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 ${}^{15}N$.
In another experiment, some bacteria from the medium with ${}^{15}N$ were transferred into a medium of ${}^{14}N$. The bacteria were allowed to divide three times. The DNA of some of these bacteria was extracted and analysed.
What is the composition of this DNA?
1 )
2 )
4 )
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