Semi conservative replication evidence

• Semi-conservative replication


  DNA replicates with one new strand and one original strand
• Meselson-Stahl experiment


  1. Grew bacteria in 15N medium

  2. Transferred to 14N medium

  3. Extracted DNA at intervals

  4. Analysed DNA by density-gradient centrifugation
• Immediately before changing the medium, DNA occupied a single band corresponding to 'heavy' DNA
• After one generation, DNA was of 'intermediate' density, being concentrated in a single band a little higher up the tube
• After two generations, there were two bands, with 50% of DNA 'intermediate' and 50% light
• The results and their interpretation are shown in Figure 20