8. λ-phage insert their DNA into bacterial host. The site for recombination on host is termed as-
(1) attp (2) attB
(3) Xis (4) Int
λ-phage inserts its DNA into the bacterial host at the attB site, making option (2) the correct answer. This site-specific recombination enables lysogenic integration of the phage genome into the E. coli chromosome.
Option Analysis
Detailed breakdown of each option clarifies their roles in λ-phage recombination:
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(1) attP: Phage attachment site on λ DNA (POP′ structure); pairs with attB for integration but is not the host site.
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(2) attB: Bacterial attachment site (BOB′) on host chromosome; specific recombination locus targeted by λ-phage Int protein. Correct as the host integration point.
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(3) Xis: Excisionase protein from phage; promotes prophage excision (attL × attR) by aiding Int, not host integration site.
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(4) Int: Integrase enzyme encoded by phage; catalyzes recombination between attP and attB, but is a protein, not the host site.
Recombination Mechanism
Integration requires supercoiled attP and linear attB, mediated by Int plus host IHF for bending. This forms attL and attR hybrids post-recombination. Excision reverses via Xis, Int, FIS, and IHF.
