Which of the following events is not an example of an mRNA modification that occurs in eukaryotes?

Question

BlackTom AI · Accepted Answer

The 5′ cap on eukaryotic mRNA is a 7-methylguanosine (written m7G) linked by a 5′–5′ triphosphate bridge; there is no biologically relevant “13′-methylguanosine” cap. Eukaryotic pre-mRNAs commonly undergo three cardinal co- and post-transcriptional modifications: addition of the 5′ cap (m7G), removal of introns by splicing, and addition of a 3′ polyadenylate (poly(A)) tail.

Relevant knowledge points and concepts:
- 5′ capping: the guanosine added to the 5′ end is methylated at the N7 position (m7G). Additional ribose 2′-O-methylations can produce cap1 and cap2 structures. The ribose carbons are numbered 1′–5′; there is no “13′” position.
- Splicing: the process by which the spliceosome removes introns and joins exons to produce mature mRNA.
- Polyadenylation: cleavage of the 3′ end followed by addition of a poly(A) tail for stability and export.

Step-by-step reasoning:
1. Evaluate “Splicing”: this is a canonical eukaryotic mRNA modification—correct example.
2. Evaluate “PolyA-tail”: addition of a polyadenylate tail is a standard modification—correct example.
3. Evaluate “13’-methylguanosine cap”: known cap is 7-methylguanosine (m7G); “13′” notation is nonstandard and chemically impossible for ribose numbering (only 1′–5′ exist). No documented eukaryotic mRNA cap is called “13′-methylguanosine.” Therefore this is not a genuine mRNA modification.
4. Evaluate “All of the above are types of RNA modifications in eukaryotes.” Because two choices are genuine and one is nonexistent, this option is false.

Why the correct answer is correct:
“13′-methylguanosine cap” names a non-existent or misnumbered modification; the real cap is a 7-methylguanosine (m7G). Thus it is not an example of an actual eukaryotic mRNA modification.

Why other options are incorrect:
- Splicing and PolyA-tail are bona fide eukaryotic mRNA processing events.
- “All of the above” is incorrect because one listed item is invalid.

Conclusion: The correct choice is “13’-methylguanosine cap,” which matches the provided answer.

Which of the following events is not an example of an mRNA modification that occurs in eukaryotes?单项选择题

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