• Phosphoric acid (H3PO4) is attached to each sugar at the 3′ and 5′ C
positions to give rise to the sugar-phosphate backbone. Free
nucleotides in the cell have 3 phosphate residues, generally attached
to the 5′ C of the pentose. During the phosphodiester bond
formation, two phosphate groups are removed from one of the two
participating nucleotides.