Why the distance between two polynucleotide chains remain constant...??

The distance between two polynucleotide chains remain constant because of the bonding between purines and pyrimidines.

As you now that in DNA, Adenine always pairs or forms a bond with Thymine and Guanine always pairs or forms a bond with Cytosine.

The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp).

Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa. 

Similarly, Guanine is bonded with Cytosine with three H-bonds. 

As a result, always a purine comes opposite to a pyrimidine. 

This generates approximately uniform distance between the two strands of the helix