Meselson and Stahl

• watson and crick


  Watson and Crick discovered the structure of DNA in 1953 helped by Rosalind Franklin's research on x-ray diffraction.

  They hypothesised that DNA must replicate either conservatively or semi-conservatively
• Hypothesis - semi conservative


  each replicated DNA molecules contains one of the original DNA strand and one newly synthesised DNA strand
• hypothesis - conservative

  original DNA remains intact following DNA replication and the two newly synthesised strands of DNA joins together.
• experiment

  -DNA bases are nitrogenous

  -Nitrogen has two isotopes 14N and 15N. 14N is lighter and 15N is heavier

  -Bacteria take in the nitrogen isotopes to make new DNA nucleotides

  -DNA samples can be spun in a centrifuge to confirm density.

  15N DNA will settle lower down as it is more dense

  14N DNA will settle higher up as it is less dense

  Watson and Crick disproved conservative replication.