Role of the 5′ Cap in RNA: Stability and Transport

Correct Answer:

(2) stability and transport of RNA

Detailed Explanation:

The 5′ cap of eukaryotic mRNA is a modified guanine nucleotide (7-methylguanosine) added to the 5′ end of the nascent RNA transcript shortly after transcription initiation. This cap structure plays multiple critical roles in RNA metabolism, primarily:

• Stability: The 5′ cap protects RNA from degradation by 5′ to 3′ exonucleases. The unusual 5′–5′ triphosphate linkage and methylation prevent exonucleases from recognizing and degrading the RNA, significantly increasing its half-life.

• Transport: The cap is recognized by the cap-binding complex (CBC) in the nucleus, which facilitates the export of the mature mRNA from the nucleus to the cytoplasm. Without this cap, RNA export is inefficient or blocked.

Additional functions (not asked here) include promoting translation initiation and aiding in RNA processing like splicing.

Why Other Options Are Incorrect:

1. Stability of RNA only
   While stability is a key function, the 5′ cap is also essential for nuclear export (transport), so this option is incomplete.

2. Transport of RNA only
   The cap also protects RNA from degradation, so transport alone is not the full picture.

3. Methylation of RNA
   The 5′ cap itself is formed by methylation, but the cap is not required for methylation; rather, methylation is part of the cap formation process.

Summary Table

Conclusion

The 5′ cap of RNA is essential for both the stability and transport of RNA in eukaryotic cells. It protects RNA molecules from degradation and promotes their export from the nucleus to the cytoplasm, ensuring proper gene expression.