DNA is built from which of the following?

• A. Nucleosides
• B. Genes
• C. Purines
• D. Nucleotides

Answer: (D)

DNA is built from nucleotides, the basic monomers that carry a deoxyribose sugar, a nitrogenous base, and one to three phosphate groups. These nucleotides link together through phosphodiester bonds, connecting the phosphate of one nucleotide to the sugar of the next, which forms the DNA sugar–phosphate backbone. The energy for this linking comes from the high-energy phosphate groups of the nucleotides that are used during polymerization, typically as deoxyribonucleotide triphosphates. The bases then pair between strands to create the double helix.

Nucleosides lack the phosphate groups needed to form the backbone, so they’re not the units used to build DNA. Genes are sequences of DNA, not the individual building blocks. Purines are one type of base found in DNA (A and G), but DNA also contains pyrimidines (C and T), so purines alone do not constitute the building blocks.