Escherichia coli with completely heavy DNA  15N was allowed to replicate in a medium with  14NH4Cl for two generations. The percentage of bacteria with  15N in its DNA is

E. coli with heavy or  15N DNA is allowed to replicate in medium having  14NH4Cl, 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  14N and the remaining half has hybrid DNA with one light and one heavy strands. Hence the percentage of bacteria with  15N is 50%.