Why does high pH result in the denaturation of DNA?

Question

In the Southern blot method, for example, a solution of NaOH is used to denature the DNA in the sample. I find this counterintuitive since I expected that $\ ext{Na}^+$ cations would neutralize the negative charges of phosphate groups, hence stabilizing the double helix.

Answer 1

High pH denatures DNA by interfering with the proper hydrogen bonding and pairing of the nucleotide bases within the DNA double helix (A-T, G-C).