conservative vs semi conservative DNA replication

• How is dna replicated

  Through semi conservative replication
• what happens in semi conservative replication 


  • the DNA double helix separates into 2 polynucleotide strands
  • Each strand is replicated into a complementary new strand
  • At the end, one molecule of DNA has been copied into 2 molecules of DNA
• what is the key idea of semi conservative replication 


  each of the two copies contain one strand from the original dna molecule plus one new strand
• what happens during conservative replication 


  a dna double helix is formed containing two new strands- this dna molecule contains none of the original DNA
• what do all bases in DNA contain

  nitrogen which exists in two main forms/isotopes
• what are the two main isotopes of nitrogen 


  nitrogen-14 and nitrogen-15; n-14 is the most common isotope
• N-15 atoms are slightly heavier than atoms of N-14
• What method did scientists use to differentiate between semi-conservative and conservative replication?

  Centrifugation of DNA samples
• Why did scientists use bacteria with N-14 in their DNA?

  To observe the lighter isotope's behavior
• What isotope was predominantly found in the DNA of the initial bacteria sample?

  1. 14
• What was the purpose of spinning the DNA in a centrifuge?

  To separate DNA based on density
• What does the position of the DNA band in the centrifuge indicate?

  It indicates the weight of the DNA
• Where did the DNA band form when using N-14 DNA?

  Near the top of the tube
• What isotope did scientists use to culture the bacteria after the initial experiment?

  1. 15
• What happened to the N atoms in the DNA after culturing in N-15?

  They became predominantly N-15
• Where did the heavy N-15 DNA band form after centrifugation?

  Near the bottom of the tube
• What are the steps scientists took to determine the type of DNA replication?

  1. Sample bacteria with N-14 DNA
  2. Extract DNA and centrifuge
  3. Observe band position (light DNA near top)
  4. Culture bacteria in N-15 medium
  5. Extract heavy N-15 DNA and centrifuge
  6. Observe band position (heavy DNA near bottom)
• What was the result of the experiment

  the scientists found that when bacteria were grown on N-14, because their DNA was lighter, it formed a band near the top of the tube, but, when bacteria were grown on N-15, because their DNA was heavier, it formed a band near the bottom of the tube
• what was the last stage of the experiment 


  • the scientists took a sample of the bacteria which had been growing on N-15/heavy N and transferred these bacteria to N-14 and allowed them to replicate their DNA only once
  • The scientists then extracted the DNA and spun it in a centrifuge
  • What they find. Was that thus DNA produced a a band in between the two bands produced before
  • This told the scientists that thus DNA contained one strand with N-14 and one strand with N-15: this was important because it means the DNA replicated semi conservatively