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Question #45872

Difficulty: 8/10 | 🟢 Free

The Question

In a modified Meselson-Stahl experiment, a synchronized eukaryotic cell culture continuously grown in a heavy nitrogen ( $^{15}N$ ) medium is transferred to a light nitrogen ( $^{14}N$ ) medium. Simultaneously, RNA interference (RNAi) vectors are introduced to specifically knock down the expression of the main replicative DNA polymerases. After one equivalent generation time, the DNA is extracted and analyzed via CsCl density gradient centrifugation. Which of the following best describes the expected observation and its molecular rationale?

Options

A.
A single hybrid density band ( $^{15}N/^{14}N$ ) forms because DNA ligase independently polymerizes and seals the available $^{14}N$ nucleotides.
B.
A predominant heavy density band ( $^{15}N/^{15}N$ ) is observed because replication forks stall, preventing the synthesis and ligation of complete $^{14}N$ complementary strands.
C.
Two distinct bands (heavy and light) appear because the depletion of DNA polymerase forces the replication machinery into a conservative mode.
D.
A single light density band ( $^{14}N/^{14}N$ ) forms because the RNAi vectors directly degrade the heavy parental DNA strands to halt replication.

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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|RNAi Vector Source|DNA Ligase Polarity

Why This Question Is Tricky

Common Mistakes

Conceptual Pivot: Students mechanically associate 'one generation' in the Meselson-Stahl experiment with the mandatory appearance of a 'hybrid band'.

The trap is failing to recognize that the hybrid band is a physical consequence of completed semi-conservative replication.

If polymerization is blocked (via RNAi knockdown), the structural prerequisite for the hybrid density (a full $^{14}N$ strand hydrogen-bonded to a full $^{15}N$ strand) is never met.

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