What is a phagemid? Choose the most accurate description of a phagemid's genetic architecture and capability.
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AA plasmid cloning vector that also carries the filamentous phage (f1) origin, enabling recovery of single-stranded DNA
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BA plasmid carrying only the λ att site for chromosomal integration
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CA plasmid with the λ cos site but no plasmid origin of replication
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DA plasmid that contains only a bacterial plasmid origin (no phage functions)
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EA defective bacteriophage genome incapable of replication in bacteria
Answer
Correct Answer: A plasmid cloning vector that also carries the filamentous phage (f1) origin, enabling recovery of single-stranded DNA
Explanation
Introduction / Context:Phagemids combine features of plasmids and filamentous phage, allowing researchers to prepare single-stranded DNA for sequencing, mutagenesis, and display technologies while maintaining standard plasmid benefits (selection, high copy number).
Given Data / Assumptions:
- Phagemids retain a bacterial plasmid origin for dsDNA replication.
- They also include an f1 phage origin that, with helper phage, yields ssDNA.
- Selectable markers and MCS sites are typical.
Concept / Approach:The defining feature is the additional f1 origin. Upon superinfection with a helper phage, the phagemid DNA is packaged as ssDNA and secreted, which can be isolated easily. This capability distinguishes phagemids from ordinary plasmids and from λ-based cosmids.
Step-by-Step Solution:
Identify key function: production of ssDNA upon helper phage presence.Map this to architecture: plasmid ori + f1 ori in one vector.Select the option that states both plasmid backbone and f1 origin.Verification / Alternative check:Vectors such as pBluescript and pUC derivatives with an f1 origin are widely used as phagemids, validating the description.
Why Other Options Are Wrong:
- λ att or cos sites are λ features; phagemids are based on filamentous phage f1, not λ.
- Vectors lacking a plasmid origin would not replicate as plasmids.
- “Defective phage genome” does not match phagemid function.
Common Pitfalls:Confusing cosmids (λ cos-based, large insert) with phagemids (f1 ori-based, ssDNA production).
Final Answer:A plasmid cloning vector that also carries the filamentous phage (f1) origin, enabling recovery of single-stranded DNA