The phosphodiester linkages in nucleic acids. Phosphodiester bonds covalently connect the nucleotide units in DNA and RNA. The backbone of alternating sugars and phosphate groups is highly negatively charged. All the phosphodiester linkages in a polynucleotide chain have the same orientation, giving each linear nucleic acid strand a specific polarity and distinct 5′ and 3′ termini. Shown here is a strand of DNA.