Nucleotides and Nucleic Acids

A nucleotide is the monomeric unit of nucleic acids. Chemically, a nucleotide is composed of a pentose sugar attached to a heterocyclic nitrogenous base through N-glycosidic linkage and to one or more phosphate groups though phosphoester bonds (Figure 1.3A). The pentose sugar can be a ribose (the nucleotide is called a ribonucleotide) or a deoxyribose (the nucleotide is called a deoxyribonucleotide). Nucleic acids are the linear polymers of nucleotides; the polymer of ribonucleotides is called a ribonucleic acid (RNA) while that of deoxyribonucleotides is called a deoxyribonucleic acid (DNA). The nitrogenous bases present in nucleic acids are derivatives of purines and pyrimidines (Figure 1.3B).

 

Figure 1.3 Structures of nucleotides and nucleic acids. Panel A represents the structure of Adenosine triphosphate (ATP). Panel B shows the structures of the sugars and the bases present in the nucleic acids. Panel C shows the double-helical structure of DNA; notice the directionality of the two complementary strands. Panel D shows the structure of a transfer RNA (tRNA) molecule (PDB ID: 2Z9Q).

The bases present in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G). In RNA molecules, thymine is replaced by uracil (U). DNA is usually made up of two antiparallel chains coiled around each other in a double helical structure (double-stranded DNA or dsDNA). Adenine-thymine, adinine-uracil, and guanine-cytosine are said to constitute the complementary base pairs in nucleic acids . The two strands of the DNA molecules are antiparallel and complementary to each other i.e. thymines and cytosines on one strand are paired with adenines and guanines, respectively on the other one (Figure 1.3C). RNA molecules are typically single stranded but can be double-stranded as well e.g. dsRNA viruses. RNA molecules can have intramolecular complementary sequences causing them fold into defined three-dimensional structures e.g. transfer RNA molecules (Figure 1.3D).

Nucleotides such as adenosine triphosphate (ATP), guanosine triphosphate (GTP), flavin adenine dinucleotide (FAD), nicotinamide adenine dinucleotide (NAD⁺), nicotinamide adenine dinucleotide phosphate (NADP⁺), and coenzyme A (CoA) play critical roles in metabolism and intracellular signaling.