Question

Imagine the Meselson and Stahl experiments had supported conservative replication instead of semiconservative replication. What results would you predict to observe after two rounds of replication? Be specific regarding per cent distributions of DNA incorporating 15N and 14N in the gradient.

Short answer

Centrifugation in calcium chloride can separate the two isotopes of nitrogen, N (heavy) and 14N (normal), based on their densities (CsCl). 25% of the DNA would be in a lower (heavier) band (15N) and 75% of the total DNA would be located in a second, higher (lighter) band (14N).

Step-by-step solution

Introduction 

In 1958, Matthew Meselson and Franklin Stahl conducted the Meselson-Stahl experiment, which proved Watson and Crick's theory that DNA replication was semiconservative. Centrifugation in calcium chloride can separate the two isotopes of nitrogen, N (heavy) and 14N (normal), based on their densities (CsCl). Over several generations, Meselson and Stahl raised E. coli in a medium containing 15NH4Cl. 15N thus became part of the bacterial DNA.

The explanation for the answer. 

Two bands would be seen during ultracentrifugation after two rounds of conservative replication. 25% of the DNA would be in a lower (heavier) band that is at the 15N density. 75% of the total DNA would be located in a second, higher (lighter) band at the 14N density.