All the DNA in a bacterial culture had been labelled with the heavier nitrogen isotope, $^{15}\text{N}$. A sample was removed and placed in a centrifuge. The diagram shows where the DNA band is located at Z in the centrifuge tube. The culture was then permitted to replicate using nucleotides that contained the usual nitrogen isotope, $^{14}\text{N}$. Samples were taken and centrifuged after one generation and again after two generations. Which pattern would show the DNA after the first generation and after the second generation?
- AAfter first generation: half at X and half at Y; after second generation: quarter at X, quarter at Z and half at Y
- BAfter first generation: half at X and half at Z; after second generation: quarter at X, quarter at Z and half at Y
- CAfter first generation: all at Y; after second generation: half at X and half at Y
- DAfter first generation: all at Z; after second generation: half at Y and half at Z