Several nucleotides are linked together by phospho-diester bonds to form a long chain. It is called polynucleotide. In polynucleotides, the phosphate molecule is attached to the 5th carbon of the pentose sugar on one side and to the 3rd carbon of the pentose sugar on the other side. Nucleotides in polynucleotides are oriented towards each other R5 → R3 carbons. Each single helix of a DNA molecule is a polynucleotide chain.