Biology 9700 · AS & A Level · Replication and division of nuclei and cells

Replication and division of nuclei and cells — practice question

Meselson and Stahl studied DNA in bacteria. They first grew bacteria in a culture containing only heavy nitrogen, $^{15}\text{N}$, until every bacterial DNA molecule was heavy. The bacteria were then transferred out of the heavy nitrogen culture and grown in a medium containing only light nitrogen, $^{14}\text{N}$. A sample of bacteria was taken from the first generation in the light nitrogen medium and their DNA was examined. Hybrid DNA consists of both heavy DNA and light DNA. Which row gives the percentage of heavy DNA strands and the percentage of hybrid DNA molecules in the first generation produced in the medium containing light nitrogen?

  • A25% heavy DNA strands, 50% hybrid DNA molecules
  • B50% heavy DNA strands, 50% hybrid DNA molecules
  • C50% heavy DNA strands, 100% hybrid DNA molecules
  • D75% heavy DNA strands, 100% hybrid DNA molecules

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