Question

Describe the Meselson-Stahl experiment. Explain the biological question that was asked, outline the methods used to answer it, and provide the conclusion drawn from the experiment

Answer 1

Meselson and Stahl experiment was done to prove semiconservative mode of DNA replication. Watson and Crick had proposed the semi conservative replication of DNA, wherein, each newly synthesised DNA molecule contains one strand from the original helix and the other strand is newly synthesised.

The biological question asked was how it is possible for the DNA to replicate itself, while the unwinding of the whole helix is complex and impossible.

Meselson and Stahl used density gradient centrifugation to seperate molecules in a solution to seperate DNA, there appeared bands of DNA in the tube, those bands were represented on a graph in form of peaks to show different densities of molecules, more peaks translated to different DNA densities. Based on this the original strand and newly synthesised strand were differentiated.

Meselson and Stahl experiment involved Ecoli grown on media containing high density N15 isotope. several generations were grown in this medium which resulted in double strands of DNA containing the heavy N15 isotope. Then these bacteria were subsequently grown on a media containing a lesser density N14 isotope. And several generations were grown in similar conditions.ansd periodic samples were drawn during each generation and the DNA was extracted and purified

And the results of the second generation indicated bands of a mixture of the N14 and N15 bands depositing in positions one above the other which is indicative of two types of bands in the DNA, thus proving the semiconservative mode of DNA replication and conservation.