d=basepair_apex_distance(chain[(' ',resseq,icode_res)],chain[(' ',resnum,icode_pair)])#calculation of the distance between the tips of the paired nucleotides
d=basepair_apex_distance(chain[(' ',resseq,icode_res)],chain[(' ',resnum,icode_pair)])# calculation of the distance between the tips of the paired nucleotides