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