Nucleic Acids
Introduction:
Nucleic acids constitute an important class of bio-molecules which are found in the nuclei of all the living cells in the form of nucleoproteins which have nucleic acid as the prosthetic group. These macromolecules derived their name as they were first isolated from nucleus of pus cells. They are the genetic materials of the cells and are responsible for the transmission of hereditary characteristics and biosynthesis of proteins. The genetic information coded in nucleic acid controls the structure of all proteins including enzymes and so governs the entire metabolic activity in the living organism. Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA) are chain-like macromolecules that function in the storage and transfer of genetic information.
Nucleic acids are also called as poly-nucleotides as the repeating structural unit of nucleic acid is a nucleotide. Nucleotide is made up of three parts-a sugar molecule, a heterocyclic nitrogenous base and phosphoric acid and on hydrolysis gives rise to these.
In general a nucleotide may be represented as:
Composition: Four different Deoxy-ribonucleotides serve as major components of DNA, they differ from each other in their nitrogenous base components. The four bases characteristic of Deoxy-nucleotide units of DNA are the Purine derivatives - Adenine and Guanine and the Pyrimidine derivatives - Cytosine and Thymine.
Four different ribonucleotides are present in RNA –they contain Purine bases -Adenine and Guanine and Pyrimidine bases - Cytosine and Uracil. The difference between Deoxy-ribonucleotide and Ribonucleotide is in their sugar-pentose component, it is 2-deoxy-D-ribose in one and D-ribose in the other.
Nucleoside: When the phosphate group of a nucleotide is removed by hydrolysis, the structure remaining is called a nucleoside.
Pyrimidines and Purines:
The parent compounds of the two classes of nitrogenous bases found in nucleotides are the heterocyclic compounds Pyrimidine and Purine. Purine is a derivative of Pyrimidine, it consists of a pyrimidine ring and an imidazole ring fused together. Three Pyrimidine derivatives- Uracil, Thymine, Cytosine and two purine derivatives-Adenine, Guanine constitute the major nitrogenous bases found in nucleotides. The dimensions of the bases, capacity for hydrogen bonding are crucial for the biological function of nucleic acids. Free Pyrimidine and Purine bases are relatively insoluble in water. They are weakly basic and may exist in tautomeric forms depending on the pH.
Types of Nucleic acids:
Deoxyribonucleic acid(DNA) consists of covalently linked chains of Deoxy-ribonucleotides and Ribonucleic acid(RNA)consists of chains of ribonucleotides.DNA and RNA share a number of chemical and physical properties because in both of them the successive nucleotide units are covalently linked in identical fashion by phospho-diester bridges. Apart from four major bases (Adenine, Guanine, Thymine, Cytosine) small amounts of methylated derivatives of these bases are also present in DNA.
