Explain how nucleotide, nucleoside, and nucleic-acid strand structures differ.

### What a good answer covers - A **nucleoside** consists of a nitrogenous base and pentose sugar. - A **nucleotide** adds one or more phosphate groups to a nucleoside. - A nucleic-acid strand is a polymer of nucleotides. - Phosphodiester bonds connect the sugar-phosphate backbone and give the strand 5′ and 3′ ends. ### The concept These names describe progressively larger structures. The base carries chemical identity, the sugar links the base to phosphate, and phosphate groups make the polymer backbone possible. This structural sequence helps explain why the terms are related but not interchangeable. ### Terms - **Nucleic acid**: a polymer of nucleotides, such as DNA or RNA. - **3′ and 5′**: labels for positions on a sugar that indicate strand direction. ### Remember A nucleoside is the base-sugar unit; a nucleotide is the phosphorylated form.