Which type of bond links nucleotides in the backbone?

• A. Hydrogen bonds
• B. Phosphodiester bonds
• C. Glycosidic bonds
• D. Peptide bonds

Answer: (B)

The backbone of nucleic acids is held together by covalent phosphodiester bonds that link the sugar of one nucleotide to the phosphate of the next. Specifically, the 5' phosphate of one nucleotide forms a phosphodiester linkage with the 3' hydroxyl of the adjacent sugar, creating a continuous sugar–phosphate backbone with directionality from 5' to 3'. This strong covalent connection provides the scaffold for the nucleotide chain, while other interactions, like hydrogen bonds, occur between bases across strands rather than in the backbone. Hydrogen bonds stabilize base pairing, glycosidic bonds connect a base to its sugar within a nucleotide, and peptide bonds join amino acids in proteins. Hence, phosphodiester bonds are the bonds that link nucleotides in the backbone.