During DNA replication, why can DNA polymerase only add nucleotides to the 3′ end of a primer?

Answer and explanation

Correct answer: A. It only extends a growing strand at its 3′ end (5′ to 3′ synthesis)

  1. Step 1: DNA polymerase adds nucleotides only to a free 3′ hydroxyl.
  2. Step 2: So the new strand grows 5′ to 3′, and the lagging strand is made as fragments.
Answer
It only extends a growing strand at its 3′ end (5′ to 3′ synthesis).
Why not the others: It can add bases to the 5′ end: new nucleotides are added at the 3′ end. It needs the 3′ end to unwind the helix: unwinding is done by helicase. It adds nucleotides to the 3′ end: primers are short RNA or DNA strands.

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