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Question #47722
The Question
During a blue-white screening procedure, a student plates transformed E. coli on a medium containing Ampicillin, X-gal, IPTG, and a high concentration of Glucose. After incubation, the student observes that all surviving colonies are white, including the non-recombinants. Which of the following best explains the failure of the non-recombinant colonies to produce a blue color?
Options
- A.
Glucose competitively inhibits the binding of IPTG to the Lac repressor, keeping the operon in a permanently repressed state.
- B.
High glucose concentration induces spontaneous insertional inactivation of the lacZ gene, preventing β-galactosidase synthesis.
- C.
Glucose lowers cellular cAMP levels, preventing the formation of the CAP-cAMP complex required for efficient RNA polymerase binding at the promoter.
- D.
The metabolic burden of glucose assimilation causes the rapid extracellular degradation of X-gal before it can be cleaved by β-galactosidase.
Check Your Answer
Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Evolutionary_Exception
Tackle problems where standard rules evolve or change based on specific environmental constraints.
Concepts Behind This Question
Gene Therapy Germline vs Somatic|pBR322 Selection Logic|Plasmid vs Chromosomal DNA
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often memorize the heuristic 'IPTG + intact lacZ = Blue colonies' but forget the dual-control mechanism of the Lac operon.
This question tests the synthesis of Biotechnology (Blue-White screening) with Molecular Genetics (Catabolite Repression).
The trap is failing to recognize that the presence of a preferred carbon source (glucose) overrides the presence of an inducer (IPTG) by shutting down the positive regulatory pathway (CAP-cAMP).
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Biotechnology : Principles and Processes - Class 12 |
| Concept | pBR322 Selection Logic |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |