Bacteria were grown in a medium containing heavy isotope of nitrogen ($N^{15}$) for many generations and all their $DNA$ contained many heavy nitrogen only. A bacterium of this type was transferred to normal medium and allowed to duplicate. After two divisions of heavy $DNA$ is likely to be that

  • [AIPMT 1993]
  • A

    Only one daughter cell will have heavy $DNA$

  • B

    Two daughter cells have normal $DNA$ and other two have both normal and heavy $DNA$

  • C

    All daughter cells have heavy $DNA$

  • D

    Half daughter cells have heavy $DNA$ and other half have normal $DNA$

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