Questions: Specialized and Generalized Transduction

Question 1

Lambda phage in E. coli always transfers either the gal or bio operon during specialized transduction, but never genes from the middle of the bacterial chromosome. What explains this specificity?

• A. Lambda phage preferentially packages genes involved in carbon metabolism, and gal and bio are both metabolic operons
• B. Lambda phage packaging machinery has a sequence preference for these genes because they share homology with phage DNA
• C. Lambda integrates at a fixed chromosomal att site flanked by gal and bio; imprecise excision can only capture genes immediately adjacent to the integration site
• D. Lambda phage transfers only the genes it acquired from its ancestral bacterial host during coevolution

Answer: (C)

This is the defining feature of specialized transduction: the specificity comes entirely from the fixed location of phage integration. Lambda always inserts at the same att site on the E. coli chromosome, which happens to be flanked by the gal operon on one side and the bio operon on the other. When the prophage excises imprecisely — the recombination boundary shifts so it includes flanking chromosomal DNA instead of leaving cleanly — it can only pick up what is immediately adjacent. There is no sequence preference or metabolic logic; any gene flanking any phage's att site would be the one transferred. A different phage with a different insertion site would transfer a completely different set of genes.

Question 2

A transducing particle produced by generalized transduction infects a new bacterial cell. What does this particle contain, and what happens to the DNA it delivers?

• A. It contains phage DNA plus the bacterial gene of interest, which integrates and causes lysogenic conversion
• B. It contains only bacterial chromosomal DNA, which can recombine into the recipient's chromosome by homologous recombination
• C. It contains a mixture of phage and bacterial DNA, allowing either integration or replication depending on conditions
• D. It contains bacterial plasmid DNA packaged accidentally, which replicates autonomously in the recipient

Answer: (B)

During generalized transduction, the phage packaging machinery accidentally grabs a headful of bacterial chromosomal DNA instead of phage DNA. The resulting transducing particle looks like a normal phage — it has a protein capsid and can inject DNA — but carries only bacterial DNA, no phage genes. Inside the new host, the injected bacterial DNA cannot replicate (it lacks phage replication origins) but can recombine into the recipient's chromosome via homologous recombination if sufficient sequence similarity exists. Cells that successfully incorporate the donor gene are called transductants.

Question 3

In specialized transduction, the prophage captures bacterial genes at random from any location on the chromosome because imprecise excision is a random event.

• T. True
• F. False

Answer: False

Imprecise excision is random in the sense that it is an error — but the DNA that gets captured is not random at all. Because the prophage integrates at a fixed att site, imprecise excision can only incorporate bacterial DNA immediately flanking that specific site. The randomness is in whether excision is imprecise; the specificity is in which genes are captured when it is. This is exactly what distinguishes specialized from generalized transduction: specialized transduction transfers the same genes every time (those flanking the att site), while generalized transduction can transfer any gene.

Question 4

A transducing particle produced by generalized transduction contains only bacterial DNA, yet it can enter a new bacterial cell in the same way a normal phage does.

• T. True
• F. False

Answer: True

The transducing particle is assembled in the phage capsid using the phage's own tail fibers and coat proteins — it is indistinguishable from a normal phage particle from the outside. The phage coat determines host range and injection mechanism, not the DNA content inside. The particle binds the receptor on the new host and injects its contents normally. The difference only becomes apparent once inside: no phage genes means no phage replication, so the particle cannot produce a lytic infection. The bacterial DNA must instead integrate via recombination to be maintained.

Question 5

Explain the mechanistic difference between specialized and generalized transduction, and why specialized transduction always transfers the same bacterial genes while generalized transduction can transfer any gene.

Model answer: In generalized transduction, the phage packaging machinery (operating during the lytic cycle) occasionally grabs a fragment of bacterial chromosomal DNA instead of phage DNA. Because packaging is essentially random with respect to chromosomal location, any chromosomal gene can end up in a transducing particle. In specialized transduction, the mechanism is entirely different: an integrated prophage excises imprecisely, incorporating bacterial DNA from the chromosomal region immediately flanking its integration site. Because the integration site is fixed, imprecise excision always captures the same flanking genes. Specificity is an automatic consequence of the fixed integration location.

This contrast illuminates a broader principle: the same outcome (horizontal gene transfer between bacteria) can arise from completely different molecular mechanisms, with very different specificity profiles. Understanding the mechanism predicts what can be transferred — which is essential for understanding how virulence factors and antibiotic resistance genes spread.