Difficulty: Easy
Correct Answer: Semiconservative replication
Explanation:
Introduction:The Meselson–Stahl experiment (E. coli grown in heavy nitrogen followed by a light nitrogen shift) is a landmark demonstration of how DNA duplicates during the cell cycle. The question asks which replication model their data supported.
Given Data / Assumptions:
Concept / Approach:Semiconservative replication predicts that each daughter DNA molecule contains one parental (heavy) strand and one newly synthesized (light) strand after one generation in light nitrogen, yielding hybrid density DNA.
Step-by-Step Solution:
1) Grow cells in 15N to label parental DNA heavy.2) Shift to 14N; allow one round of replication.3) Observe a single intermediate-density band (hybrid), not separate heavy and light bands.4) After two rounds, see both hybrid and light bands, consistent with semiconservative replication.Verification / Alternative check:Conservative replication would show distinct heavy and light bands after one generation; dispersive would maintain only intermediates without a pure light band even after several generations.
Why Other Options Are Wrong:
a) Predicts separate heavy and light duplexes after one replication; not observed.c) Predicts all hybrids each generation without pure light; data contradict.d) A chromosomal disorder, not a replication model.e) Rolling-circle occurs in some systems but was not the universal model demonstrated here.Common Pitfalls:Confusing labeling (heavy vs light) with strand composition; forgetting the generational pattern of bands.
Final Answer:Semiconservative replication.
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