Meselon and Stahl

• what three theories explain DNA replication?

  • fragmentary (all DNA strands made of mixture of original parent/new nucleotides)
  • semi-conservative (each DNA molecule contains one original parent strand and one new strand)
  • conservative (DNA mol has 2 original parent DNA strands and other molecule has 2 new strands)
• what bacteria did they use?

  E.coli, placed in growth medium containing heavy isotope of nitrogen (15) which was used to make nucleotides which made DNA.

  • DNA was extracted and centrifuged in a density gradient. All DNA was 'heavy' so band of DNA forms at bottom of the tube.

  if grown with light nitrogen (14) and centrifuged all DNA will be light so band of DNA forms at top of test tube
• what happened after first replication of DNA?

  • single medium band of DNA formed, conservative theory rejected.
• what happened after second replication of DNA?

  allowed to replicate twice instead of once like first time:

  • two bands produced, one medium one light so semi-conservative accepted
  • NOTE: after DNA replication number of DNA molecules doubles !