How does magnesium concentration affect PCR?

Magnesium Chloride (MgCl2) is an essential cofactor for optimizing PCR protocols. A concentration range of 1.5 to 4.5 mM of MgCl2 is ideal for creating optimal PCR conditions. Very high or very low magnesium concentrations can both affect PCR negatively.  

Higher magnesium chloride concentrations in PCR creates ideal conditions for non-specific binding of primers, which results in errors in DNA replication. There’s also a higher risk of primer dimer formation at higher magnesium chloride concentrations. 

At lower magnesium chloride concentrations primers are unable to base pair with the DNA template, resulting in weak amplification or total failure of the reaction.