66. Which combination of the following statements about specialized transduction is TRUE?
(P) Specialized transducing phages can transport only certain genes between bacteria
(Q) Specialized transducing phages can transport any gene between bacteria
(R) Phage P22 is a specialized transducing phage
(S) Phage lambda ( 𝜆 ) is a specialized transducing phage
(A) P and S only
(B) Q and R only
(C) P and R only
(D) Q and S only

Mastering Specialized Transduction: Explained with Phage Lambda and P22 Details

Specialized transduction involves temperate phages transferring specific bacterial genes adjacent to their integration site, unlike generalized transduction. The correct answer for this question is option (A) P and S only, as confirmed by classic microbiology principles.

Correct Answer

Option (A): P and S only.

Phage lambda (λ) undergoes improper excision from the bacterial chromosome near the gal or bio operons, packaging only those nearby genes into transducing particles. This matches statement P, which states specialized transducing phages transport only certain genes, and S, confirming lambda as a prime example.

Option Breakdown

• (P) True: Specialized transduction limits transfer to genes flanking the phage attachment site (att), such as gal-bio in lambda lysogens.

• (Q) False: Only specific genes near integration sites transfer; random genes require generalized transduction.

• (R) False: Phage P22 performs generalized transduction in Salmonella, packaging any chromosomal DNA via headful mechanism.

• (B) False: Q incorrect, R incorrect.

• (C) False: R incorrect despite P true.

• (D) False: Q incorrect despite S true.

Key Differences Table

Why Lambda Excels

Lambda integrates at attB between gal and bio, enabling λdgal (defective gal-transducing) or λdbio particles during rare excision errors. This site-specificity defines specialized transduction, foundational for bacterial genetics research.