What is the function of MgCl2 in PCR?

In PCR, MgCl₂ is an essential cofactor that enhances the activity of Taq DNA polymerase, which in turn increases the amplification rate of DNA. It is important to note, however, that higher concentrations of MgCl₂ can result in decreased specificity. In addition, MgCl₂ also helps primers anneal to the correct location on a DNA template; this is due to Mg²⁺ binding to the negatively charged phosphate groups in DNA, thereby decreasing electrostatic repulsion between strands.