Practice question

Divalent magnesium ions serve as an essential cofactor for DNA polymerases in PCR. Mg2+ coordinates with phosphate oxygens of dNTPs at the active site, facilitates nucleophilic attack by 3' hydroxyl, stabilizes negative charges during phosphodiester bond formation, and influences primer-template duplex stability. Concentration critically affects enzyme activity, fidelity, specificity, and primer annealing. Unlike an enzyme that catalyzes reaction itself, buffer that maintains pH, or template that encodes sequence, Mg2+ is a non-protein chemical activator indispensable for catalytic function and typically used at 1.5-2.5 mM.