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Question #45874
The Question
A researcher attempts to replicate the Meselson-Stahl experiment using E. coli grown in 15N medium, followed by one generation in 14N medium. However, during the CsCl density gradient centrifugation, a high concentration of Ethidium Bromide (EtBr) is mistakenly added to the gradient. Given that the E. coli strain possesses highly repetitive AT-rich sequences, which of the following is the most likely consequence on the experimental results?
Options
- A.
The buoyant density of the DNA decreases non-linearly due to preferential AT-rich intercalation, creating artifactual bands that obscure the expected single intermediate 15N-14N hybrid band.
- B.
The intercalation of EtBr increases the overall molecular mass of the DNA, shifting the hybrid band to a higher density position identical to pure 15N DNA.
- C.
EtBr induces the denaturation of the 15N-14N hybrid DNA double helix, resulting in two distinct bands corresponding to heavy and light single strands.
- D.
EtBr acts solely as a fluorescent tag in the CsCl gradient without altering the buoyant density, yielding the standard single intermediate band under UV light.
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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
Meselson-Stahl Density Gradient|Ethidium Bromide Visualization|Human Genome Repetitive DNA
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often compartmentalize NCERT facts, knowing EtBr as a 'DNA stain' (Biotechnology) and CsCl as a 'density gradient' (Molecular Basis of Inheritance).
The trap is assuming EtBr acts solely as a passive fluorescent tag (Option D) or that adding a molecule increases density (Option B).
The synthesis requires understanding that intercalation increases the physical volume of the DNA helix disproportionately to its mass, thereby decreasing buoyant density and creating artifacts if sequence-specific affinities (AT-rich) are present.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Molecular Basis of Inheritance - Class 12 |
| Concept | Meselson-Stahl Density Gradient |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |