NEET 2025 Botany Question 129 Solution

In eukaryotes, RNA Polymerase II transcribes heterogeneous nuclear RNA (hnRNA), which contains both coding exons and non-coding introns and is non-functional until post-transcriptional processing:

• • Splicing (Event B): The non-coding introns are excised by spliceosomes and the coding exons are ligated together in a specific, defined sequence. This occurs entirely within the nucleoplasm.
• • Capping (Event C): An unusual nucleotide, 7-methylguanosine triphosphate (m⁷G), is added to the 5'-end of the primary transcript. This cap protects the mRNA from exonucleases and assists in ribosome binding during translation.
• • Tailing (Event D): Polyadenylation involves the template-independent enzymatic addition of 200–300 adenylate residues (poly-A tail) to the 3'-end of hnRNA.

Analyzing incorrect statements:

• • Statement A claims "Transport of pre-mRNA to cytoplasm prior to splicing". Eukaryotic nuclear pore complexes enforce strict quality control: unspliced pre-mRNAs are retained within the nucleus until splicing and processing are complete. Only mature, fully spliced mRNA is exported to the cytoplasm.
• • Statement E claims "Base pairing of two complementary RNAs". This is the mechanism of RNA interference (RNAi) or antisense regulation, not a regular step in eukaryotic mRNA maturation.

The genuine post-transcriptional modification events are B (splicing), C (capping), and D (tailing). Hence, B, C, D only are correct.