What is the role of primase in prokaryotic replication?

• A. It unwinds the DNA double helix.
• B. It seals nicks in the backbone.
• C. It synthesizes RNA primers to provide starting points for DNA polymerase.
• D. It removes RNA primers.

Answer: (C)

Primase’s job is to lay down RNA primers. DNA polymerase can’t start a new strand from scratch; it can only extend an existing 3′ end. So primase synthesizes short RNA segments that provide a starting point for DNA polymerase to add DNA nucleotides. This is essential for both the continuous synthesis on the leading strand and the discontinuous synthesis of Okazaki fragments on the lagging strand. After synthesis, these RNA primers are removed and the gaps filled with DNA, and the backbone is sealed.

The other roles described belong to different enzymes: a helicase unwinds the helix, a ligase seals nicks in the backbone, and nucleases remove RNA primers after they’ve served their purpose.