Bacteria were grown in a medium containing ${}^{15}N$. After several generations, all of the DNA contained ${}^{15}N$. Some of these bacteria were transferred to a medium containing the common isotope of nitrogen, ${}^{14}N$. The bacteria were allowed to divide once. The DNA of some of these bacteria was extracted and analysed. This DNA was all hybrid DNA containing equal amounts of ${}^{14}N$ and ${}^{15}N$.

The remaining bacteria were left in the medium with ${}^{14}N$and allowed to divide one more time. The DNA of some of these bacteria was extracted and analysed.

What is the composition of this DNA?