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 the bacteria in a medium containing only heavy nitrogen, $^{15}\text{N}$, until every bacterial DNA molecule was heavy. The bacteria were then transferred from the heavy nitrogen medium to a medium containing only light nitrogen, $^{14}\text{N}$. A sample of bacteria was taken from the first generation and the DNA was examined. Hybrid DNA is made up of heavy DNA and light DNA. Which row gives the percentage of light DNA strands and the percentage of hybrid DNA molecules in the first generation produced in the medium containing light nitrogen?

  • Apercentage of light DNA strands: 25; percentage of hybrid DNA molecules: 50
  • Bpercentage of light DNA strands: 50; percentage of hybrid DNA molecules: 50
  • Cpercentage of light DNA strands: 50; percentage of hybrid DNA molecules: 100
  • Dpercentage of light DNA strands: 75; percentage of hybrid DNA molecules: 100

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