The importance of nucleic acids in controlling life processes is well known. Under the suitable conditions, DNA molecules are responsible for replication of themselves and acts as templates for RNA synthesis in transcription. In translation, RNA molecules control protein synthesis and thus various metabolic activities in living cells. All these processes are monitored by sequences of bases in polynucleotide chains. Nucleosides consist of a sugar group and a heterocyclic nucleobase that bound to the sugar group by a P-N-glycosidic linkage. The sugar group in the ribonucleosides is a D-ribose, and that in the deoxyribonucleosides is a 2‘-deoxy-Dribose. The nucleobases are amino- and hydroxyl-substituted purine and pyrimidine derivatives, including adenine (A), guanine (G), cytosine (C), uracil (U), thymine (T). The corresponding ribonucleosides are designated adenosine (rA), guanosine (rG), cytidine (rC), uridine (rU). Deoxyribonucleosides are prefixed by deoxy- and their abbreviations are dA, dG, dC, dT. In addition to being building blocks of DNA and RNA, nucleosides play key roles in neurotransmission, signaling transduction, and regulation of cardiovascular activity. Because of their ability to interfere with the synthesis of nucleic acids and cellular repair systems, nucleoside derivatives serve as anti-tumor agents and as antimicrobials. In addition, nucleoside molecules have been developed for use in agricultural chemistry as herbicides or insecticides.
The nucleotides are phosphorylated nucleosides that contain an additional phosphoester group at the 3'- or 5'-hydroxyl function of the sugar group. Nucleotides and their derivatives are involved in multiple important biological processes. They play diverse roles in energy metabolism, enzymatic regulation, and signal transduction and serve as structural components of coenzymes. Nucleoside triphosphates and nucleoside diphosphate, especially adenosine triphosphate (ATP) and adenosine diphosphate (ADP), facilitate energy transfer in biological systems. ATP and ADP also play key roles in the phosphorylation of various enzymes. Adenine nucleotides are components of various coenzymes such as nicotinamide adenine dinucleotide (NAD), flavin adenine dinucleotide (FAD), and coenzyme A (CoA). Nucleotides also act as mediators in metabolic regulation, such as the cyclic nucleotides cyclic adenosine monophosphate (cAMP) and guanosine monophosphate (cGMP) as messengers of hormonal regulation. Adenosine Monophosphate (AMP), ADP, cytidine triphosphate (CTP), and guanosine triphosphate (GTP) are allosteric regulators of enzymes. GTP and Guanosine diphosphate (GDP) are active in signal transduction cascades.
