The DNA polymerase reaction. (a) DNA polymerases require a primer strand and a template strand (i.e., a primed template). As dNTPs are added to the 3′-OH group of the primer strand, this strand grows in the 5′→3′ direction. Incoming dNTPs are complementary to the template strand. (b) The insertion and postinsertion sites in DNA polymerase properly align the primed template for sequential addition of incoming nucleotides.