COS cell line — origin and purpose in SV40-based expression What accurately describes the COS (COS-1/COS-7) mammalian cell line commonly used for transient expression from SV40-origin plasmids?

Introduction / Context:
COS cells (COS-1 or COS-7) are workhorse mammalian cell lines used for high-level, transient protein expression. They are particularly useful for plasmids carrying the SV40 origin of replication because they provide the SV40 large T antigen in trans.

Given Data / Assumptions:

• COS cells derive from CV-1 African green monkey kidney cells.
• They express SV40 large T antigen, which recognizes and drives replication from SV40 ori.
• The question contrasts COS cells with the phage λ cos sequence and unrelated cell backgrounds.

Concept / Approach:

SV40 large T antigen is necessary to initiate bidirectional DNA replication at the SV40 origin. By stably expressing large T, COS cells amplify SV40-ori plasmids episomally, boosting transgene expression without needing viral infection. This distinguishes the cell line from the similarly named λ cos DNA packaging site.

Step-by-Step Solution:

Verification / Alternative check:

Standard expression protocols specify COS-7 for SV40-ori plasmids; omission of large T antigen support in other cell lines yields lower copy number, confirming the unique benefit of COS cells.

Why Other Options Are Wrong:

Phage λ cos DNA sites (options b and c) are sequence elements, not cell lines. Insect cells (option d) pertain to baculovirus systems. Human fibroblasts with telomerase (option e) are unrelated to SV40-ori dependence.

Common Pitfalls:

Confusing COS (cell line) with cos (λ DNA packaging site). The similar names refer to very different concepts.

Final Answer:

Derivative of permissive CV-1 African green monkey cells engineered to express SV40 large T antigen