Metabolism of Nucleic acids

Nucleosides & Nucleotides
Nucleosides are derivatives of purines and pyrimidines that have a sugar linked to a ring nitrogen. Numerals with a prime (eg, 2? or 3?) distinguish atoms of the sugar from those of the heterocyclic base. The sugar in ribonucleosides is D-ribose, and in deoxyribonucleosides it is 2-deoxy-D-ribose. The sugar is linked to the heterocyclic base via a ?-N-glycosidic bond, almost always to N-1 of a pyrimidine or to N-9 of a purine
Mononucleotides are nucleosides with a phosphoryl group esterified to a hydroxyl group of the sugar. The 3?- and 5?-nucleotides are nucleosides with a phosphoryl
group on the 3?- or 5?-hydroxyl group of the sugar, respectively. Since most nucleotides are 5?-, the prefix “5?-” is usually omitted when naming them. UMP and dAMP thus represent nucleotides with a phosphoryl group on C-5 of the pentose. Additional phosphoryl groups linked by acid anhydride bonds to the phosphoryl group of a mononucleotide form nucleoside diphosphates and triphosphates .Steric hindrance by the base restricts rotation about the ?-N-glycosidic bond of nucleosides and nucleotides. Both therefore exist as syn or anti conformers. While both conformers occur in nature, anti conformers predominate. Table 33–1 lists the major purines and pyrimidines and their nucleoside and nucleotide derivatives. Single-letter abbreviations are used to identify adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U), whether free or present in nucleosides or nucleotides. The prefix “d” (deoxy) indicates that the sugar is 2?-deoxy-D-ribose (eg, dGTP)