E. coli with heavy or ${ }^{15}N$ DNA is allowed to replicate in medium having ${ }^{14}N{H}_{4}Cl$, after first generation the number of DNA molecules is doubled and each DNA molecule has one light DNA strand and one heavy strand and the DNA is described as hybrid. In the second generation there is further doubling of DNA and half of the DNA is light with ${ }^{14}N$ and the remaining half has hybrid DNA with one light and one heavy strands. Hence the percentage of bacteria with ${ }^{15}N$ is 50%.