Answer:
3'
Explanation:
Nucleotides are joined together by phosphodiester bonds in a DNA strand. Formation of phosphodiester bond occurs when a free 3' OH end of existing DNA strand sets a nucleophile attack on the 5' carbon of the incoming nucleotide. Here, the release of pyrophosphate from the incoming nucleotide drives the reaction in the direction of bond formation. Therefore, the presence of free 3' OH end at the existing DNA strand or RNA primer is required for DNA strand synthesis.
