meselson and stahl dna replication

• Watson and Crick discovered the structure of DNA in 1953 with significant help from Rosalind Franklin's research on x-ray diffraction
• Semiconservative replication


  When DNA replicates, one of the original strands is conserved and combined with a newly synthesized strand
• Conservative replication


  The original DNA remains intact and the newly synthesized DNA combines together
• DNA bases are nitrogenous, meaning they contain nitrogen
• 14N


  Lighter isotope of nitrogen
• 15N


  Heavier isotope of nitrogen
• Bacteria incorporate the nitrogen isotopes from their growth medium into the DNA nucleotides
• DNA samples containing 15N isotopes will settle at the bottom of a centrifuge, while DNA samples containing 14N isotopes will settle higher up
• Meselson and Stahl's experiment

  1. Grow E. coli in 15N medium

  2. Transfer to 14N medium and allow one round of division

  3. Analyze DNA composition and centrifugation results

  4. Allow further rounds of division in 14N medium

  5. Analyze DNA composition and centrifugation results
• Conservative replication


  Would result in two distinct bands in centrifugation, one for 14N DNA and one for 15N DNA
• Semiconservative replication


  Results in a gradual shift of the DNA band in centrifugation, with a mixture of 14N and 15N DNA strands
• Meselson and Stahl's experiment disproved conservative replication and provided evidence for semiconservative replication
• There are many practice questions available on the topic of DNA replication and Meselson and Stahl's experiment at www.anki.com