A culture of bacteria had all its DNA labelled with the heavy isotope of nitrogen (${}^{15}N$). A sample was taken and spun in a centrifuge.
The diagram shows the position of the DNA band at Z in the centrifuge tube.
The culture was then allowed to reproduce using nucleotides containing the normal isotope of nitrogen (${}^{14}N$). Samples were taken and spun in a centrifuge after one generation and again after two generations.
In which pattern would the DNA be found after the first and after the second generations?
1 )
A
2 )
B
C
4 )
D
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