A DNA molecule of E.coli is heavy i.e. fully labelled with ${}^{15}\mathrm{N}$ and is allowed to duplicate in a medium containing ${}^{14}\mathrm{N}$. In first generation, the two daughter DNA molecules-

From the image given below it is clear that the two daughter DNA molecules produced in the first generation have  ${}^{15}\mathrm{N}$ in one strand and ${}^{14}\mathrm{N}$ in another. Therefore, the two daughter DNA molecules have same density but do not resemble their parent DNA that has ${}^{15}\mathrm{N}$ in both the strands.