Two batches of bacteria were cultured in different nitrogen-containing growth media. For one batch, the medium contained the ‘heavy’ isotope of nitrogen, $^{15}\text{N}$, and the bacteria were kept there until every DNA molecule became labelled. The other batch was cultured in a medium containing the ‘light’ isotope of nitrogen, $^{14}\text{N}$, until all the DNA was labelled. DNA was taken from each batch of bacteria and then centrifuged. The diagram indicates where this DNA appeared in the centrifuge tubes. Bacteria whose DNA was labelled with $^{15}\text{N}$ were moved into a medium containing $^{14}\text{N}$ and allowed to divide once. DNA was then extracted from the resulting generation of bacteria and centrifuged. Which tube shows where the DNA from this new generation of bacteria is found?
Biology 9700 · AS & A Level · Replication and division of nuclei and cells