Definition of cosmids Cosmid vectors are best described as which of the following?

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:

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