Bacterial genetics: practice question

A Microbiology 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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Microbiology · Bacterial genetics

Sample item

A non-toxigenic strain of Corynebacterium diphtheriae isolated from a carrier starts producing diphtheria toxin after infection with a temperate bacteriophage. Which process is this?

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

The answer and the reasoning

C. Lysogenic (phage) conversion

The diphtheria toxin gene (tox) is carried by a prophage, beta phage. When a temperate phage integrates into the bacterial chromosome as a prophage, it can confer new traits (lysogenic conversion). Botulinum toxin of some strains, cholera toxin and Shiga-like toxin are similarly phage-encoded.

Option by option

  • A. Transformation by naked DNA uptake

    Transformation requires uptake of free DNA without a phage.

  • B. Conjugation through an F plasmid

    Conjugation requires direct cell contact and a plasmid transfer, not a phage.

  • C. Lysogenic (phage) conversion (correct)

    Correct. A temperate phage brings the toxin gene and integrates as a prophage.

  • D. Generalised transduction with random bacterial DNA

    Generalised transduction transfers random host DNA by a faulty phage, not a defined phage gene.

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 Microbiology question

What is the answer to this Microbiology question?

The best answer is C, Lysogenic (phage) conversion. The diphtheria toxin gene (tox) is carried by a prophage, beta phage.

Why is "Transformation by naked DNA uptake" not the answer?

Transformation requires uptake of free DNA without a phage.

Why is "Conjugation through an F plasmid" not the answer?

Conjugation requires direct cell contact and a plasmid transfer, not a phage.

Why is "Generalised transduction with random bacterial DNA" not the answer?

Generalised transduction transfers random host DNA by a faulty phage, not a defined phage gene.

What concept does this question test?

It tests Bacterial genetics in Microbiology. When a temperate phage integrates into the bacterial chromosome as a prophage, it can confer new traits (lysogenic conversion). Botulinum toxin of some strains, cholera toxin and Shiga-like toxin are similarly phage-encoded.

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 Bacterial genetics?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology 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.