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Nucleosides and nucleotides are the fourth and final major group of biochemical molecules and are essential for numerous biological functions in humans, including maintaining and transferring genetic information, playing a major role in energy storage, and acting as signaling molecules. These molecules can be divided into two major families—purines, which include adenosine and guanine, and pyrimidines, which include cytosine, thymidine, and uracil. The unique structures and interactions of these molecules serve as the building blocks of RNA and DNA molecules and allow fundamental processes of gene replication and protein synthesis to occur. Many other functions of the various nucleosides and nucleotides will be explored in later chapters.


Nucleosides and nucleotides are closely involved in the preservation and transmission of the genetic information of all living creatures. In addition, they play roles in biological energy storage and transmission, signaling, regulation of various aspects of metabolism, and even an important role as an antioxidant. Mistakes or deficiencies in their synthesis usually lead to death. Overproduction or decreased elimination of nucleic acid derivates also lead directly to medical conditions.

Nucleosides have a nitrogenous base and a five-carbon carbohydrate group, usually a ribose molecule (see Chapter 2). Nucleotides are simply a nucleoside with one or more phosphate groups attached (Figure 4-1). The resulting molecule is found in ribonucleic acid or RNA. If one hydroxyl (OH) group has been removed from the ribose, the deoxy versions of the nucleoside and nucleotide form the building blocks of deoxyribonucleic acid or DNA (Figure 4-1). Each component of nucleosides and nucleotides is discussed below.

Figure 4-1.

Basic Structure of Nucleosides and Nucleotides. Five major nucleoside bases are common in human biology, including the purines (two-ring structure) adenine and guanine (top) and the pyrimidines (one-ring structure) cytosine, uracil, and thymine (middle). Nucleosides (bottom) are made of a nitrogenous base, usually either a purine or pyrimidine, and a five-carbon carbohydrate ribose. A nucleotide is simply a nucleoside with an additional phosphate group or groups (blue); polynucleotides containing the carbohydrate ribose are known as ribonucleotide or RNA. If 2′ hydroxyl group (OH) is removed, the polynucleotide deoxyribonucleic acid (DNA) results. [Adapted with permission from Naik P: Biochemistry, 3rd edition, Jaypee Brothers Medical Publishers (P) Ltd., 2009.]


  1. Nitrogenous base—The nitrogenous base of a nucleoside or nucleotide (named because of the nitrogen atoms found in its structure) may be either a purine or a pyrimidine. Purines, including inosine (I), adenine (A), and guanine (G), are two-ring structures and pyrimidines, including uracil (U), cytosine (C), and thymine (T), have only one ring (Figure 4-1). Both purine and pyrimidine nitrogenous bases are made, in part, from amino acids as shown in Figures 4-2 and 4-3, ...

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