[priyasaravanan_1406]

The Ribonucleic acid (RNA) exists as three forms in a cell. They are transfer RNA or tRNA, Ribosomal RNA or rRNA and Messenger RNA or mRNA. The messenger RNA as the name implies is carrier of information from DNA to the protein factory of the cell called as the ribosome. In ribosome, the information carried by the mRNA is read by rRNA and they participate in the conversion of the received information into proteins through a process called translation with the help of the tRNA.

Transfer RNA (tRNA): tRNAs are tiny in nature and acts as a tool in translation of mRNA into proteins by linking the base pairs of mRNA and amino acid sequence on a polypeptide. Transfer RNAs are amino acid specific and it scans and detects the parts of mRNA coding the type of aminoacid and enables the exact placement of the aminoacid in the polypeptide chain. The physique of the tRNA molecule resembles that of a clover leaf with several extended loops. They are acceptor arm, Dihydrouracil arm, anticodon arm and TⱷC arm each having a special function.

The acceptor arm as the name indicates acts as the site for aminoacid attachment and the anticodon arm detects the codons in mRNA and aids in their binding. RNA polymerase III is the active enzyme in the process of tRNA synthesis which involves transcription of genes corresponding to tRNA. The sequential array of the nucleotides in tRNA is susceptible to modification by chemical groups which contribute to methylation, saturation of double bond, deletion of amino group, replacement by sulfur group and so on.

Ribosomal RNA (rRNA): rRNAs are the native RNAs of the cell organelle Ribosome (protein factory) and hence the name Ribosomal RNA. They signify their presence by deriving the information from mRNA and participating in protein synthesis. The cell relies on ribosomes for all its protein requirement and the amount of protein synthesized in a cell is directly proportional to the number of ribosome molecules present in the cell. S value denotes the size of the ribosome and they exist as 70s in prokaryotes and 80s in eukaryotes. 70s ribosome is the combination of 50s subunit and 30s subunit. The 50s subunit of prokaryotes has 2 rRNAs and the 30s subunit has 1 rRNA. Whereas the 60s subunit in eukaryotes possess 3 rRNAs and 40s subunit has 1 rRNA.

The occurrence of inter RNA molecule base pairing and intra molecular base pairing stabilizes the structure of the rRNA molecule. The functional proteins are found attached to the rRNAs in ribosomes. Few RNAs possess the characteristics of an enzyme and are called as ribozymes.
The process of formation of rRNA is complex involving several steps before the final product of mature rRNA. In prokaryotes, the RNA polymerase mediated transcription of rRNA genes results in the formation of pre-rRNA. The pre-rRNA exists in folded form and base pairing occurs resulting in the formation of stem-loop structure. This is followed by binding of ribosomal proteins to the folded pre-rRNA and modification of bases by methylation and action of RNAse III on specific points on rRNA causing cleavage and finally trimming the 5′and 3′of the rRNA by the M5, M16 and M23 ribonucleases resulting in the formation of mature rRNA. In eukaryotes, the steps involved in the formation of mature rRNA are similar to prokaryotes except for the additional step of ribonuclease activated trimming in prokaryotes.

Messenger RNA (mRNA): The carriers of information from DNA to the ribosome and poses as the template for synthesis of proteins. RNA polymerase II activated transcription of genes addressing proteins in nucleus results in the formation of mRNA. The format of coding regions separated by the non coding region exists in eukaryotes. The coding regions are called as Exons and the non-coding regions are called as Introns. Like the other two RNAs, mRNA formation is also initiated by the formation of pre-mRNA by transcription of both the coding and non-coding regions present as such. This is followed by a process called as splicing which removes the introns allowing the continuity of the Exons, making it an exact template for protein synthesis. Capping and polyadenylation occurs post splicing. Capping process protects the 5′ end of mRNA from the action of exonucleases and polyadenylation protects the 3′ end of the mRNA. All this described processes are skipped by the prokaryotes as the information is translated much earlier even before the completion of the transcription itself.

The transfer RNA and ribosomal RNA are considered stable whereas the life span of the messenger RNA is short.