The melting temperature of a DNA double helix is affected by several factors that affect the hydrogen bonds and /or the other interactions involved in maintaining the helical structure. Two DNA strands of a helix are held together by the hydrogen bonds between the complementary nitrogenous bases. The pH of the solution would affect the hydrogen bonds. If the pH is too acidic, the hydrogen bonds acceptor atoms will accept protons from the solution and would not be able to make hydrogen bonds.
The ionic strength of the solution represents the ions present in it. If the solution has positively charged ions, the negative charges of phosphate residues of the DNA backbone will be neutralized. This would make the double helix more stable. Similarly, a longer DNA strand would have a higher melting temperature as more heat will be required to break more hydrogen bonds as compared to a shorter DNA strand.
