DNA replication requires an RNA primer to begin synthesis on a new strand.

DNA replication requires an RNA primer to begin synthesis on a new strand.

• A. A DNA primer
• B. A free 3' hydroxyl group
• C. An RNA primer
• D. A protein primer

Answer: (D)

DNA polymerases can only extend an existing strand; they can’t start a new one from scratch. To begin synthesis on a new strand, an RNA primer is laid down by primase. This RNA piece provides a free 3' hydroxyl group, which is the spot where DNA polymerase can add the first nucleotide and continue elongation. Later, the RNA primer is removed and replaced with DNA, and the fragments are joined. So the RNA primer is required to start replication. A DNA primer wouldn’t work because polymerases can’t initiate DNA synthesis without a primer, a protein primer isn’t used, and merely having a 3' OH isn’t enough without the RNA primer to supply a starting point.

DNA polymerases can only extend an existing strand; they can’t start a new one from scratch. To begin synthesis on a new strand, an RNA primer is laid down by primase. This RNA piece provides a free 3' hydroxyl group, which is the spot where DNA polymerase can add the first nucleotide and continue elongation. Later, the RNA primer is removed and replaced with DNA, and the fragments are joined. So the RNA primer is required to start replication. A DNA primer wouldn’t work because polymerases can’t initiate DNA synthesis without a primer, a protein primer isn’t used, and merely having a 3' OH isn’t enough without the RNA primer to supply a starting point.