Escherichia coli fully labelled with ¹⁵N is allowed to grow in${ }^{14}\mathrm{N}$ medium. The two strands of DNA molecule of the first generation bacteria have

Meselson and Stahl (1958) cultured (Escherichia coli) bacteria in a culture medium containing ${ }^{15}\mathrm{N}$. After these had been replicated for a few generations in the medium both the strands of their DNA contained ${ }^{15}\mathrm{N}$ as constituents of purines and pyrimidines. When these bacteria with ${ }^{15}\mathrm{N}$ were transferred in cultural medium containing ${ }^{14}\mathrm{N}$, it was found that DNA separated from fresh generation of bacteria possesses one strand heavier than the other. The heavier strand represents the parental strand and lighter one is the new one synthesized from the culture indicating semi conservative mode of DNA replication. So after one generation among two strands of DNA molecule one strand is with ¹⁵N,another strand is with¹⁴N(hybrid DNA) .