please explain the messelson and stahl's experiment
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.