In DNA replication, in which direction does DNA polymerase add nucleotides?

• A. 5' to 3'
• B. 3' to 5'
• C. 3' to 3'
• D. 5' to 5'

Answer: (A)

DNA polymerase can only add nucleotides to the 3' end of a growing DNA strand, so new DNA is synthesized in the 5' to 3' direction. Each incoming nucleotide is added to the 3' hydroxyl group of the previous nucleotide, forming a phosphodiester bond and releasing energy from the incoming nucleoside triphosphate. This requirement also means the template strand is read in the opposite direction (3' to 5') to ensure proper base pairing. A short RNA primer is needed to start the process because polymerase cannot begin synthesis without a pre-existing 3' end. On the leading strand the synthesis is continuous toward the fork, while on the lagging strand it happens in short Okazaki fragments, but each fragment is still built 5' to 3'.