Primer

Term used in biotechnology for a piece of DNA or RNA that DNA polymerases require to initiate the synthesis of mononucleotides. In addition to the polydeoxyribonucleotides (dNTPs), DNA polymerases require an initiating oligonucleotide (or polynucleotide), the so-called primer, which carries a hydroxyl group at the 3′ end. The primer is used as a starting point for chain growth.

A primer can be a short or long piece of DNA or RNA that carries a free 3′-OH group. Primers provide the DNA polymerase with a double-stranded structure by attaching to a complementary region of the DNA or RNA strand, the so-called template. The DNA polymerase moves along the DNA (or RNA) template and extends the primer in the 5′ → 3′ direction according to the Watson-Crick base pairing rule, i.e. A pairs with T (or U) and C with G. The polarity of the newly synthesized chain is opposite (or antiparallel) to that of the template. The insertion of a non-complementary nucleotide is considered an "error". The frequency of errors is an important characteristic of a polymerase.