Key feature of cosmids: Which essential lambda functions do cosmids intentionally lack while retaining cos sites and plasmid elements?

Introduction / Context:
Cosmids are engineered to harness lambda packaging efficiency without carrying the full lambda genome. Understanding what they lack clarifies why cosmids behave as plasmids inside bacteria yet are packaged like lambda during transduction.

Given Data / Assumptions:

• Cosmids include cos sites to enable in vitro lambda packaging.
• They retain plasmid ori and selection markers for propagation in E. coli.
• They do not encode the majority of lambda proteins needed for a viral life cycle.

Concept / Approach:
By omitting most lambda genes, cosmids avoid phage replication and lytic growth while still benefiting from headful packaging. After transduction, the cosmid circularizes and replicates as a plasmid, carrying large DNA inserts efficiently without phage biology complications inside the host.

Step-by-Step Solution:

Verification / Alternative check:
Cosmid maps show cos sequences flanking a plasmid backbone with ori and antibiotic resistance but no λ structural/replication cassettes.

Why Other Options Are Wrong:

• B/C/D: Cosmids retain ori, markers, and MCS—these are core plasmid features.
• E: Cosmids do replicate autonomously in bacteria via their plasmid ori.

Common Pitfalls:
Assuming “cos” implies a full phage genome; cosmids only borrow packaging signals.

Final Answer:
Genes coding for viral structural and replication proteins (most λ genes are absent)