Semi-conservative DNA replication

• What does semi-conservative replication mean?

  Each new DNA molecule has one original strand
• What was the purpose of the Meselson and Stahl Experiment?

  • To demonstrate semi-conservative replication of DNA
  • Used heavy nitrogen (¹⁵N) and light nitrogen (¹⁴N)
• What happened to bacteria in the Meselson and Stahl Experiment after being transferred to lighter nitrogen?

  They replicated their DNA with lighter nitrogen
• What was observed after one round of replication in the experiment?

  A single band at intermediate density was observed
• What did the single band at intermediate density indicate?

  Each DNA molecule had one strand from ¹⁵N
• What appeared after the second round of replication?

  Two bands appeared: one intermediate, one light
• What do the two bands after the second round of replication represent?

  ¹⁵N/¹⁴N hybrid DNA and pure ¹⁴N DNA
• What model of DNA replication was confirmed by the Meselson and Stahl Experiment?

  The semi-conservative model of DNA replication
• What models of DNA replication were disproved by the experiment?

  • Conservative model: DNA stays fully intact
  • Dispersive model: Random mixing of old and new DNA
• What is semi-conservative replication?

  DNA replication method conserving one original strand
• How does semi-conservative replication occur?

  Original DNA splits, creating new complementary strands
• What happens to the original DNA molecule during replication?

  It splits into two strands for new strands
• What are the steps of semi-conservative replication?

  1. Unwinding the DNA by DNA helicase
  2. Priming the DNA with RNA primers
  3. Building new strands with DNA polymerase
  4. Completion by removing RNA primers and filling gaps
• What enzyme unwinds the DNA double helix?

  DNA helicase
• What is the role of primase in DNA replication?

  It adds RNA primers to initiate strand synthesis
• What does DNA polymerase do during replication?

  Adds new nucleotides to form new strands
• How does DNA polymerase add nucleotides?

  It adds complementary nucleotides to the template strand
• What is the difference between the leading and lagging strands?

  The leading strand is continuous; lagging is fragmented
• What are Okazaki fragments?

  Short sections of DNA on the lagging strand
• What enzyme joins Okazaki fragments together?

  DNA ligase
• What happens after DNA polymerase finishes adding nucleotides?

  RNA primers are removed and gaps filled
• Why is it called "semi-conservative" replication?

  Half of the original DNA is conserved in each molecule
• What are the key points to remember about semi-conservative replication?

  • DNA helicase unwinds the double helix
  • DNA polymerase adds complementary bases
  • Leading strand is continuous; lagging strand is fragmented
  • Each new DNA molecule has one old and one new strand
• How can you visualize semi-conservative replication?

  Using half of original string for new pieces
• What is the final result of semi-conservative replication?

  Two identical DNA molecules with one old strand
• What does each new DNA molecule consist of?

  One old strand and one new strand
• What is the summary of semi-conservative replication?

  • Original DNA strands separate
  • Each serves as a template for new strands
  • Results in two DNA molecules
  • Each contains one old strand and one new strand
• What isotopes of nitrogen were used in the experiment?

  Heavy and light isotopes of nitrogen
• What technique was used to separate the isotopes in the experiment?

  Ultracentrifugation
• How are the bands in the centrifuge tube formed?

  By separating heavy and light DNA strands
• What happens to the DNA strands in terms of isotopes for each generation?

  Heavy and light strands are produced
• How are the relative amounts of DNA in each band explained?

  By the presence of heavy and light isotopes
• What are the characteristics of conservative replication?

  • Always some heavy DNA present
  • Increasing mass of light DNA
  • No intermediate DNA
  • Original heavy DNA is not split
• What is the implication of having some heavy DNA present in conservative replication?

  It indicates original DNA remains intact
• What are the characteristics of dispersive replication?

  • All DNA would be intermediate in mass
  • Getting lighter between G0 and G3
  • Original heavy DNA splits among new molecules
• How does the molecular mass of DNA change from G0 to G3 in dispersive replication?

  It gets lighter
• What does it mean that all DNA would be intermediate in molecular mass in dispersive replication?

  It indicates mixing of heavy and light DNA
• Why is there no intermediate DNA in conservative replication?

  Because the original heavy DNA is not split
• What is DNA considered to be?

  The molecule of life
• What is the structure of DNA identified as?

  A double helix