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)?
-
AIt always contains an origin of replication so the plasmid can be maintained in the host
-
BIt usually carries a selectable marker such as an antibiotic-resistance gene for host selection
-
CIt always provides DNA regulatory segments (promoter and related elements) to drive mRNA production
-
DAll of the above
-
EOnly a multiple-cloning site; the host supplies the rest
Answer
Correct Answer: All of the above
Explanation
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:
1) Origin of replication: enables autonomous plasmid replication in the host (e.g., ColE1-derived ori for E. coli).2) Selectable marker: allows survival on a selective medium (e.g., ampicillin, kanamycin), ensuring maintenance of the vector in culture.3) Expression control: a promoter suited to the host (e.g., T7, lac, CMV), plus additional elements such as operator, RBS/Kozak, terminator/poly(A) for eukaryotes, and often tags for purification.4) Therefore, a, b, and c are all correct; the comprehensive answer is “All of the above.”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:
e) A cloning site without ori/marker/regulatory elements cannot support stable expression or selection.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.