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Question #45877
The Question
In a recombinant DNA experiment, a plasmid vector containing an Ampicillin resistance gene and a lacZ marker is restricted and mixed with a foreign DNA insert. Due to suboptimal conditions, DNA ligase fails to seal several phosphodiester gaps, leaving nicks at the cloning site within the lacZ gene. Upon transformation into an E. coli host and plating on an Ampicillin/X-gal medium, the researcher observes numerous white colonies. However, sequencing reveals that many of these white colonies do NOT contain the foreign insert. Which of the following best explains this anomalous result?
Options
- A.
The host cell's error-prone DNA repair mechanisms attempted to fix the unsealed nicks, introducing deletions that caused a frameshift mutation in the lacZ gene.
- B.
The unsealed 5'-PO4 and 3'-OH ends were recognized by DNA Polymerase III, which initiated de novo synthesis and suppressed β-galactosidase activity.
- C.
The presence of nicks completely prevented plasmid replication, causing the host cells to lose the plasmid and appear white due to the absence of the lacZ gene.
- D.
The unligated plasmid fragments underwent homologous recombination with the host's nucleoid, silencing the lacZ gene through RNA interference.
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Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Regulatory_Failure
Understand how NOT, EXCEPT, and logical constraints are used to manipulate exam results.
Concepts Behind This Question
DNA Polymerase Directionality|DNA Ligase Polarity|Blue-White Screening Mechanism
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often memorize 'White colony = Recombinant (successful insertion)' without understanding the underlying genetics.
The trap tests the realization that white colonies simply mean a non-functional lacZ gene, which can also result from frameshift mutations caused by the host's error-prone repair of unligated nicks (a classic false-positive scenario in molecular cloning).
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Molecular Basis of Inheritance - Class 12 |
| Concept | DNA Polymerase Directionality |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |