Two sets of bacteria were grown using different types of nitrogen-containing growth media.
One set was grown in a medium containing the ‘heavy’ isotope of nitrogen, ${}^{15}N$, until all the DNA was labelled. The other set were grown in a medium containing the ‘light’ isotope of nitrogen, ${}^{14}N$, until all the DNA was labelled.
The DNA from each set of bacteria was extracted and centrifuged. The diagram shows the position in the centrifuge tubes of this DNA.
Bacteria with ${}^{15}N$ labelled DNA were transferred to a medium containing ${}^{14}N$ and allowed to reproduce once. The DNA of the new generation of bacteria was extracted and centrifuged.
Which tube shows the position of DNA from this new generation of bacteria?
1 )
A
2 )
B
3 )
C
D
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