Cosmids and cos sites: Which statements correctly describe the cos site used in cosmid cloning and packaging?

Introduction / Context:
Cosmids are hybrid vectors that include the λ cos site, allowing plasmid-sized constructs to be packaged into λ phage heads for efficient delivery. Understanding the structure and role of the cos site clarifies how large inserts can be cloned and transported like phage, yet maintained as plasmids in the host.

Given Data / Assumptions:

• Cos sites are the cohesive ends used by λ phage during packaging and circularization.
• Cosmids carry a plasmid origin plus cos sequences.
• After infection, cosmids circularize to replicate as plasmids.

Concept / Approach:
The λ cos site is a 12-base cohesive end that mediates concatemer cleavage and subsequent circularization in the host. In cosmids, cos enables λ head packaging of large DNA constructs, improving library construction efficiency compared with direct transformation of large plasmids.

Step-by-Step Solution:

Verification / Alternative check:
Cosmid protocols highlight using λ extract for packaging and subsequent recovery of circular plasmids in E. coli, all reliant on the cos sites.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing cos with general restriction sites; underestimating how cos improves transformation efficiency for large constructs via packaging.

Final Answer:
Both (a) and (b) are correct.