Which of the following is true about a nucleoside?

• A. It consists of a five-carbon sugar and a phosphate group.
• B. It consists of a phosphate group and a nitrogenous base.
• C. It consists of a five-carbon sugar and a nitrogenous base.
• D. It lacks a sugar component.

Answer: (C)

A nucleoside is formed from a five-carbon sugar bonded to a nitrogenous base. The sugar can be ribose in RNA or deoxyribose in DNA, and the base can be a purine or a pyrimidine. Importantly, a phosphate group is not part of a nucleoside; adding one or more phosphates to a nucleoside yields a nucleotide, which is what builds DNA and RNA. So the statement describing a nucleoside as consisting of a five-carbon sugar and a nitrogenous base is the correct description. Choices that include a phosphate group or that omit the sugar do not describe a nucleoside.