Difficulty: Easy
Correct Answer: One
Explanation:
Introduction / Context:
The Meselson–Stahl experiment demonstrated semiconservative replication by monitoring DNA density changes after switching isotopic media. Understanding the banding pattern after each generation reveals the replication mechanism.
Given Data / Assumptions:
Concept / Approach:
Semiconservative replication yields daughter molecules with one parental (15N) strand and one newly synthesized (14N) strand after the first generation. Both DNA duplexes have identical hybrid densities and thus co-migrate.
Step-by-Step Solution:
1) Start: all DNA is heavy–heavy (15N/15N).2) After one replication: each duplex is heavy–light (15N/14N).3) Density gradient: both molecules have the same intermediate density, forming a single band.
Verification / Alternative check:
After two generations, bands split into one intermediate and one light band, confirming the semiconservative model rather than conservative or dispersive replication.
Why Other Options Are Wrong:
Two: occurs after two generations, not one.Three or smear: inconsistent with discrete semiconservative outcomes under standard conditions.
Common Pitfalls:
Expecting both heavy and light bands after one generation, or confusing the timeline of band formation.
Final Answer:
One
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