A subunit of nucleic acids formed from a simple sugar, a phosphate group, and a nitrogenous base is called

• A. Nucleoside
• B. Polynucleotide
• C. Sugar-phosphate complex
• D. Nucleotide

Answer: (D)

Nucleic acids are built from nucleotides, each of which 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 backbone of the polymer through phosphodiester bonds, while the base provides the genetic information through base pairing. A nucleoside has only sugar and base, lacking the phosphate, so it isn’t the complete subunit of nucleic acids. A polynucleotide refers to a chain of many nucleotides, i.e., the polymer, not a single unit. A sugar-phosphate complex isn’t the standard term for the subunit. Therefore, the described building block is a nucleotide.