Molecular cloning — What core features define an expression vector used to express a cloned gene in a host cell (consider replication, selection, and transcriptional control)?

Introduction:
An expression vector is a specialized cloning vector designed not merely to carry DNA but to express it as RNA and protein inside a chosen host (for example, Escherichia coli, yeast, or mammalian cells). This question tests whether you can identify the essential architectural features that make expression possible and selectable.

Given Data / Assumptions:

• The host must replicate the plasmid to inherit it.
• Researchers must select cells that actually contain the plasmid.
• Transcription and often translation require appropriate regulatory DNA sequences.

Concept / Approach:
Evaluate each proposed feature as part of a minimal, practical expression system. A workable vector must be replicated (origin), selected (antibiotic or auxotrophic marker), and expressed (promoter plus elements like ribosome-binding site or Kozak sequence). The multiple-cloning site alone is insufficient.

Step-by-Step Solution:

Verification / Alternative check:
Commercial expression vectors (pET, pGEX, pcDNA) universally include ori + selectable marker + promoter/terminator modules, illustrating the standard design principles.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing general cloning vectors with expression vectors; forgetting that promoters are host-specific (a bacterial promoter will not drive expression in mammalian cells without the correct eukaryotic elements).

Final Answer:
All of the above.