Distance between two polynucleotide chains in DNA remains almost constant because

Distance between two polynucleotide chains in DNA remains almost constant because always purine pairs with pyrimidine. This complementary base pairing ensures that the distance between the two polynucleotide chains in the DNA double helix remains relatively constant. The consistent pairing of a purine (A or G) with a pyrimidine (T or C) ensures that the width of the DNA molecule remains uniform because the purine-pyrimidine base pairs have a similar structural geometry that fits the helical structure of DNA.