Phagemid vectors — What is the most accurate description of their structure and functional advantage in cloning workflows?
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AHybrid plasmids carrying a filamentous phage origin (f1/M13) to allow single-stranded DNA rescue
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BCombinations of lambda head–tail genes with plasmid replication for high-capacity packaging
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CBacteriophages that occasionally behave like plasmids in eukaryotic cells
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DPure cosmids that include cos sites but no plasmid origin
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EMinimal plasmids that cannot replicate unless packaged into phage heads
Answer
Correct Answer: Hybrid plasmids carrying a filamentous phage origin (f1/M13) to allow single-stranded DNA rescue
Explanation
Introduction / Context:Phagemids merge convenient plasmid features (ori, antibiotic resistance, MCS) with a filamentous phage origin of replication (f1/M13). This enables recovery of single-stranded DNA after helper phage infection—critical for Sanger sequencing, mutagenesis, and phage display scaffolds.
Given Data / Assumptions:
- Phagemids are primarily plasmids in growth behavior.
- An f1/M13 origin permits packaging of generated ssDNA into filamentous phage particles when a helper phage is present.
- They are distinct from lambda, cosmid, and BAC systems.
Concept / Approach:A phagemid replicates as a double-stranded plasmid under normal growth. Upon superinfection with a helper phage, the f1 ori directs production of circular ssDNA that is secreted in filamentous particles. This duality combines ease of manipulation with the sequencing advantages of ssDNA templates.
Step-by-Step Solution:
Identify key hallmark: f1/M13 origin on a plasmid backbone.Relate to function: helper phage rescue produces ssDNA for downstream applications.Choose the option that captures both structure and function.Verification / Alternative check:Vector manuals for pBluescript, pGEM, and similar phagemids describe helper phage rescue protocols to obtain ssDNA for Sanger sequencing or mutagenesis.
Why Other Options Are Wrong:
- B: Describes lambda-based systems, not phagemids.
- C: Incorrect biology; phages do not replicate as plasmids in eukaryotes.
- D: Cosmids lack the f1 origin and are not phagemids.
- E: Phagemids replicate autonomously in bacteria via plasmid ori; phage packaging is optional.
Common Pitfalls:Confusing cosmid packaging (dsDNA) with filamentous phage ssDNA rescue; they serve very different purposes.
Final Answer:Hybrid plasmids carrying a filamentous phage origin (f1/M13) to allow single-stranded DNA rescue