Bacteria were cultured in a medium containing $^{15}\text{N}$. After several generations, every DNA molecule contained $^{15}\text{N}$. Some of these bacteria were then moved to a medium containing the common isotope of nitrogen, $^{14}\text{N}$. The bacteria were allowed to divide once. DNA from some of these bacteria was removed and analysed. This DNA consisted entirely of hybrid DNA with equal amounts of $^{14}\text{N}$ and $^{15}\text{N}$ ($^{14}\text{N}/^{15}\text{N}$). In a different experiment, some bacteria from the medium with $^{15}\text{N}$ were transferred into a medium of $^{14}\text{N}$. The bacteria were allowed to divide three times. DNA from some of these bacteria was extracted and analysed. What would be the composition of this DNA?
Biology 9700 · AS & A Level · Replication and division of nuclei and cells