Lecture Notes: Meselson-Stahl Experiment and DNA Replication

Overview

• Lecture continuation on DNA replication
• Focus on Meselson-Stahl experiment as evidence for semiconservative replication
• Background:
  • Watson and Crick discovered DNA structure in 1953
  • Hypotheses:
    • Semiconservative replication: Each new DNA conserves one original strand
    • Conservative replication: Original DNA remains intact and new DNA consists of entirely new strands

Key Concepts

DNA Structure

• DNA bases are nitrogenous (contain nitrogen)
• DNA nucleotide components:
  • Phosphate group
  • Deoxyribose pentose sugar
  • Nitrogenous bases: adenine, thymine, cytosine, guanine

Isotopes in DNA Experiment

• Isotopes of nitrogen used:
  • 14N: Lighter
  • 15N: Heavier
• Isotopes differ in number of neutrons, not protons or electrons

Bacterial Growth and Isotope Incorporation

• Bacteria need growth medium with nutrients and nitrogen source
• Bacteria incorporate nitrogen isotopes into DNA nucleotides
• 14N medium: DNA is lighter
• 15N medium: DNA is heavier

Meselson-Stahl Experiment

Background Information

• DNA density can be observed using centrifugation
• Experiment steps involve growing bacteria in different nitrogen isotope mediums

Experiment Steps

1. Initial Condition (Generation 0)

   • Bacteria grown in 15N medium
   • DNA contains two strands of 15N
   • Centrifuge shows DNA at bottom (high density)

2. First Round of Replication (Generation 1)

   • Bacteria moved to 14N medium
   • DNA strands split and replicate using 14N
   • Result: One strand 15N, one strand 14N
   • Centrifuge shows middle-band position (intermediate density)

3. Second Round of Replication (Generation 2)

   • Continued growth in 14N medium
   • Mix of 15N/14N strands and 14N/14N strands
   • Centrifuge: Two bands observed (middle and higher position)

4. Third Round of Replication (Generation 3)

   • Further growth in 14N medium
   • Proportions: 25% 15N/14N, 75% 14N/14N
   • Centrifuge shows two bands with different thickness

Conclusion

• Meselson-Stahl experiment proved semiconservative replication
• Disproved conservative replication hypothesis

Importance

• Proved Watson and Crick’s semiconservative hypothesis
• Demonstrated incorporation of isotopes into DNA bases
• Highlighted use of centrifugation to determine DNA density and composition

Resources

• Practice questions available on related topics
• Additional study materials and assessment booklets are recommended for deeper understanding