4. Eukaryotic mRNA are modified to possess a 5′ cap structure. Which one of the following is an INCORRECT statement about the function of the 5′ cap structure?
(1) It protects the mRNA from 5′ exo-ribonuclease attack.
(2) It facilitates splicing of the nascent transcripts
(3) It protects the transcripts from degradation by RNAse Ill family enzymes.
(4) It facilitates attachments to 40S subunit of ribosome

Identifying the Incorrect Statement About the 5′ Cap Function in Eukaryotic mRNA

Question Recap:

Which one of the following is an INCORRECT statement about the function of the 5′ cap structure in eukaryotic mRNA?

1. It protects the mRNA from 5′ exo-ribonuclease attack.

2. It facilitates splicing of the nascent transcripts.

3. It protects the transcripts from degradation by RNAse III family enzymes.

4. It facilitates attachment to the 40S subunit of the ribosome.

Correct Answer:

(3) It protects the transcripts from degradation by RNAse III family enzymes.

Detailed Explanation:

The 5′ cap on eukaryotic mRNA is a modified guanine nucleotide (7-methylguanosine) linked via a 5′–5′ triphosphate bridge to the first nucleotide of the mRNA. It plays several crucial roles:

• Protection from 5′ exonucleases: The cap physically blocks 5′ to 3′ exonucleases, preventing degradation from the 5′ end and thus stabilizing the mRNA (Statement 1 is correct).

• Facilitation of splicing: The cap interacts with components of the spliceosome and promotes efficient removal of 5′ proximal introns during pre-mRNA splicing (Statement 2 is correct).

• Promotion of translation initiation: The cap is recognized by the eukaryotic initiation factor eIF4E, which helps recruit the 40S ribosomal subunit to the mRNA, facilitating translation initiation (Statement 4 is correct).

Why Statement 3 is Incorrect:

• RNAse III family enzymes are double-stranded RNA-specific endonucleases involved in processing and degradation of double-stranded RNA, including microRNA precursors and rRNA. The 5′ cap does not protect mRNA from RNAse III enzymes, which cleave internally and do not rely on the 5′ end structure.

• The cap mainly protects against 5′ exonucleases and decapping enzymes, not endonucleases like RNAse III.

Summary Table

Conclusion

The incorrect statement about the 5′ cap function is that it protects transcripts from degradation by RNAse III family enzymes. The 5′ cap protects against 5′ exonucleases and promotes splicing and translation initiation but does not defend against RNAse III, which cleaves double-stranded RNA internally.