Gene editing: CRISPR-Cas9: 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 · Gene editing: CRISPR-Cas9

Sample item

A research group uses CRISPR-Cas9 to knock out a gene in cultured cells. Which description of the mechanism is correct?

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

The answer and the reasoning

D. A guide RNA directs Cas9 to a complementary DNA sequence next to a PAM site, and Cas9 cuts both strands, with error-prone repair causing indels

The single guide RNA base-pairs with a target DNA sequence adjacent to a protospacer adjacent motif, and Cas9 creates a blunt double-strand break. Repair by non-homologous end joining often introduces insertions or deletions that disrupt the reading frame, while homology-directed repair can insert a supplied template. This principle underlies approved therapy for sickle cell disease.

Option by option

  • A. Cas9 inserts a retroviral cassette using its own reverse transcriptase activity

    Cas9 is a nuclease and has no reverse transcriptase activity.

  • B. Cas9 methylates the target promoter and silences the gene without cutting DNA

    Methylation without cutting is not the mechanism of standard Cas9 knockout.

  • C. A guide DNA oligomer directs Cas9 to cut single-stranded mRNA in the cytoplasm

    Cas9 uses an RNA guide and targets DNA, not mRNA.

  • D. A guide RNA directs Cas9 to a complementary DNA sequence next to a PAM site, and Cas9 cuts both strands, with error-prone repair causing indels (correct)

    Guide RNA, PAM recognition and a double-strand break define the technique.

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.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, A guide RNA directs Cas9 to a complementary DNA sequence next to a PAM site, and Cas9 cuts both strands, with error-prone repair causing indels. The single guide RNA base-pairs with a target DNA sequence adjacent to a protospacer adjacent motif, and Cas9 creates a blunt double-strand break.

Why is "Cas9 inserts a retroviral cassette using its own reverse transcriptas…" not the answer?

Cas9 is a nuclease and has no reverse transcriptase activity.

Why is "Cas9 methylates the target promoter and silences the gene without cut…" not the answer?

Methylation without cutting is not the mechanism of standard Cas9 knockout.

Why is "A guide DNA oligomer directs Cas9 to cut single-stranded mRNA in the…" not the answer?

Cas9 uses an RNA guide and targets DNA, not mRNA.

What concept does this question test?

It tests Gene editing: CRISPR-Cas9 in Biochemistry. Repair by non-homologous end joining often introduces insertions or deletions that disrupt the reading frame, while homology-directed repair can insert a supplied template. This principle underlies approved therapy for sickle cell disease.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 Gene editing: CRISPR-Cas9?

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.