Question

Identify the nucleotide cap that is attached at the 5'end of mRNA

• A. 5-methyl guanosine
• B. 7-methyl guanosine
• C. 5- acetyl guanosine
• D. 7- acetyl guanosine

Hint:

The eukaryotic 5' cap is universally designated as \(\text{m}^7\text{G}\).
The "7" indicates that the methyl group is attached to the 7th nitrogen of the guanine base.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Eukaryotic messenger RNA (mRNA) undergoes several post-transcriptional modifications inside the nucleus before export to the cytoplasm.
One of the first modifications is 5' capping, which protects the transcript from nuclease degradation and assists in ribosome binding during translation.

Step 2: Detailed Explanation:
The 5' cap is added to the pre-mRNA transcript shortly after transcription initiation.
The capping process occurs in three enzymatic steps:
1. RNA triphosphatase removes the terminal 5'-phosphate group from the nascent pre-mRNA chain.
2. Guanylyltransferase adds a GMP molecule from GTP, creating a unique 5'-to-5' triphosphate linkage.
3. Methyltransferase transfers a methyl group from S-adenosylmethionine (SAM) to the nitrogen at position 7 of the terminal guanine ring.
This produces a 7-methylguanosine (\(\text{m}^7\text{G}\)) cap at the 5' end of the mRNA transcript.
The other options, including 5-methylguanosine (A) and acetylated forms (C and D), do not occur naturally as the 5' cap structure on eukaryotic mRNA transcripts.

Step 3: Final Answer:
Therefore, the nucleotide cap attached at the 5' end of mRNA is 7-methyl guanosine, corresponding to option (B).