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GATE 2019 BT Q.36: solution and answer

GATE 2019 · BTQ.36 · 2 marks · MCQ

Which one of the following covalent linkages exists between 7-Methyl guanosine (m^7G) and mRNAs?

  1. (A)

    2'–3' triphosphate

  2. (B)

    3'–5' triphosphate

  3. (C)

    5'–5' triphosphate

  4. (D)

    2'–5' triphosphate

See the question in the paper
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Answer and solution

ANSWER

The correct answer is (C) 5'–5' triphosphate: the m^7G cap is joined to the mRNA's original 5' end through an unusual triphosphate bridge linking the two 5' carbons, formed by guanylyltransferase.

Official answer: CSource: Official answer key, GATE 2019 BT (IIT Madras)

  1. The question asks for the covalent linkage joining the 7-methylguanosine (m^7G) cap to the 5' end of the mRNA transcript.

  2. During co-transcriptional capping, RNA triphosphatase removes the terminal (γ) phosphate from the 5' end of the nascent transcript, leaving a 5' diphosphate.

  3. Guanylyltransferase then transfers GMP from GTP onto this 5' diphosphate end. This joins the 5'-carbon of the incoming guanosine to the 5'-carbon of the transcript through a bridge of three phosphates — an unusual 5'–5' triphosphate linkage, not the standard 3'–5' phosphodiester bond used elsewhere along the RNA chain.

  4. Guanine-N7 methyltransferase subsequently methylates this terminal guanosine at N7 to produce m^7G; this methylation step modifies the base, not the linkage already formed.

  5. Option (A) 2'–3' triphosphate: incorrect — no such bond exists in cap chemistry.

  6. Option (B) 3'–5' triphosphate: incorrect — this describes ordinary internucleotide phosphodiester geometry (normally one phosphate), not the cap junction.

  7. Option (C) 5'–5' triphosphate: correct — the hallmark 'head-to-head' bond created by guanylyltransferase linking the cap GMP's 5' carbon to the transcript's 5' carbon via three phosphates.

  8. Option (D) 2'–5' triphosphate: incorrect — 2'–5' linkages occur elsewhere in biology (e.g., the 2-5A system, lariat branch points) but not in the cap structure.

  9. Therefore the covalent linkage between m^7G and the mRNA is the 5'–5' triphosphate bridge.

KEY IDEA

mRNA 5' capping installs m^7G via a unique 5'–5' triphosphate linkage (not the standard 3'–5' phosphodiester bond of the rest of the chain), formed when guanylyltransferase adds GMP to the diphosphate 5' end left after RNA triphosphatase removes the γ-phosphate; N7-methylation of the added guanosine follows and does not alter the linkage.

WATCH OUT FOR

Assuming the cap is attached by the usual 3'–5' phosphodiester bond used for all other internucleotide linkages in RNA, rather than recognizing the atypical 5'-to-5' triphosphate bridge unique to capping.

From the GATE 2019 BT paper. Every question of this paper · Download the full paper · All 21 Gene expression questions, with notes

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