Why Meselson grow E.coli in N15 for DNA replication then we put it in N14?

Meselson and Stahl grew E. coli in nitrogen-15 (N-15) to label the DNA with heavy nitrogen, allowing them to track the incorporation of nitrogen isotopes during replication. By then transferring the bacteria to a medium containing nitrogen-14 (N-14), they could observe how the newly synthesized DNA incorporated the lighter isotope. This experimental setup provided crucial evidence for the semi-conservative model of DNA replication, as it allowed them to distinguish between original and newly synthesized DNA strands based on their density.