DNA is synthesized in the 5' to 3' direction. The template is antiparallel to the strand being synthesized. The attacking nucleophile is the 3'-hydroxyl at the end of the growing strand (sometimes called the primer terminus). The electrophile is the phosphorus atom of the α-phosphoryl group of the incoming nucleotide. A proofreading exonuclease activity, a separate activity and active site in most DNA polymerases, comes into play if an incorrect nucleotide is added. Refer to Figure 25-5a.