Structure and types of RNA

Structure of RNA

RNAs are present in the nucleus, ribosomes and cytoplasm of eukaryolic cells. They are involved in the transfer and expression of genetic information. They act as primer for DNA formation. Some act as enzymes and as coenzymes. RNA also function as genetic material for viruses.

RNAs are also polynucleotides. In RNA polymer, purine and pyrimidine nucleotides are linked together through phosphodiester linkages. The sugar present is ribose. The nitrogenous bases present in RNA are adenine and guanine (purine bases), uracil and cytosine (pyrimidine bases). The nucleotides present in RNA are adenylic acid, quanidylic acid, cytidylic acid and uridylic acid.

Types of RNA

There are mainly three types of RNAs in all prokaryotic and eukaryotic cells. They are (1) Messenger RNA (mRNA) 2) Transfer RNA (tRNA) 3) Ribosomal RNA (rRNA). They differ from each other by size, formation and stability.

1. Messenger RNA

It accounts for 1-5% of cellular RNA. They have a primary structure. They are single standed linear molecules. They consist of 1000-10,000 nucleotides. They have a free or phosphorylated 3’ and 5’ end. They have different life span ranging from few minutes to days.

mRNA molecules are capped at 5’ end by methylated guanine triphosphate. Capping protects mRNA from nuclease attack. At 3’ end a polymer of adenylate (poly A) is found as the tail. Poly A tail protects mRNA from nuclease attack.

Intrastrand base pairing among complementary bases allows folding of the linear molecule. As a result, haripin or loop like secondary structure is formed.

Functions

·           mRNA is a direct carrier of genetic information from the nucleus tothe cytoplasm.

·           It contain information required for the synthesis of protein molecules.

2. Transfer RNA

It accounts for 10-15% of total cell RNA. They are the smallest of all the RNAs. Usually they consist of 50-100 nucleotides. They are single standard molecules. They contain unusual bases such as methylated adenine, guanine, cytosine and thymine,dihydrouracil and pseudouridine. These unusual bases are important for binding 6-RNA to intra chain base pairing. Further some bases are not involved in base pairing, resulting in loops and arms formation in tRNA. These folding in the primary structure generates a secondary structure (Fig.7.8).

Secondary structure of t-RNA is in the form of a clover leaf. The important feature of the clover-leaf structure are,

1.        An acceptor arm with base sequence “CCA” 3’-OH of adenosine moiety of t-RNA.

2.        An anticodon arm which recognises codon on mRNA.

3.        TφC arm which contain unusual base cytosine.

4.        D- arm which contain many dihydrouracil residues.

Functions

It is the carrier of amino acids to the site of protein synthesis.

There is at least one t-RNA molecule to each of 20 amino acids required for protein synthesis.

3. Ribosomal RNA

This accounts for 80% of the total cellular RNA. It is present in ribosomes. In ribosomes, r-RNA is found in combination with protein. It is known as ribonucleoprotein. The length of rRNA ranges from 100-600 nucleotides. rRNA molecules have a secondary structure. Intra strand base pairing between complementary bases generate double helical segments or loops.

Functions

1.        They are required for the formation of risosomes

2.        They are involved in the initiation of protein synthesis.