Which structure consists of 5' to 3' phosphodiester bonds to form a sugar-phosphate backbone?

• A. DNA backbone
• B. DNA double helix
• C. RNA backbone
• D. Protein backbone

Answer: (A)

The main idea is that a nucleic acid’s backbone is a sugar-phosphate chain built by phosphodiester bonds linking nucleotides in a 5' to 3' direction. Each bond connects the 3' hydroxyl of one sugar to the 5' phosphate of the next, creating a continuous, directional backbone that stabilizes the molecule and leaves a distinct 5' end and 3' end.

This description matches the DNA backbone, which refers to the sugar-phosphate framework that underlies DNA’s structure. The full DNA double helix describes the overall shape and base pairing, not just the backbone. While RNA also has a similar sugar-phosphate backbone, the option given points to the DNA backbone as the structure formed by those 5' to 3' phosphodiester bonds. The protein backbone, on the other hand, is the peptide chain formed by peptide bonds, not nucleic acids.