1. 3 DNA is a double helix ucleotide subunit CH <py imidine base Fioure 5 CH2 A polynucleotide Purine base chain consists of a eries of 5C-3C ugar-phosphate links that form a backbone from which the bases Sugar-phosphate backbone protrude 消当Figure 1.5 A polynucleotide chain consists of a series of 5￠-3￠ sugar-phosphate links that form a backbone from which the bases protrude 1.3 DNA is a double helix