Which term describes the basic structural unit of nucleic acids consisting of a sugar, phosphate, and a base?

• A. Nucleotide
• B. Purines
• C. Base pairs
• D. Double helix

Answer: (A)

Nucleotides are the basic building blocks of nucleic acids. Each nucleotide contains a five‑carbon sugar (deoxyribose in DNA or ribose in RNA), a phosphate group, and a nitrogenous base. The sugar and phosphate form the repeating backbone of the strand, linked by phosphodiester bonds, while the bases project inward and pair with their complementary partners on the opposite strand to store genetic information. Purines are a type of base (adenine and guanine), not the whole building block. Base pairs describe the paired bases across strands, not a single unit. A double helix is the overall shape of DNA, not the unit itself. Thus, the term that describes the basic structural unit is nucleotide.