinding and primer placement, and on the lagging strand it repeatedly lays down primers for each Okazaki fragment. Eventually the RNA primer is removed and replaced with DNA, and ligase seals the remaining nicks. The other enzymes have different roles: DNA polymerase extends from the primer but cannot start it; helicase unzips the DNA; ligase seals DNA nick after fragments are joined.