Q.Give an account of post transcriptional modifications of a eukaryotic mRNA.

Post-Transcriptional Modifications of Eukaryotic mRNA

In eukaryotes, the primary transcript (hnRNA or heterogeneous nuclear RNA) formed during transcription undergoes extensive processing before it becomes a mature, functional mRNA ready for translation. These modifications occur in the nucleus and are essential for mRNA stability, transport, and efficient translation.

The first modification is capping at the 5′ end. An unusual nucleotide, methyl guanosine triphosphate (cap), is added to the 5′ end of hnRNA through a 5′–5′ triphosphate linkage. This cap protects the mRNA from degradation by exonucleases and helps in ribosome binding during translation initiation.

The second modification is tailing at the 3′ end. A long stretch of adenylate residues (about 200–300 adenine nucleotides), called the poly-A tail, is added to the 3′ end of the transcript. This tail also provides stability to the mRNA and facilitates its export from the nucleus to the cytoplasm. …

When a eukaryotic gene is transcribed, the RNA polymerase II produces what we call a primary transcript or heterogeneous nuclear RNA (hnRNA). This molecule is not yet ready to leave the nucleus or be translated into protein. It must undergo a series of chemical alterations collectively known as post-transcriptional modifications. These changes are essential for mRNA stability, export from the nucleus, and efficient translation.

The first modification happens at the 5′ end of the transcript almost as soon as it emerges from the RNA polymerase. A modified guanine nucleotide is added in an unusual 5′-to-5′ linkage, forming what is called the 7-methylguanosine cap or simply the 5′ cap. This cap serves multiple purposes: it protects the mRNA from degradation by exonucleases, helps the ribosome recognize and bind to the mRNA during translation initiation, and assists in the export of the mature mRNA from the nucleus to the cytoplasm.

At the other end of the molecule, the 3′ end undergoes polyadenylation. The primary transcript is cleaved at a specific site downstream of the coding sequence, and a stretch of approximately 200–300 adenine nucleotides — the poly(A) tail — is added by the enzyme poly(A) polymerase. This tail does not come from the DNA template; it is added enzymatically after transcription. The poly(A) tail enhances mRNA stability, protects against degradation, and plays a role in translation and mRNA localization within the cell.

The most dramatic modification is splicing, the process by which non-coding sequences called introns are removed from the primary transcript, and the coding sequences called exons are joined together. Eukaryotic genes are typically split: the coding information is interrupted by stretches of DNA that do not code for amino acids. After transcription, these intervening sequences must be precisely excised.

Splicing is carried out by a large ribonucleoprotein complex called the spliceosome, which recognizes specific sequences at the boundaries of introns and exons. The spliceosome cuts the RNA at the intron-exon junctions and ligates the exons together in the correct order. This process must be extraordinarily accurate — even a single nucleotide error can shift the reading frame and produce a completely different, often nonfunctional, protein. …

Instead of listing capping, tailing, and splicing purely in temporal order, match each one to the one problem it exists to solve: introns exist because eukaryotic genes are "split", so splicing solves the problem of removing non-coding interruptions; nuclease attack threatens both raw ends of a linear transcript, so capping and tailing solve that same protection problem at the 5' …