Question

This question is a bit more advanced for any molecular biologists...
Some DNA polymerases used in PCR reactions require the presence of Mg ions to function properly. Does anyone know why?

Answer 1

Put simply DNA polymerases need Magnesium ions as a cofactor to allow them to unction effectively.
This means it increases its affinity for the nucleotides it binds to so that it can function fully. Without its presence the polymerase (in PCR this is the taq polymerase, from Thermophilus aquaticus) would not bind to the DNA properly and so the PCR reaction would not occur, or would be very low efficiency at best.

Answer 2

The whole principal behind enzyme action is that a protein folds into a certain shape to produce an "active site" on the protein with the appropriately shaped and charged amino acids at that active site. The binding of an Mg2+ ion to the protein is essential to ensure the shape of the protein is correct to allow for binding and catalytic activity of the enzyme on the substrate.