pyrimidine ring: With one or more phosphate groups at the 5' position of the sugar, the molecule is a nucleotide (Figure 1B). When nucleoside triphosphates (NTP) are linked through the 5' phosphate groups to the 3' position of the previous residue on the growing chain, the chemical energy for the polymerization is provided by the removal of the second and third phosphate groups.

In DNA the pentose is 2'-deoxyribose (Figure 1A) and the bases are adenine (A), guanine (G), cytosine (C), and thymine (T). Two strands are wound in opposing chemical directions in the well-defined double-helix structure of DNA, with each nucleotide of one strand linked by hydrogen bonding to a complementary nucleotide on the other (A-T and

G-C). The deoxyribose and phosphate groups form the outer sides of the 'ladder.'

The RNA molecule is chemically single stranded, but double-helical regions arise when stretches of complementary sequences allow hairpin loops to form. In addition, the base uracil (U) is found instead of thymine, and the pentose is ribose.

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