Definition of cosmids Cosmid vectors are best described as which of the following?
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APlasmids that contain a fragment of λ DNA including the cos site
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BPhages that lack the cos site required for packaging
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CPlasmids that have no selectable marker
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DCryptic plasmids with no known function
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EMini-chromosomes integrated into the host genome
Answer
Correct Answer: Plasmids that contain a fragment of λ DNA including the cos site
Explanation
Introduction / Context:Cosmids bridge properties of plasmids and λ phage by harnessing the λ cos site while retaining plasmid replication. They became popular for constructing genomic libraries with larger inserts than standard plasmids.
Given Data / Assumptions:
- Cos site enables λ headful packaging of DNA into phage particles.
- Cosmids replicate in cells as plasmids once introduced.
- They provide selectable markers for maintenance in bacteria.
Concept / Approach:By inserting λ cos sequences into a plasmid backbone, cosmids allow in vitro packaging of large recombinant DNA fragments (often 30–45 kb) into λ particles for efficient transduction, followed by plasmid-like replication in E. coli.
Step-by-Step Solution:
Identify the hybrid nature: plasmid backbone + λ cos site.Recognize function: packaging via λ system, replication as a plasmid in host.Select the option that states this definition accurately.Verification / Alternative check:Classic cosmid vectors (e.g., pJB8) illustrate this architecture and behavior.
Why Other Options Are Wrong:
- Phages lacking cos cannot package via λ system.
- Lack of selectable markers contradicts common cosmid design.
- “Cryptic plasmids” or “mini-chromosomes” are unrelated categories.
Common Pitfalls:Confusing cosmids with phagemids; phagemids use filamentous phage (f1) ori for ssDNA production, not λ cos.
Final Answer:Plasmids that contain a fragment of λ DNA including the cos site