Nucleic Acids: Distribution, Constituents and Structure

Let us make an in-depth study of the nucleic acids. After reading this article you will learn about 1. Distribution of Nucleic Acids and 2. Constituents and Structure of Nucleic Acids.

Distribution of Nucleic Acids:

There are two types of nucleic acids i.e., DNA (deoxy-ribose nucleic acid) and RNA (ribose nucleic acid):

(i) DNA which is the seat of all the hereditary characters is chiefly found in chromatin in the nucleus where it is associated with proteins called as histones. Some DNA is also found in mitochondria and chloroplasts.

(ii) RNAs which are intimately associated with protein synthesis are found chiefly in nucleolus in the nucleus, ribosomes and cytoplasm. Besides this, RNA also occurs in mito­chondria and chloroplasts. All plant viruses contain RNA.

Constituents and Structure of Nucleic Acids:

Nucleic acids are un-branched long-chain polymers of nucleotides.

Each nucleotide con­sists of 3 parts:

(i) A purine or pyrimidine base.

(ii) An aldopentose sugar.

(iii) orthophosphoric acid (H₃PO₄).

Different purine and pyrimidine bases found in the nucleic acids are:-

Combination of a base and pentose sugar is called as a nucleoside. If one of the C atoms (either 2, 3 or 5) of the pentose sugar in nucleoside is phosphorylated by H₃PO₄, it is called as a nucleotide Fig. 9.9.

The nucleosides and nucleotides of the different bases are named as follows:

The structure formulae of bases, nucleosides and nucleotides are given in Fig. 9.10, 9.11 and 9.12 respectively.

(Note: In DNA, the nucleosides and nucleotides of Adenine, Guanine, Thymine and Cytosine will contain P-D-2-deoxy ribose sugar.)

The nucleotides are joined together by phosphorous ester links between carbon no. 3 and 5 of the pentose sugars to form long polynucleotide chains that constitute the nucleic acid (Fig. 9.13). That end of the polynucleotide chain at which the 3rd C atom of the sugar is free (not having phosphorous ester link) is called as 3’terminal end (or 3 prime end).

The other end with free 5th C-atom is called as 5’terminal end (or 5 prime end). In polynucleotide chain, the sequence of nucleotides is read from 5′ end to 3′ end. ‘Upstream’ are the nucleotide sequences more towards the 5′-end. ‘Downstream’ are the sequences more towards the 3′-end.