Molecular biology and genetics: trinucleotide repeats: 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: trinucleotide repeats

Sample item

A boy has intellectual disability, a long face and macro-orchidism. Analysis shows more than 200 CGG repeats in the FMR1 gene. Through which mechanism does this cause disease?

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

The answer and the reasoning

D. Methylation of the promoter silences the gene

In fragile X, the CGG expansion lies in the 5' untranslated region, and the full mutation is hypermethylated, silencing FMR1. Loss of FMRP causes the phenotype. Polyglutamine toxicity applies to CAG repeats such as Huntington disease.

Option by option

  • A. Repeats cause a frameshift in the coding region

    The CGG repeat is not in the coding region and does not cause a frameshift.

  • B. Excess repeats increase gene transcription

    Transcription is reduced, not increased.

  • C. Expanded repeats produce a toxic polyglutamine protein

    A polyglutamine protein arises from CAG repeats in coding regions.

  • D. Methylation of the promoter silences the gene (correct)

    Correct. Hypermethylation silences FMR1.

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 D, Methylation of the promoter silences the gene. In fragile X, the CGG expansion lies in the 5' untranslated region, and the full mutation is hypermethylated, silencing FMR1.

Why is "Repeats cause a frameshift in the coding region" not the answer?

The CGG repeat is not in the coding region and does not cause a frameshift.

Why is "Excess repeats increase gene transcription" not the answer?

Transcription is reduced, not increased.

Why is "Expanded repeats produce a toxic polyglutamine protein" not the answer?

A polyglutamine protein arises from CAG repeats in coding regions.

What concept does this question test?

It tests Molecular biology and genetics: trinucleotide repeats in Biochemistry. Loss of FMRP causes the phenotype. Polyglutamine toxicity applies to CAG repeats such as Huntington disease.

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: trinucleotide repeats?

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.