please explain the messelson and stahl's experiment

Dear Student,

Messelson and Stahl performed the experiment to prove that DNA replicates  semi-conservatively. The salient features of their experiment include

  • E.coli was grown in a medium containing heavy isotope 15N as the nitrogen source.
  • 15N was incorporated into newly synthesised DNA as well and the DNA became heavy DNA.
  • Heavy DNA molecule can be differentiated from normal DNA by density gradient centrifugation using cesium chloride as the gradient.
  •  Then, cells were again transferred into a medium with 14N as nitrogen source. Samples were taken from this media and their DNA was extracted.

  • E .coli divides every 20 minutes. Therefore, the DNA extracted after 20 minutes had a hybrid density.
  • DNA extracted after 40 minutes had equal amount of hybrid and light intensities.
  •  This implies that the newly synthesised DNA obtained one of its strands from the parent. Thus, replication is semi-conservative.
Hope this answers your question,


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