Meselson & Stahl experiment

• What was the setup of Meselson and Stahl's experiment?

  -they used e.coli bacteria as their source of DNA

  -'new' and 'original' nucleotides were distinguished by the isotope of nitrogen in the bases

  -N15 is heavier than N14, so DNA molecules containing these can be separated by mass in centrifugation
• What was the first step of the experiment?

  E.coli was grown in a culture medium with only N15 isotopes until all of the bases contained N15. When a sample was centrifuged, the band of DNA molecules was near the bottom of the tube, this is because N15 is heavier.
• What was the second step of the experiment?

  A sample of the E.coli was transferred to a new culture medium containing N14 nucleotides and allowed to replicate once.
• What was the third step of the experiment?

  A sample was withdrawn from the flask and centrifuged. The DNA was found at an intermediate position between N15 and N14 molecules. This first generation showed that replication was not conservative, but that each new DNA molecule produced by replication consisted of half new (N14) nucleotides and half original (N15) nucleotides.
• How was the dispersive model eliminated?

  The E.coli were allowed to replicate two more times, leading to generations 2 and 3. In generation 2, the bands of DNA appeared at the light and intermediate positions, which showed that replication was not dispersive. In the dispersive model, all the DNA molecules would have more N14 than N15, so the DNA bands would be between the intermediate and light positions.
• What were the results of the experiment?

  Gen 0: 100% heavy

  Gen 1: 100% intermediate

  Gen 2: 50% light, 50% intermediate

  Gen 3: 75% light, 25% intermediate

  Gen 4: 88% light, 12% intermediate