In both prokaryotes and eukaryotes there are three different types of RNAs found namely – mRNA, rRNA and tRNA. All the three RNAs are necessary for the synthesis of proteins in a cell. The mRNA acts as the template for protein, the rRNA performs structural and catalytic functions and the tRNA brings amino acids.

This is the process of transcription which is accompanied by a set of polymerases and initiation and termination factors. Next is the step of translation which is entirely different in prokaryotes and eukaryotes. In bacteria, the site of transcription and translation is the same (since there is no distinguished separation between cytosol and nucleus). However, in eukaryotes the transcription takes place in the nucleus and translation takes place in the cytoplasm.

Before transporting the primary transcript (obtained from transcription) to the cytoplasm, it needs to undergo various processes that are known as post-transcriptional modifications. The three processes that a primary transcript undergoes before moving out of the nucleus is capping, tailing and splicing. Let us study the three modifications in detail.

Capping

This is the first processing event an mRNA undergoes before translation. Capping is the process of adding a 7-methylguanosine cap to the 5’ end of the primary transcript by phosphate linkage. Three enzymes are involved in this process – 7-methyltransferase, RNA triphosphatase and guanylyltransferase.

The process of capping is initiated by the removal of phosphate groups from the terminal nucleotide to create a site for the addition of the methylated guanine residues. Guanylyltransferase first adds guanine residues to the site created (the 5’ terminal), which is then followed by the addition of methyl groups by the enzymes methyltransferases.

Tailing

Tailing, also known as polyadenylation, is the process of cleavage of the 3’ end and addition of adenylate residues to the same end of the pre-mRNA. A conserved sequence 5’-AAUAAA-3′ is present near the 3’ end of the primary transcript that is a polyadenylation signal sequence. The endonucleolytic cleavage takes place 10-30 base pairs down this signal sequence, leaving the AAUAAA sequence in the transcript. A GU rich region is also present downstream this signal sequence that helps in cleavage and polyadenylation.

The enzyme poly-A polymerase then adds adenine nucleotides (200-250 in number) to the cleaved pre-mRNA. This addition of adenine residues is called poly-A tailing or polyadenylation.

Splicing

There are many non coding sequences present in the primary transcript that can intervene in the process of translation. The removal of these non-coding sequences (known as introns) and the reattachment of the coding sequences into a continuous molecule is the process of splicing. The coding sequences are referred to as exons.

The whole splicing mechanism takes place by recruitment of the spliceosome machinery. This machinery consists of proteins and small nuclear RNAs (snRNA) that recognise the splicing sites. In eukaryotes, the pre-mRNAs are spliced in various combinations to produce different mature RNAs, this process is referred to as alternative splicing.

The whole recruitment and functioning of the spliceosome is a tedious process and beyond the scope of your syllabus.

What is the Need for Post-Transcriptional Modification?

Capping: The process of capping on the primary transcripts gives stability to the molecule. Additionally, these caps also prevent the mRNA from degradation by the ribonucleases present in the cytoplasm. Last but not least, the caps also help in the recognition of the mRNA by the translation machinery.

Tailing: The polyadenylated tail gives extended stability to the pre-mRNA. It further signals the end of transcription to the cellular factors for transportation of the transcript to the cytoplasm.

Splicing: The process of splicing helps in evolution by creating new combinations of exons. It also helps in the regulation of gene expression and protein content of the cell.

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Also Read:

• Short Notes Of Biology For NEET tRNA Structure
• MCQs On Structure Of RNA
• Difference Between mRNA tRNA and rRNA