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Question #47720
The Question
A researcher synthesizes a modified lacZ alpha-fragment for a novel cloning vector. To theoretically optimize expression, they utilize the degeneracy of the genetic code to replace native codons with synonymous, highly GC-rich codons. However, when testing the empty vector (without any foreign DNA insert) in E. coli on X-gal/IPTG plates, the colonies appear white instead of the expected blue. Which of the following best explains this anomalous 'pseudo-white' phenotype?
Options
- A.
High GC content in the modified transcript formed stable mRNA stem-loops, obstructing the ribosome and preventing translation.
- B.
Synonymous codon substitution altered the amino acid sequence of the repressor protein, permanently switching off the lac promoter.
- C.
The GC-rich sequence induced spontaneous insertional inactivation of the lacZ gene by host genomic DNA.
- D.
The modified codons depleted the host's rare tRNA pool, causing RNA polymerase to stall during transcription.
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
Griffith’s Transforming Principle|pBR322 Selection Logic|Plasmid vs Chromosomal DNA
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often assume that because the genetic code is degenerate, synonymous mutations are strictly 'silent' and have no phenotypic effect.
This question tests the advanced NCERT-Plus concept that mRNA primary sequence dictates its physical secondary structure, which critically regulates translation efficiency.
It bridges Molecular Basis of Inheritance (Degeneracy/Translation) with Biotechnology (Blue-White Screening).
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 |