Nucleotides consists of a sugar (ribose in RNA and 2'-deoxyribose in DNA), a heterocyclic base, and at least one phosphate group. The heterocyclic base – adenine (A), guanine (G), cytosine (C), and uracil (U) in RNA, and A,G, C, and thymine (T) in DNA – attaches to the 1' carbon of the sugar via what is called a glycosidic bond (see here). The phosphate group attaches to the 5' hydroxyl group of the sugar. These structural features give nucleotides the following properties:

1. The phosphate group of a nucleotide acts as a strong acid (pKa 1), and this is why DNA and RNA are called nucleic acids.

2. The amine groups of the purine and pyrimidine bases can be protonated.

3. The bases can tautomerize; that is, the bases can redistribute positions of hydrogens and double bonds (Figure 4.4).

4. Nucleotides absorb light strongly in the near-ultraviolet region of the spectrum. This makes it possible to use spectrophotometry to quantitate DNA and RNA (Figure 4.5).

See also: Nucleoside and Nucleotide Naming, Phosphodiester Bonds, Polynucleotide Structures, Nucleosides