the salient features of the Double-helix structure of DNA include that it is made of two polynucleotide chains with anti-parallel polarity. This means if one chain has the polarity 5' → 3', the other has 3' → 5' . Furthermore, Chargaff's rules state that for double-stranded DNA, the ratio between Adenine and Thymine is constant and equals one, and the total amount of purines equals pyrimidines .