Nucleic acids are colourless, complex, amorphous, compounds made up of three units: bases, sugar and phosphoric acid. These are obtained by the hydrolysis of nucleoproteins which is a class of conjugated proteins. Nucleic acid constitutes the prosthetic group of the nucleoproteins. There are macro-molecules of high molecular weight and are present in every living cell. Nucleic acids are of two types.
(i) Pentose nucleic acids or ribonucleic acids (R.N.A.).
(ii) Deoxypentose nucleic acids or deoxyribonucleic acids (D.N.A.)
Nucleic acids can be hydrolysed in stages to nucleotides, nucleosides and phosphoric acid and ultimately to base and sugar.
Nucleic acid → Nucleotides → Nucleosides + H3PO4 → Base + Sugar
Sugars : Two sugars are found to be present in nucleic acids, namely D-ribose in R.N.A. and 2-deoxyribose in D.N.A.
Bases Two types of bases have been isolated from the hydrolysis products of nucleosides; viz. purines and pyrimidines. Important purine bases are adenine and guanine; while pyrimidine bases are uracil, thymine and cytosine. Adenine, guanine and cytosine are present in RNA as well as in DNA, while thymine is present only in DNA and uracil only in RNA.
Nucleosides : These are condensation products of of base with sugar. The five important nucleosides are named as adenosine, guanosine, cytidine, uridine and thymidine.
Nucleotides : These are condensation products of nucleosides with phosphoric acid. The five important nucleotides are named as adenylic acid, guanylic acid, cytidylic acid, uridylic acid and thymidylic acid. In nucleotides and hence nucleic acids the three important units are present as
Base-Sugar-Phosphate
The double stranded helical structure was given by Wastson and Crick. The two polynucleotide chains of DNA molecule are twisted around a common axis but run in opposite directions to form a right-handed helix. The two chains are joined together by specific hydrogen bonds (adenine to thymine and guanine to cytosine).
