They grew cells of the bacterium Escherichia coli in the presence of 15N (heavy nitrogen). Therefore the DNA of the bacterium cells contained 15N. After that, the cells were transferred to a medium containing 14N (simple nitrogen) and allowed to go through cell division. As a result, the researchers were able to demonstrate that, in the DNA molecules of the daughter cells, one strand contained only 15N, and the other strand contained 14N. This proved the semiconservative mode of replication.