Which enzyme creates an RNA primer to initiate DNA synthesis?

Which enzyme creates an RNA primer to initiate DNA synthesis?

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

Answer: (A)

At the start of DNA synthesis, a new strand needs a starting point. DNA polymerases can only add nucleotides to an existing 3' end; they can’t begin a new strand from scratch. The enzyme that provides that starting point is primase, which makes a short RNA primer complementary to the DNA template at the replication fork. This primer offers the essential 3' hydroxyl group for DNA polymerase to extend the growing DNA strand. Once the primer is in place, DNA polymerase can add DNA nucleotides to synthesize the new strand. After synthesis, the RNA primer is removed and replaced with DNA, and ligase then seals the backbone. The other enzymes have different roles: helicase unwinds the DNA to open the fork, ligase seals nicks after short fragments are joined, and DNA polymerase builds the new DNA strand but cannot start without the primer.

At the start of DNA synthesis, a new strand needs a starting point. DNA polymerases can only add nucleotides to an existing 3' end; they can’t begin a new strand from scratch. The enzyme that provides that starting point is primase, which makes a short RNA primer complementary to the DNA template at the replication fork. This primer offers the essential 3' hydroxyl group for DNA polymerase to extend the growing DNA strand.

Once the primer is in place, DNA polymerase can add DNA nucleotides to synthesize the new strand. After synthesis, the RNA primer is removed and replaced with DNA, and ligase then seals the backbone. The other enzymes have different roles: helicase unwinds the DNA to open the fork, ligase seals nicks after short fragments are joined, and DNA polymerase builds the new DNA strand but cannot start without the primer.