Hello Arnab,

DNA is made of three types of molecules in equal proportions - basic nucleotides, sugar deoxyribose and acidic phosphate groups. The bases are on the inside of the helix and partly hidden from the outside. Deoxyrybose and phopshates are on the outside, forming a backbone.

Though the proportions are equal, the nucleotides are weak bases, so the overall pH is acidic.

Hope it helps!!!