Q.61 The following is the list of steps, involved in the transfer of bacterial DNA by generalized transduction. Which option
represents the correct sequence of steps ?
(A). Packaging of host genomic DNA fragment, into P1 phage head.
(B). Injection of DNA of transducing particle, into bacterial host.
(C). Injection of P1 phage DNA, into bacterial host.
(D). Release of transducing particle, into bacterial host.
(E). Fragmentation of host genomic DNA, by phage-encoded endonuclease.
Choose the correct answer from the options given below:
1. (C), (A), (B), (E), (D).
2. (C), (E), (A), (D), (B).
3. (A), (D), (B), (E), (C).
4. (A), (E), (B), (D), (C).
Mastering Generalized Transduction: Correct Sequence of Steps in Bacterial DNA Transfer
Generalized transduction is a key mechanism of horizontal gene transfer in bacteria, where a bacteriophage like P1 accidentally packages host DNA instead of its own and delivers it to a new host. The correct sequence follows the phage lytic cycle: host DNA fragmentation, packaging into phage heads, release of transducing particles, and injection into a recipient bacterium. This ensures precise understanding for biology exams and research.
Correct Answer
Option 2: (C), (E), (A), (D), (B) is the right choice.
The logical flow starts with P1 phage infection via injection of phage DNA (C). Phage-encoded endonuclease then fragments host genomic DNA (E). A host DNA fragment gets packaged into the phage head (A), forming a transducing particle. This particle is released upon host lysis (D). Finally, the transducing particle injects the host DNA into a new bacterial host (B).
Step-by-Step Process
Generalized transduction relies on the lytic cycle of temperate phages like P1.
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(C) Injection of P1 phage DNA: Phage attaches to donor bacterium and injects its DNA to initiate infection.
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(E) Fragmentation of host genomic DNA: Phage enzymes degrade the host chromosome into fragments.
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(A) Packaging of host genomic DNA fragment: By error, a bacterial DNA piece is packaged into some phage heads instead of phage DNA.
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(D) Release of transducing particle: Lysis releases both normal phages and transducing particles containing bacterial DNA.
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(B) Injection of DNA of transducing particle: Transducing phage infects recipient bacterium, injecting bacterial DNA for potential recombination.
This sequence matches the standard lytic pathway in generalized transduction.
Why Other Options Are Incorrect
Option 1: (C), (A), (B), (E), (D)
Injection (C) cannot precede fragmentation (E) or packaging (A), as phage DNA must first enter to trigger host DNA breakdown. Packaging before fragmentation defies biology, placing release (D) last wrongly.
Option 3: (A), (D), (B), (E), (C)
Starts illogically with packaging (A) sans infection or fragmentation. Places phage injection (C) last, ignoring that transduction needs prior phage entry into the donor.
Option 4: (A), (E), (B), (D), (C)
Packaging (A) before fragmentation (E) is impossible; DNA must fragment first. Injection of transducing DNA (B) cannot precede release (D), and original phage injection (C) ends the sequence erroneously.
Key Takeaways for Exams
| Step Label | Description | Occurs In |
|---|---|---|
| (C) | Phage DNA injection | Donor infection start |
| (E) | Host DNA fragmentation | Phage replication phase |
| (A) | Bacterial DNA packaging | Head assembly error |
| (D) | Transducing particle release | Lysis of donor |
| (B) | Transducing DNA injection | Recipient infection |
This table highlights the chronological order, aiding quick recall for competitive exams like NEET PG. Focus on the lytic cycle dependency for generalized transduction questions.
