In the experiments performed by Meselson and Stahl, E. coli bacteria were grown for many generations in a medium containing heavy nitrogen isotope (15N) then for one generation in a medium containing a lighter nitrogen isotope (14N).
To show that DNA replication is semi-conservative, Meselson and Stahl used density gradient centrifugation to separate the DNA molecules based on their density. The heavy DNA containing 15N would be expected to settle at a lower point on the gradient than lighter DNA containing 14N.
After one generation of replication in the 14N medium, the DNA was extracted and analyzed using density gradient centrifugation. There were two possible outcomes that would support semi-conservative replication:
1. If replication was conservative, the DNA extracted from the E. coli cells would have two distinct bands on the density gradient: one band with heavy DNA (15N) and one band with light DNA (14N). However, the results showed only one band of intermediate density. This could not support conservative replication.
2. If replication was semi-conservative, the DNA extracted from the E. coli cells would have one band of intermediate density. This is exactly what Meselson and Stahl observed, which supported the hypothesis of semi-conservative replication.
Therefore, the results of the experiments showed that the DNA of the last generation of E. coli had one band of intermediate density, indicating that DNA replication was semi-conservative.