DNA molecule was proven to replicate via semi-conservative method where one of the two strand is a new one and other one is from the parent. When ${\mathrm{N}}^{15}$ labelled E. coli is grown in medium with ${\mathrm{N}}^{14}$ , after completion of first round of replication, the new duplicated DNA will have one strand of ${\mathrm{N}}^{15}$ labelled bases and the new strand having ${\mathrm{N}}^{14}$ labelled bases. Since, the new DNA is a hybrid its density will be lesser than the parent DNA. The DNA sequence will not differ from parent DNA. Also ${\mathrm{N}}^{15}$ is heavy but it is not radioactive. Thus neither the parent DNA nor daughter DNA is radioactive.