In DNA adjacent nucleotides are joined together by phosphodiester bonds to form a polynucleotide chain. In a nucleotide, N-base is attached to deoxyribose sugar at C-1 and phosphoric acid is attached at C-5 of sugar. Similarly in a polynucleotide chain, adjacent nucleotides are joined together by ‘phosphodiester bond’ between C-3 and C-5 of different deoxyribose sugars of two adjacent nucleotides. Further, one end of the polynucleotide chain has a sugar residue with C-3 not linked to another nucleotide and the
other end with sugar residue with C-5 linked to a phosphate group, not linked to another nucleotide. These are named as 3' and 5' ends of polynucleotide chain respectively. This chain is not straight but helical in structure.