What is an RNA primer in DNA replication?

• A. A short DNA segment that acts as a template
• B. A short segment of RNA used to initiate synthesis of a new strand of DNA during replication
• C. An enzyme that unwinds DNA
• D. A protein that stabilizes the replication fork

Answer: (B)

In DNA replication, enzymes can only add nucleotides to an existing 3' end; they cannot start a new strand from scratch. An RNA primer provides that starting point by being a short RNA segment synthesized by primase and complementary to the template strand. This primer gives DNA polymerase a 3' hydroxyl to extend from, allowing the new DNA strand to be synthesized.

On the leading strand, a single primer can support continuous synthesis, while on the lagging strand, multiple primers are laid down to produce Okazaki fragments that are later filled in and joined. After synthesis, the RNA primer is removed and replaced with DNA, and the fragments are sealed by ligase.

The other options refer to different components: a short DNA segment used as a template is a template, not a primer; an enzyme that unwinds DNA is helicase; and a protein that stabilizes the replication fork is usually a single-strand binding protein.