Q.E. coli cells grown on 15N medium are transferred to 14N medium and allowed to grow for two generations. DNA extracted from these cells is ultracentrifuged in a caesium chloride density gradient. What density distribution of DNA would you expect in this experiment?

Step 1. The starting DNA is fully heavy (15N/15N). After the FIRST generation in 14N medium, every DNA molecule is a 15N/14N hybrid, so all the DNA settles at one intermediate density -- no heavy band remains and no fully light band has yet appeared.

Step 2. After the SECOND generation, each hybrid molecule again separates and templates a new light (14N) strand. Half of the resulting molecules are still hybrid (15N old strand + 14N new strand); the other half are now fully light (14N old strand from the hybrid + 14N new strand). …

Apply the Meselson-Stahl logic generation by generation: each round of semi-conservative replication halves the surviving fraction of hybrid molecu …

• Expecting a heavy band to still be present -- once the culture is switched to 14N medium, no fully-15N molecule survives past the first replication. …