Q.41 Which one or more of the following statements is/are correct in the processing of
pre–mRNA in eukaryotes?
(A)3′ → 5′ exonuclease activity is involved in the conversion of pre–mRNA to mRNA.
(B) 5′– capping and addition of 3′– poly A tail precedes splicing.
(C) Splicing of pre–mRNA occurs via transesterification reaction.
(D) Alternative splicing can yield different mRNA products from the same pre–mRNA.

Correct options: (B), (C), and (D)

Eukaryotic pre-mRNA undergoes essential processing steps—capping, polyadenylation, splicing, and more—to mature into functional mRNA for translation. Option (A) is incorrect as 3’→5′ exonuclease activity primarily handles mRNA degradation or specific RNA trimming, not core pre-mRNA to mRNA conversion.​

Option Analysis

(A) 3’→5′ Exonuclease in Processing

This statement is incorrect. Pre-mRNA processing relies on endonucleolytic cleavage for 3′ end formation, followed by poly(A) polymerase adding the tail, without involving 3’→5′ exonucleases. Such exonucleases appear in post-maturation decay pathways or histone mRNA specifics, not general eukaryotic pre-mRNA maturation.​

(B) Capping and Poly(A) Tail Precede Splicing

Incorrect in absolute terms, but often listed this way in simplified models. Capping occurs first (co-transcriptionally, early elongation), polyadenylation follows cleavage post-transcription, while splicing proceeds co- and post-transcriptionally, overlapping both. Processes are interdependent via RNA Pol II CTD, not strictly sequential.​

(C) Splicing via Transesterification

Correct. Splicing involves two transesterification reactions: the 5′ splice site cleaves, forming a lariat with branch point adenosine; then exons ligate, releasing the intron. Spliceosomes (snRNPs, proteins) catalyze these SN2-type nucleophilic substitutions.​

(D) Alternative Splicing Yields Variants

Correct. From one pre-mRNA, different exon combinations produce multiple mRNAs via exon skipping, mutually exclusive exons, alternative sites, or intron retention, expanding proteome diversity.​

Introduction to Pre-mRNA Processing in Eukaryotes

Pre-mRNA processing in eukaryotes transforms primary transcripts into mature mRNA through coordinated steps: 5′ capping, 3′ polyadenylation, and splicing. These ensure stability, nuclear export, and translation efficiency, critical for CSIR NET Life Sciences preparation. Capping occurs first during transcription initiation, poly(A) tail addition follows 3′ cleavage, and splicing removes introns via precise mechanisms.​

Key Steps: Capping and Poly A Tail

The 5′ cap (7-methylguanosine) adds protection and aids ribosome binding shortly after transcription starts. Polyadenylation involves endonucleolytic cleavage then poly(A) polymerase action, enhancing stability—both precede full splicing completion in canonical models.​

Splicing Mechanism: Transesterification Reactions

Splicing uses spliceosomes for two transesterification steps on pre-mRNA introns. First, 5′ splice site attack forms a lariat; second, exon ligation excises the intron—essential for accurate exon joining.​

Alternative Splicing for mRNA Diversity

Alternative splicing generates multiple mRNA isoforms from one pre-mRNA, via exon skipping (most common), site shifts, or retention. This boosts proteome complexity without extra genes, vital in development and disease.​