Cosmid vectors are best suited for which general cloning task based on their hybrid plasmid–lambda design and packaging strategy?

Introduction / Context:
Cosmids combine a plasmid backbone with lambda cos sites, enabling in vitro packaging into lambda heads for efficient transduction and selection. Their primary value lies in accommodating larger inserts than typical plasmids—bridging the gap between plasmids and BAC/YAC systems.

Given Data / Assumptions:

• Cosmids retain plasmid replication and selection markers.
• Presence of cos sites allows lambda headful packaging.
• Insert sizes of roughly 35–45 kb are common in cosmid libraries.

Concept / Approach:
Lambda packaging imposes genome size constraints; cosmids exploit these to efficiently deliver large inserts while maintaining plasmid-like maintenance in E. coli. This makes them well-suited for intermediate-scale genomic library construction from both prokaryotic and eukaryotic sources.

Step-by-Step Solution:

Verification / Alternative check:
Genomic library protocols frequently specify cosmid vectors for 35–45 kb inserts, enabling robust coverage without moving to BAC/YAC complexity.

Why Other Options Are Wrong:

• A: Too small; plasmids suffice for small fragments.
• C/D: Cosmids are agnostic to source; both prokaryotic and eukaryotic DNA can be cloned.
• E: Cosmids are not primarily expression tools; they are for large-fragment cloning.

Common Pitfalls:
Confusing cosmid capacity with lambda replacement vectors or BACs; cosmids occupy the middle ground.

Final Answer:
Cloning large DNA fragments (approximately 35–45 kb) using lambda packaging