Question

Explain the processing the hnRNA needs to undergo before becoming functional mRNA in eukaryotes.

Answer 1

In eukaryotes, the hnRNA (heterogeneous nuclear RNA) is the direct product of transcription and contains both coding exons and non-coding introns. To become functional mRNA, it must undergo three essential processing steps within the nucleus:

1. Splicing: This is the most critical step, where the non-functional introns are removed, and the functional exons are joined together in a specific order. This process is carried out by a complex called the spliceosome.
2. Capping: A unique nucleotide, methyl guanosine triphosphate, is added to the 5’ end of the hnRNA. This “cap” protects the RNA from degradation and helps the ribosome recognise it during translation.
3. Tailing (Polyadenylation): About 200–300 adenylate residues are added to the 3’ end of the RNA, forming what is known as a poly-A tail. This tail assists in the export of the mRNA from the nucleus and further stabilises the molecule.

Once these three modifications are complete, the processed transcript is called mature mRNA and is then transported out of the nucleus into the cytoplasm for protein synthesis.