During DNA replication, which enzyme lays down RNA primers?

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

Answer: (D)

Primase is the enzyme that lays down RNA primers during DNA replication. DNA polymerase can only add nucleotides to an pre-existing 3' end, so it cannot start a new strand from scratch. Primase, as an RNA polymerase, makes a short RNA primer complementary to the DNA template, providing the free 3' hydroxyl group that DNA polymerase needs to begin synthesis. On the lagging strand this happens repeatedly to create Okazaki fragments, which are later replaced with DNA and joined by ligase. The primer itself is made of ribonucleotides and is eventually removed and replaced with DNA.