PCR primers are typically 16-28 nucleotides long. If the length is too short, it is difficult to design gene-specific primers and choose optimal annealing temperature. On the other hand, very long oligos unnecessarily increase oligo synthesis cost. In addition, longer primers are more likely to form secondary structures that result in decreased PCR efficiency or promote primer dimer formation, since the primers constitute the nucleic acid sequences at highest concentration in the reaction.
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