Molecular biology and genetics: tryptophan operon attenuation: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Molecular biology and genetics: tryptophan operon attenuation

Sample item

In E. coli, when tryptophan is plentiful, transcription of the trp operon stops after initiation, before the structural genes are transcribed. Which mechanism accounts for this?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Attenuation: a terminator hairpin forms in the leader RNA when the ribosome moves quickly through the tryptophan codons

In attenuation, translation of the leader peptide with its tryptophan codons is coupled to transcription. When tryptophan-tRNA is plentiful, the ribosome passes quickly, allowing a terminator hairpin to form and RNA polymerase to stop. When tryptophan is scarce the ribosome stalls, an anti-terminator forms and transcription continues.

Option by option

  • A. Methylation of the promoter by DNA methyltransferase

    DNA methylation of the promoter is a eukaryotic style of silencing and does not explain attenuation.

  • B. Attenuation: a terminator hairpin forms in the leader RNA when the ribosome moves quickly through the tryptophan codons (correct)

    Correct. A terminator hairpin forms in the leader when tryptophan-tRNA is plentiful, ending transcription early.

  • C. Binding of cAMP-CAP to the promoter

    cAMP-CAP regulates catabolite repression of the lac operon, not the trp operon.

  • D. Removal of the repressor by allolactose

    Allolactose is the inducer of the lac operon, not of the trp operon.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Attenuation: a terminator hairpin forms in the leader RNA when the ribosome moves quickly through the tryptophan codons. In attenuation, translation of the leader peptide with its tryptophan codons is coupled to transcription.

Why is "Methylation of the promoter by DNA methyltransferase" not the answer?

DNA methylation of the promoter is a eukaryotic style of silencing and does not explain attenuation.

Why is "Binding of cAMP-CAP to the promoter" not the answer?

cAMP-CAP regulates catabolite repression of the lac operon, not the trp operon.

Why is "Removal of the repressor by allolactose" not the answer?

Allolactose is the inducer of the lac operon, not of the trp operon.

What concept does this question test?

It tests Molecular biology and genetics: tryptophan operon attenuation in Biochemistry. When tryptophan-tRNA is plentiful, the ribosome passes quickly, allowing a terminator hairpin to form and RNA polymerase to stop. When tryptophan is scarce the ribosome stalls, an anti-terminator forms and transcription continues.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Molecular biology and genetics: tryptophan operon attenuation?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.