Meselson and Stahl (1958) grew E. coli bacteria in a culture medium enriched in heavy isotopes of 14N and 15N. After several generations, the DNA of E. coli bacteria is labeled with 15N. The 15N enriched bacteria were then transferred back to the 14N medium. The next generation showed that the newly formed bacterial DNA double helix had one strand of 14N and the other strand of 15N. That is, one old formula and the other new formula of the double helix. This proves that DNA replication occurs in a semi-conservative manner.