In PCR, the two primers hybridize to the DNA target. Forward primers anneals to the DNA target at the 3' toward 5' terminus where the 3 carbon bonds by a covalent hydroxyl bond with the phosphates of a succeed nucleotide to extend the complement amplified strand. On the second strand of DNA template the complement reverse primer hybridize at 3' toward 5' end where another hydroxyl covalent bond adds a complement nucleotide to the 3 carbon.