Introduction
A nucleoside consists of a nitrogenous base linked to a
pentose (sugar). A nucleotide is composed of a nucleoside
plus one ormore phosphate groups. The nitrogenous bases
of nucleosides and nucleotides are either purines or
pyrimidines. Figure 1 shows the structures of themajor
purines (adenine, guanine) and pyrimidines (cytosine,
uracil, thymine). Figure 2 shows the structures of four
common ribonucleosides inwhich a base is linked to
ribose. Figure 3 shows the structures of three important
nucleotides, ATP, ADP, andAMP, which are composed of
the nucleoside adenosine plus three, two and one
phosphate group(s), respectively.
