DNA helix made up of two polynucleotide chains,each constitued by sugar-phosphate-bases,the chains are anti parallel in polarity.(5,—>3’ and 3’—>5’) the bases are linked with H-bonds, Adenine pair with Thymine with two H-bonds while Guanine pair with Cytosine with three H-bonds, Coiling of the chain are in right handed fashion, pitch of the helix ic 3.4 nm and there are 10 bp per turn, the plane of one base pair stack over the other in a double helix.