Difficulty: Medium
Correct Answer: Phagemids routinely contain a λ att site for integration into the host chromosome
Explanation:
Introduction / Context:Phagemids are versatile vectors that combine features of plasmids and filamentous phage (such as M13). They are widely used for phage display, site-directed mutagenesis, and production of single-stranded DNA. Understanding their core genetic elements helps distinguish them from λ phage–based vectors and standard plasmids.
Given Data / Assumptions:
Concept / Approach:The key to phagemid function is the f1 origin derived from filamentous phage, not λ. With a helper phage, phagemids can be packaged into filamentous phage particles, allowing purification of single-stranded DNA. They are not λ-based integration vectors and therefore do not require a λ att site (used by λ for site-specific integration into the E. coli chromosome).
Step-by-Step Solution:
Check origin composition → plasmid ori + f1 ori (true).Check propagation modes → plasmid state in standard hosts; phage particle packaging in helper strains (true).Evaluate λ att site claim → phagemids are not λ integration vectors; this is not true.Confirm selectable marker and ssDNA utility → common and accurate features.Verification / Alternative check:Phagemid manuals and classic vector maps (for example, pBluescript, pUC-derivatives with f1 ori) show the f1 origin and standard antibiotic markers; no λ att is present.
Why Other Options Are Wrong (i.e., they are true statements):
Plasmid + f1 origins: Core architecture enabling ds and ss forms.Dual propagation modes: Fundamental to helper-phage rescue.Selectable marker: Required for maintenance and selection.ssDNA recovery: A hallmark application of phagemids.Common Pitfalls:Confusing λ-based replacement/insertion vectors with filamentous phage–based phagemids; assuming all phage-related vectors need λ att sites.
Final Answer:Phagemids routinely contain a λ att site for integration into the host chromosome.
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