DNA Replication

• What is the name of the process of DNA replication?

  Semi-conservative
• Process of DNA Replication

  • Enzyme DNA helicase breaks hydrogen bonds and unwinds the double helix
  • Each strand acts as a template (parental strands)
  • Free nucleotides are attracted to their complementary bases
  • DNA polymerase joins adjacent nucleotides together forming phosphodiester bonds and the sugar phosphate backbone
  • 2 identical molecules of DNA are created
  • They both have a parent strand and a new strand
• How is the phosphodiester bond in DNA formed?

  A condensation reaction between the deoxyribose and phosphate groups
• What isotope of nitrogen was used in Meselson and Stahl's experiment?

  Heavy (15N) nitrogen isotope
• Why is nitrogen important in the context of DNA?

  DNA contains nitrogen in its bases
• How did bacteria use nitrogen from the broth during replication?

  To make new DNA nucleotides
• What was the result of the bacteria growing in the 15N broth?

  DNA containing only heavy (15N) nitrogen
• What method was used to analyze the DNA after extraction from the 15N culture?

  Centrifugation
• What did the centrifugation reveal about the DNA containing heavy nitrogen?

  It settled near the bottom of the tube
• What happened to the bacteria after being removed from the 15N broth?

  They were added to a broth with 14N nitrogen
• What was the purpose of allowing one round of DNA replication in the 14N broth?

  To observe the nitrogen composition of DNA
• What would occur if conservative DNA replication had happened?

  Original DNA would settle at the bottom
• What would indicate semi-conservative replication in the centrifuge results?

  DNA settling in the middle of the tube
• What did Meselson and Stahl confirm about bacterial DNA replication?

  It underwent semi-conservative replication
• What did the second round of centrifugation show about the DNA molecules?

  They contained a mixture of 15N and 14N
• How does the ratio of 15N to 14N change with more rounds of replication?

  It increases from 1:1 to 3:1 to 7:1
• What is the ratio of 15N to 14N after the first round of replication?

  1:1
• What is the ratio of 15N to 14N after the second round of replication?

  3:1
• What is the ratio of 15N to 14N after the third round of replication?

  7:1