DNA polymerase can only add DNA nucleotides off an existing _____.

DNA polymerase can only add DNA nucleotides off an existing _____.

• A. 5' phosphate group
• B. 3' hydroxyl group
• C. 2' deoxyribose
• D. 5' triphosphate group

Answer: (B)

DNA polymerase can only extend a DNA strand from its end that has a free 3' hydroxyl group. The growing strand ends with a 3' OH, which acts as a nucleophile to attack the incoming deoxynucleoside triphosphate's alpha phosphate. This forms a new phosphodiester bond and releases pyrophosphate. Because the reaction adds nucleotides onto the 3' end, a primer with a 3' end is needed to start synthesis. The incoming nucleotide does bring a 5' triphosphate, but the crucial reactive site for bond formation is the 3' hydroxyl on the existing strand, not the 5' phosphate or the sugar itself.

DNA polymerase can only extend a DNA strand from its end that has a free 3' hydroxyl group. The growing strand ends with a 3' OH, which acts as a nucleophile to attack the incoming deoxynucleoside triphosphate's alpha phosphate. This forms a new phosphodiester bond and releases pyrophosphate. Because the reaction adds nucleotides onto the 3' end, a primer with a 3' end is needed to start synthesis. The incoming nucleotide does bring a 5' triphosphate, but the crucial reactive site for bond formation is the 3' hydroxyl on the existing strand, not the 5' phosphate or the sugar itself.