Question

Gunaosine, which is a leading nucleotide of the 'CAP' of mRNA is methylated at _____ position.

• A. N1
• B. N3
• C. N7
• D. N9

Hint:

The 5' cap of eukaryotic mRNA is universally designated as $m^7\text{G}$ (7-methylguanosine).
The "7" explicitly denotes the N7 nitrogen atom of the guanine base where the methyl group is covalently attached.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Eukaryotic pre-mRNAs undergo a series of modifications at their ends.
The first modification occurs co-transcriptionally at the 5' end, where a protective structure called the 5' cap is added.
Detailed Explanation:
- The 5' cap consists of a guanosine nucleotide linked to the 5'-terminal nucleotide of the pre-mRNA via an unusual 5'-to-5' triphosphate linkage.
- Immediately after this linkage is formed by guanylyltransferase, the terminal guanine residue is methylated by the enzyme methyltransferase.
- This enzyme transfers a methyl group from S-adenosylmethionine (SAM) to the N7 position of the guanine purine ring.
- This modification forms 7-methylguanosine ($m^7\text{G}$), which is the hallmark of the "Cap 0" structure.
- The $m^7\text{G}$ cap protects the mRNA from 5'-to-3' exonucleolytic degradation and is recognized by the cap-binding complex (CBC) and translation initiation factors (e.g., eIF4E) to promote translation.

Step 2: Final Answer:
The guanosine of the mRNA cap is methylated at the N7 position, corresponding to option (C).