Final answer:
Meselson and Stahl used the 15N isotope to differentiate between new and old DNA strands in E. coli, confirming the semiconservative model of DNA replication.
Step-by-step explanation:
Meselson and Stahl determined that DNA replication in E. coli is semiconservative by using an additive to distinguish new DNA from old DNA. The correct additive they supplied to the medium was the 15N isotope. They initially grew E. coli in a medium containing heavy nitrogen (15N), which was incorporated into the DNA.
After several generations, the E. coli was switched to a medium with the more common light nitrogen (14N). When DNA replication occurred, the DNA that incorporated 14N showed a density that was intermediate between DNA with only 15N and DNA with only 14N, confirming semiconservative replication where each new DNA molecule consists of one old and one new strand.