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Question #46785
The Question
An E. coli culture, grown for several generations in a medium containing 15N (heavy nitrogen), is transferred to a normal 14N medium. Simultaneously, a temperature-sensitive mutation is activated that completely abolishes the recognition of the 'ori' sequence by initiator proteins. After 20 minutes (one normal generation time), the DNA is extracted and subjected to CsCl density gradient centrifugation. Which of the following correctly describes the expected banding pattern and the primary biochemical reason?
Options
- A.
A single intermediate density band (14N-15N); DNA Polymerase III bypasses the 'ori' sequence and utilizes random 3'-OH ends to complete semi-conservative replication.
- B.
A single heavy density band (15N-15N); failure to form the pre-replication complex prevents template unwinding, depriving DNA Polymerase of the required 3'-OH ends from RNA primers.
- C.
Two bands, one heavy (15N-15N) and one light (14N-14N); the mutation forces the replication machinery to adopt a conservative mode of replication to bypass the 'ori' defect.
- D.
A single heavy density band (15N-15N); the replication fork forms but immediately collapses because DNA Polymerase III lacks 3' to 5' exonuclease activity in the mutant.
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 Polymerase Directionality|Ori Sequence Stringency|Meselson-Stahl Density Gradient
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often rote-memorize the Meselson-Stahl experiment results (20 mins = 100% intermediate band) without integrating the biochemical prerequisites of DNA replication.
This question traps students who fail to realize that without 'ori' recognition, the entire downstream cascade (helicase loading -> primer synthesis -> 3'-OH availability for DNA Pol III) fails.
Thus, replication does not even begin, leaving the original heavy DNA intact.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Biomolecules - Class 11 |
| Concept | Peptide Bond Geometry |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |