The best known role of nucleic acids is that of carriers of genetic information in DNA, but they have other functions. We have seen that adenosine triphosphate (ATP) is a carrier of chemical energy. Similar small nucleotides carry energy, reducing power, and chemical groups during synthesis.

Five organic nitrogen bases are at the heart of small nucleotides and nucleic acids, and these are shown (right). Adenine and guanine are derivatives of the double-ring base purine, and cytosine, thymine, and uracil are derivatives of pyrimidine. They are so important that they often are identified only by the letters A, G, C, T, and U. When bonded to the C1` position of the b-D-ribose, as shown on Page 31, they form nucleosides: adenosine, guanosine, cytidine, thymidine (with an -H for the -OH at position C2`), and uridine. (Position numbers in the sugar usually are primed to avoid confusion with positions around the rings of the nitrogen bases.) These nucleosides in turn can form esters with phosphate at any of the three -OH positions on the ribose ring: C2`, C3`, or C5`. A nucleoside esterified with phosphate is called a nucleotide. Esters at the 5` position are the most common. As shown on Page 545, the nucleoside adenosine is esterified with phosphate at the 5` position to form the nucleotide adenosine-5`-monophosphate (AMP), and to this two more phosphate groups are attached to form adenosine-5`-diphosphate (ADP), and adenosine-5`-triphosphate (ATP).