What differentiates nucleosides from nucleotides?

• A. Nucleotides contain a phosphate group; nucleosides do not.
• B. Nucleotides lack a sugar; nucleosides have a sugar.
• C. Nucleosides are polymers; nucleotides are monomers.
• D. Nucleotides are proteins; nucleosides are carbohydrates.

Answer: (A)

The key distinction is the phosphate group. A nucleoside is just a base attached to a sugar (ribose or deoxyribose) with no phosphate. Add one or more phosphate groups and you obtain a nucleotide. Those phosphate groups are what link nucleotides together to form the backbone of DNA and RNA via phosphodiester bonds, and they also play central roles in energy transfer and signaling (like ATP or cyclic nucleotides). So the presence of a phosphate group is what sets nucleotides apart from nucleosides. The other statements don’t fit because both nucleosides and nucleotides contain a sugar and a base, nucleotides can polymerize into nucleic acids, and nucleotides are not proteins.