Describe the structure of a nucleotide and base pairing.

• A. A nucleotide is made of a sugar and a phosphate only.
• B. A nucleotide consists of a sugar, a phosphate, and a nitrogenous base; A pairs with T, C with G.
• C. A nucleotide is made of a sugar, a phosphate, and a lipid tail.
• D. A nucleotide consists of a sugar and a base only.

Answer: (B)

The main idea is that nucleotides are the building blocks of nucleic acids, and how bases pair determines the structure and genetic information carried.

A nucleotide has three parts: a sugar (deoxyrose in DNA), a phosphate group, and a nitrogenous base. The sugar and phosphate form the backbone, while the bases stick out to pair with a partner strand. Base pairing follows specific rules: adenine pairs with thymine (two hydrogen bonds) and cytosine pairs with guanine (three hydrogen bonds). This complementary pairing holds two DNA strands together in the classic double-helix and allows accurate copying during replication and correct transcription into RNA. In RNA, thymine is replaced by uracil, so A pairs with U instead of T.

Descriptions that omit a sugar or a phosphate, or replace a base with something unrelated like a lipid tail, don’t capture what a nucleotide actually is.