DNA replication (experiment with N)

• What organism did Meselson and Stahl use as the source of DNA?

  1. E.coli bacteria
• Why are E.coli bacteria suitable for Meselson and Stahl's experiment?

  They can be easily grown in a flask of culture medium and replicate every 20 minutes
• How often do E.coli replicate their cells under optimal conditions?

  Every 20 minutes
• How were 'new' and 'original' nucleotides distinguished in the experiment?


  By the isotope of nitrogen in the nitrogenous bases
• Which nitrogen isotope is heavier, 15N or 14N?


  15N
• What method did Meselson and Stahl use to separate DNA molecules containing different nitrogen isotopes?


  Centrifugation
• What was the result of centrifuging the DNA sample containing only 15N?

  The band of DNA molecules was near the bottom of the tube
• Why did the DNA band containing 15N settle at the bottom of the tube?

  Because 15N is heavier, so it settles at the bottom
• What type of nucleotides were used in the new culture medium after transferring E.coli?

  14N (light) nucleotides
• What was observed after the E.coli replicated once in the new culture medium?

  The DNA was found at an intermediate position between 15N and 14N molecules
• What did the first generation of DNA molecules demonstrate about replication?

  Replication was not conservative; each new DNA molecule consisted of half new (14N) and half original (15N) nucleotides
• What are the key steps in Meselson and Stahl's experiment?

  • Grow E.coli in 15N culture medium
  • Centrifuge to observe DNA band at bottom
  • Transfer E.coli to 14N culture medium
  • Allow replication and centrifuge again
  • Observe DNA band at intermediate position
  • Conclude that replication is semi-conservative