2.9—Nucleotides, DNA, RNA and base pairing

A nucleotide contains a pentose sugar, phosphate group and nitrogenous base. DNA nucleotides contain deoxyribose and A, T, C or G; RNA nucleotides contain ribose and A, U, C or G.

Nucleotides join by phosphodiester bonds between the phosphate and sugars, forming a sugar–phosphate backbone. Complementary bases pair through hydrogen bonds: A with T in DNA or U in RNA, and C with G.

The base sequence stores information while the backbone gives the polymer continuity. The 2′ hydroxyl in ribose makes RNA chemically less stable than DNA and suits its shorter-lived information-carrying roles.

A DNA template with the base A pairs with a T in the complementary strand; an RNA transcript uses U instead. Replacing T with U is a molecule-level difference, not a change in the genetic message by itself.

A nucleotide is not just a base, and DNA/RNA strands are not held together by phosphodiester bonds between strands. Keep covalent backbone links separate from hydrogen-bonded base pairing.