Practice question

Primer length balances specificity, thermodynamics, and economy. Oligonucleotides of 18 to 22 nucleotides provide adequate complexity to anneal uniquely within mammalian genome of 3 billion bases, with melting temperature around 55-65°C compatible with Taq polymerase extension. Shorter primers of 10-15 bases lack uniqueness and bind at multiple loci, causing non-specific amplification. Excessively long primers of 25-40 bases increase propensity for secondary structures, hairpins, and primer-dimers, raise cost, and may reduce efficiency. Hence 18-22 bases is considered optimal for standard PCR applications.