Correct option is A
Explanation:
- mRNA export from the nucleus is a highly regulated process ensuring that only properly processed transcripts leave for translation in the cytoplasm.
- Key modifications required for nuclear export:
- 5' Cap (7-methylguanosine cap): Protects RNA from degradation and aids in nuclear export.
- 3' Poly(A) Tail: Provides stability and facilitates translation.
- Splicing: Removal of introns ensures a mature mRNA transcript ready for translation.
- Association with export factors (e.g., TREX, NXF1/TAP): Facilitates mRNA passage through the nuclear pore complex (NPC).
- Option 1 is correct because a fully spliced mRNA with a cap and poly(A) tail is properly processed and recognized by export factors, ensuring its transport to the cytoplasm.
Information Booster
- mRNA export is facilitated by TREX (transcription-export complex) and NXF1 (nuclear export factor 1).
- The Cap-Binding Complex (CBC) binds to the 5’ cap and interacts with export proteins to promote mRNA transport.
- Only correctly processed mRNAs are exported, while defective mRNAs are retained and degraded.
- The exon-junction complex (EJC) marks successful splicing and promotes efficient nuclear export.
- mRNAs that fail quality control (e.g., mis-spliced, unprocessed) are degraded by nuclear exosomes.
- Nuclear pores selectively transport mature mRNAs while retaining improperly processed transcripts.
- mRNA export is an energy-dependent process requiring ATP and GTP-hydrolysis-driven transport mechanisms.
Additional Information
Option (2) Mis-spliced RNA with multiple stop codons → Incorrect
→ These mRNAs are typically retained and degraded via nonsense-mediated decay (NMD).Option (3) Spliced RNA with the associated spliceosomal complex → Incorrect
→ The spliceosome must dissociate before export. Unprocessed complexes remain nuclear.Option (4) Uncapped and unspliced RNA → Incorrect
→ These are immature transcripts that fail nuclear export and are degraded.
