Nucleic acids, in addition to polysaccharides and proteins, represent the third type of biopolymers. They occur in all living systems, playing an exclusive role in the biosynthesis of proteins and in the transmission of hereditary characteristics. The nucleic acids are the chemical carriers of the genetic code, which prescribes the specific amino acid sequences in proteins.
Two types of nucleic acids, namely ribonucleic acids (RNA) and deoxyribo-nucleic acids (DNA)1, differ in their structure and biological functions. In this chapter, the main attention will be paid to the structure and some chemical properties for better understanding biochemical behaviour of these biopolymers.
17.1. CONSTITUENTS OF NUCLEIC ACIDS
Just as polysaccharides are polymers made of monosaccharide units and as proteins are polymers made of amino acid units, nucleic acids are high molecular compounds that consist of building blocks called nucleotides. Nucleotides can be produced on enzyme-catalyzed hydrolysis of a nucleic acid (Fig. 17.1). The nucleotide, in its turn, can be hydrolyzed to yield a nucleoside and phosphoric acid. Each nucleoside can finally be cleaved into a sugar (pentose) and a pyrimidine or a purine nucleic base.
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Figure 17.1. The principal scheme of splitting of nucleic acids. The rectangles designate constituents of a product.
The sugar component of RNA is D-ribose, whereas DNA contains 2-deoxy-D-ribose, which lacks the hydroxyl group at C-2 (the names ribose and deoxyribose will be used hereafter). Both pentoses are in the furanose form.
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1 These terms are usually used in the singular, but they designate a great number of various molecules of RNA or DNA.
17.1.1. Structure of Nucleosides and Nucleotides