In the Meselson–Stahl experiment, if E. coli grown in 15N medium is shifted to 14N and allowed exactly one round of replication, how many DNA bands appear in a CsCl density gradient?

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:

  • Cells initially labeled with heavy 15N.
  • Shifted to light 14N medium.
  • Exactly one round of replication has occurred.


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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