DNA Replication- Meselson and Stahl

• What were the names of the scientists who demonstrated DNA replication was semi-conservative?

  Matthew Meselson and Franklin Stahl
• What was the main question scientists had about DNA replication?

  Whether it was conservative or semi-conservative
• What isotopes of nitrogen were used in the experiment?

  A heavy isotope and a light isotope
• What was the first step in Meselson and Stahl's experiment?

  Bacteria were grown in heavy nitrogen broth
• Why did the bacteria use nitrogen from the broth?

  To make new DNA nucleotides
• What type of nitrogen did the bacteria have after some time?

  Only ¹⁵N
• What happened when the DNA was spun in a centrifuge?

  Heavy nitrogen settled near the bottom
• What was done after extracting DNA from the ¹⁵N culture?

  It was spun in a centrifuge
• What broth was used after the ¹⁵N DNA was extracted?

  A broth containing only ¹⁴N
• What would indicate conservative replication in the centrifuge results?

  Original template heavy, new strand light
• What would indicate semi-conservative replication in the centrifuge results?

  DNA molecules contain both isotopes
• What are the expected results after multiple rounds of DNA replication?

  • Generation 2: 1 band middle - 50% ¹⁴N, 50% ¹⁵N hybrid
  • Generation 3: Light band 75% ¹⁴N, middle band 25% hybrid
• What is the difference between conservative and semi-conservative replication?

  Conservative keeps original, semi-conservative mixes
• What elements do bacteria need to replicate DNA?

  Nitrogen, phosphorus, and sugars
• Why did scientists use bacteria for the experiment?

  They replicate quickly and are simple
• What is the role of the centrifuge in the experiment?

  To separate DNA based on density
• How can density of molecules be determined in the centrifuge?

  By observing where DNA settles in the tube
• Which nitrogen isotope is considered heavy?

  ¹⁵N
• Which nitrogen isotope is considered light?

  ¹⁴N
• Why did the scientists use nitrogen isotopes in their experiment?

  To distinguish between DNA strands
• What was observed after the first spin in the centrifuge?

  Heavy DNA settled at the bottom
• What would have been expected after the second cycle of replication if it was conservative?

  Two distinct bands: one heavy, one light
• Which test tube shows the correct pattern of DNA after two divisions?

  Tube 2 with one middle band
• Describe the results of generation 2 using a diagram.

  One middle band with 50% hybrid
• How would you illustrate the DNA bands for each generation?

  • Generation 1: Tube with heavy band
  • Generation 2: Tube with middle band (50% hybrid)
  • Generation 3: Tube with light band (75% ¹⁴N, 25% hybrid)
• What does a DNA nucleotide consist of?

  • A phosphate group
  • A sugar (deoxyribose)
  • A nitrogenous base (A, T, C, or G)