In this article we will discuss about the relations among nucleic acids, nucleotides and nucleosides.
These are defined as polynucleotides composed of repeating units ribonucleotides (in RNA) or deoxyribonucleotides (in DNA). The difference between the two structure is that in deoxyribonudeotides there is no-OH group in position 2′.
The presence of three esterifiable-OH groups in the ribose portion or two in the deoxyribose portion gives rise to several possible phosphoesters. The structure shown below are the 3′-phosphoesters.
Nucleotides are building blocks of large molecules. They serve three crucial functions in cells. Some are energy carriers, other are co-enzymes and still others are carriers of hereditary information (genetic code).
Structurally, a nucleotide can be regarded as a phosphoester of a nucleoside. In turn, a nucleoside is a N-glycoside in which the sugar component is ribose or deoxyribose and aglucon is a pyrimidine or purine base. Thus, nucleotide is composed of three units : a phosphate group derived from phosphoric acid (H3PO4), attached to a pentose sugar (5 carbons), attached to a nitrogenous base (either a pyrimidine or purine).
The general structure of single nucleotide is shown above diagrammatically.
A nucleotide is formed by the reaction of the sugar portion of a nucleoside with phosphoric acid through dehydration synthesis.
Nucleoside + phosphoric acid → Nucleotide + H2O.
Nucleotides may be called acids (adenylic acid, guanylic acid and so forth) because their phosphate groups produce hydrogen ions or they may be designated as the specific nucleoside phosphate (adenosine phosphate, guanosine phosphate and so forth). The names of some nucleotides are listed in Table-1.
The four nucleotides found in DNA are combinations of adenine, guanine, cytosine or thymine with deoxyribose and phosphate.
The four nucleotides found in RNA are combinations of adenine, guanine, cytosine, or uracil with ribose and phosphate.
A nucleoside is a chemical combination of a pentose sugar and a pyrimidine or a purine base. The attachment of the two components is a general dehydration synthesis reaction in-which a molecule of water is removed from between the base and the sugar.
Nitrogenous base + Pentose sugar → Nucleoside + H2O.
Thus a nucleoside is a N-glycoside in which the sugar component is ribose or deoxyribose and the aglucon is a pyrimidine or purine base.
Nucleosides containing ribose are called ribonucleosides. Those containing deoxyribose are called deoxyribonucleosides.
The name of some nucleosides are listed in Table-2.