The RNA primer provides which chemical group for DNA polymerase to extend?

The RNA primer provides which chemical group for DNA polymerase to extend?

• A. Free 3' hydroxyl group
• B. Free 5' phosphate group
• C. Amino group
• D. Methyl group

Answer: (A)

DNA polymerase can only add new nucleotides to a pre‑existing strand that has a free 3′ hydroxyl group. The RNA primer provides exactly that 3′ OH at the end of the growing strand, allowing the enzyme to form a new phosphodiester bond as the incoming dNTPs are incorporated. The 3′ OH is the nucleophile that attacks the incoming nucleotide’s phosphate, releasing pyrophosphate and extending the chain. The 5′ phosphate at the primer’s start isn’t the site of elongation, and amino or methyl groups don’t participate in backbone formation. This is why replication requires a primer with a 3′ end to begin DNA synthesis.

DNA polymerase can only add new nucleotides to a pre‑existing strand that has a free 3′ hydroxyl group. The RNA primer provides exactly that 3′ OH at the end of the growing strand, allowing the enzyme to form a new phosphodiester bond as the incoming dNTPs are incorporated. The 3′ OH is the nucleophile that attacks the incoming nucleotide’s phosphate, releasing pyrophosphate and extending the chain. The 5′ phosphate at the primer’s start isn’t the site of elongation, and amino or methyl groups don’t participate in backbone formation. This is why replication requires a primer with a 3′ end to begin DNA synthesis.