Assertion: Methyl guanosine triphosphate is added to the 3’ end of hnRNA (pre-mRNA) during the process known as capping.
Reason: Capping is a post-transcriptional modification in eukaryotic mRNA processing, where an unusual nucleotide (methyl guanosine triphosphate) is added to the 5'-end of hnRNA.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Assertion is false, but Reason is true.
In eukaryotes the first RNA copy, hnRNA, is processed before it leaves the nucleus. The introns are removed by splicing, a cap is added at one end (capping) and a poly-A tail at the other (tailing).
Assertion — False. In capping, methyl guanosine triphosphate is added to the 5′-end of hnRNA, not the 3′-end. The 3′-end receives the tail of 200–300 adenylate residues.
Reason — True. Capping is one of the steps of processing hnRNA after transcription. In it an unusual nucleotide, methyl guanosine triphosphate, is added to the 5′-end.
The Assertion puts the cap at the wrong end, while the Reason places it correctly, so the Assertion is false and the Reason is true.
NCERT Extract:
Types of RNA and the Process of Transcription – “… hnRNA undergoes additional processing called as capping and tailing. In capping an unusual nucleotide (methyl guanosine triphosphate) is added to the 5′-end of hnRNA. In tailing, adenylate residues (200-300) are added at 3′-end in a template independent manner. It is the fully processed hnRNA, now called mRNA, that is transported out of the nucleus for translation (Figure 5.11).”