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Question #45884
The Question
A researcher excises only the lacZ structural gene (strictly excluding the native promoter and operator sequences) from the E. coli chromosome. This gene is ligated into a high-copy number plasmid immediately downstream of a strong constitutive promoter. The recombinant plasmid is then transformed into an E. coli strain that lacks its own chromosomal lac operon. Which of the following is the most accurate prediction regarding gene expression in these transformants?
Options
- A.
Beta-galactosidase will be synthesized continuously, but permease and transacetylase will not be produced.
- B.
Beta-galactosidase synthesis will remain inducible because the repressor protein will bind to the plasmid's constitutive promoter.
- C.
The entire polycistronic mRNA for lacZ, lacY, and lacA will be transcribed constitutively due to the high copy number of the plasmid.
- D.
Beta-galactosidase will not be synthesized because the absence of the native operator prevents the binding of RNA polymerase.
Check Your Answer
Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Comparative_Analysis
Sharpen your ability to evaluate multiple scenarios and distinguish between competing potential answers.
Concepts Behind This Question
Operator|Structural_Genes|Plasmid_vs_Chromosomal_DNA
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often harbor the misconception that an enzyme like beta-galactosidase is inherently 'inducible' by its very nature.
This question forces the realization that inducibility is strictly a property of the regulatory sequence (the operator) upstream of the gene, not the structural gene itself.
It also tests the precise binding locations of transcriptional machinery (RNA polymerase to promoter vs.
Repressor to operator).
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Molecular Basis of Inheritance - Class 12 |
| Concept | Transcriptional Unit Boundaries |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |