why the distance between two polynucleotide chains in dnaemains almost constant?

Each polynucleotide chain of a double helical DNA structure is attached to one another with the help of specific number of base pairs. As each single base of one chain is attached to the complementary pair of base present in another chain through hydrogen bonds (H-bonds) and the size of the bonds never change, the distance between the two polynucleotide chains in DNA remains constant.