Bacteria were cultured in a medium containing $^{15}\text{N}$. After a number of generations, every DNA molecule contained $^{15}\text{N}$. Some of these bacteria were then moved to a medium containing the usual nitrogen isotope, $^{14}\text{N}$. The bacteria were left to divide once. DNA from some of the bacteria was taken out and analysed. This DNA consisted entirely of hybrid DNA, with equal quantities of $^{14}\text{N}$ and $^{15}\text{N}$. In a different experiment, some bacteria from the $^{15}\text{N}$ medium were transferred to 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 is the composition of this DNA?
Biology 9700 · AS & A Level · Replication and division of nuclei and cells