Bacteria were grown in a medium containing $^{15}N$. After several generations, all of the bacterial 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 percentage of hybrid DNA?