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Question #47707
The Question
During a recombinant DNA experiment, a researcher notices that a batch of plasmid vectors has lost its negative supercoiling due to accidental single-strand nicks. When this relaxed plasmid is subjected to the standard heat-shock method (42°C) for transformation into competent E. coli, the transformation efficiency is exceptionally low. Which of the following is the most accurate biophysical explanation for this failure?
Options
- A.
The loss of supercoiling increases the plasmid's hydrodynamic radius, completely preventing its entry through the transient pores of the host membrane.
- B.
Thermal stress at 42°C induces local melting at the nicked sites, generating free ends that are rapidly degraded by host intracellular exonucleases.
- C.
The heat shock temperature directly cleaves the remaining intact phosphodiester bonds, linearizing the plasmid before entry.
- D.
Relaxed plasmids fail to be recognized by the host's DNA polymerase III, preventing their autonomous replication post-entry.
Check Your Answer
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 Ligase Polarity|Restriction Enzyme Cleavage Physics|Okazaki Fragment Logic
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students memorize heat shock (42°C) solely as a mechanism to create transient pores in the bacterial cell wall/membrane.
They often fail to integrate the thermodynamic effects of this heat on the DNA molecule itself.
The trap lies in ignoring that exonucleases require free ends to act; a nicked plasmid subjected to thermal stress provides these free ends via local melting, bridging biophysics with recombinant DNA technology.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Biotechnology : Principles and Processes - Class 12 |
| Concept | Restriction Enzyme Cleavage Physics |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |