What is an RNA primer?

• A. A short RNA sequence that provides a starting point for DNA synthesis
• B. A long RNA molecule that encodes RNA polymerase
• C. An enzyme that unwinds the DNA double helix
• D. A DNA primer that is extended by DNA polymerase

Answer: (A)

In DNA replication, an RNA primer is a short RNA sequence that provides a starting point for DNA synthesis. DNA polymerases can only add nucleotides after a pre-existing 3' end, so they can’t start from scratch on a bare DNA template. The RNA primer supplies that 3'-OH end, allowing DNA polymerase to extend the new DNA strand. This primer is made by primase, an RNA polymerase, and on the leading strand a single primer can suffice, while on the lagging strand multiple primers create the Okazaki fragments. After synthesis, the RNA primers are removed and replaced with DNA, with enzymes like DNA polymerase I in bacteria (or RNase H and other polymerases in eukaryotes) and then DNA ligase sealing the backbone.