Which enzyme synthesizes RNA primers during replication in bacteria, and how about in eukaryotes?

• A. Bacteria: DNA polymerase; Eukaryotes: RNA polymerase II
• B. Bacteria: primase (DnaG); Eukaryotes: the DNA polymerase alpha-primase complex synthesizes a short RNA primer followed by DNA
• C. Bacteria: Ligase; Eukaryotes: Primase
• D. Bacteria: Helicase; Eukaryotes: Primase only

Answer: (B)

RNA primers are made by primase to provide a starting 3'-OH for DNA synthesis. In bacteria, primase is the DnaG protein, working with other proteins as the primosome to lay down a short RNA primer, after which DNA polymerase III takes over to extend the new strand. In eukaryotes, primer synthesis is done by the DNA polymerase alpha-primase complex, which first creates a short RNA primer and then begins DNA synthesis with a short stretch of DNA before handing off to the main replicative polymerases. This fits the idea that primer formation is handled by a primase in bacteria and by the DNA polymerase alpha-primase complex in eukaryotes. RNA polymerase II isn’t used to initiate replication primers; it transcribes mRNA. Ligase and helicase have other roles in replication, not primer synthesis.