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?

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