Bacterial genetics: Hfr conjugation: 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: Hfr conjugation

Sample item

In a laboratory cross, an Hfr donor strain is mixed with an F-negative recipient. After mating, many recipients have received donor chromosomal genes, yet almost all of these recombinants remain F-negative. What is the best explanation?

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

The answer and the reasoning

D. The F transfer genes lie at the trailing end and rarely enter before the bridge breaks

In an Hfr cell the F factor is integrated into the chromosome, and transfer starts from the origin inside F. Part of the F sequence leads, but the remainder, including the genes needed to make the cell F-positive, is at the far end, so it enters only if the entire chromosome (about 100 minutes) is transferred. The fragile mating bridge usually breaks first, giving recombinants that remain F-negative.

Option by option

  • A. The recipient lacks pilus receptors, so it can only receive DNA by transduction

    Conjugation requires the pilus, and the recipient does receive DNA directly by conjugation, not transduction.

  • B. Chromosomal genes travel inside a bacteriophage head, which excludes plasmid DNA

    Phage-mediated transfer is transduction, whereas this cross is conjugation with a direct cell-to-cell bridge.

  • C. The F plasmid is destroyed by restriction enzymes in the recipient

    Restriction enzymes are not the explanation, and the F factor is not selectively degraded in the recipient.

  • D. The F transfer genes lie at the trailing end and rarely enter before the bridge breaks (correct)

    The trailing position of the F transfer genes explains why recombinants usually stay F-negative.

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 D, The F transfer genes lie at the trailing end and rarely enter before the bridge breaks. In an Hfr cell the F factor is integrated into the chromosome, and transfer starts from the origin inside F.

Why is "The recipient lacks pilus receptors, so it can only receive DNA by tr…" not the answer?

Conjugation requires the pilus, and the recipient does receive DNA directly by conjugation, not transduction.

Why is "Chromosomal genes travel inside a bacteriophage head, which excludes…" not the answer?

Phage-mediated transfer is transduction, whereas this cross is conjugation with a direct cell-to-cell bridge.

Why is "The F plasmid is destroyed by restriction enzymes in the recipient" not the answer?

Restriction enzymes are not the explanation, and the F factor is not selectively degraded in the recipient.

What concept does this question test?

It tests Bacterial genetics: Hfr conjugation in Microbiology. Part of the F sequence leads, but the remainder, including the genes needed to make the cell F-positive, is at the far end, so it enters only if the entire chromosome (about 100 minutes) is transferred. The fragile mating bridge usually breaks first, giving recombinants that remain F-negative.

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: Hfr conjugation?

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.