The salient features of double helix structure of DNA are as follows:There are two polynucleotide chains in a double stranded DNA.The backbone is constituted by sugar phosphate.The two strands possess anti-parallel polarity, means one chain has polarity 5’-3’ and the other has 3’-5’.The two strands of DNA are joined together by hydrogen bonds between purines and pyrimidines. Adenine always pairs with Thymine by two hydrogen bonds and Guanine always pairs with Cytosine by three hydrogen bonds.The two chains are coiled around each other where the pitch of the helix is 3.4 nm and distance between two base pairs is 3.4 A° (as in case of B-DNA)The backbone of DNA double helix is formed by sugars and phosphates where bases are projecting inwards.