BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 9. NUCLEIC ACIDS

9.2. Components of Nucleic Acids

Chemically, Nucleic Acids are polynucleotides consisting of numerous monomers known as NUCLEOTIDES (mononucleotides). Upon Treatment with acids, alkalis, or specific Enzymes (Nucleases), nucleic acids break down into simpler components—nucleotides—due to the Cleavage of phosphodiester bonds (Hydrolysis).

Nucleotides are composed of a nitrogenous base (purine or pyrimidine), a pentose sugar (ribose or deoxyribose), and orthophosphoric acid. Depending on The Structure of the pentose, they are classified into ribonucleotides and deoxyribonucleotides. The presence of phosphoric acid residues in nucleotides imparts acidic properties to them; hence they are considered acids, and their polymers are referred to as nucleic acids.

Based on their chemical structure, the nitrogenous bases of nucleic acids are divided into two groups: Purines and Pyrimidines. Pyrimidine derivatives include cytosine (2-oxo-4-aminopyrimidine), uracil (2,4-dioxypyrimidine), and thymine (2,4-dioxo-5-methylpyrimidine):

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Purine derivatives include adenine (6-aminopurine) and guanine (2-amino-6-oxopurine):

DNA and RNA both contain adenine, guanine, and cytosine, whereas thymine, with rare exceptions, is found exclusively in DNA, and uracil exclusively in RNA. Small amounts of additional methylated pyrimidine and purine bases have also been detected in nucleic acids (e.g., 1-methylguanine, 2-methyladenine, 5-methylcytosine, 5-hydroxymethylcytosine, etc.).

The metabolic products of purine bases include hypoxanthine (6-ketopurine), xanthine (2,6-diketopurine), and uric acid (2,6,8-triketopurine):

Pharmacologically active purine derivatives have been identified in plant Cells. For instance, caffeine (1,3,7-trimethylxanthine) is found in coffee beans, theobromine (3,7-dimethylxanthine) is a constituent of cocoa beans, and theophylline (1,3-dimethylxanthine) is present in tea leaves, among others: