Question

Describe Meselson and Stahl's experiment that was carried in 1958 on E.coli. Write the conclusion they arrived at after the experiment.

Answer 1

Meselson and Stahl experiment :

Grown E,coli in ¹⁵NH₄Cl for many generations to get ¹⁵N incorporated into DNA, Then the cells are transferred into ¹⁴NH₄Cl , The extracted DNA are centrifuged in CsCl and measured to get their densities, DNA extracted from the culture after one generation (20 minutes), showed intermediate hybrid density, DNA extracted after two generations (40 minutes) showed light DNA and hybrid DNA. 

A correctly labelled diagrammatic representation in lieu of the above explanation of experiment to be considered.

Observation : Since, E.coll divides every 20 minutes, the DNA extracted after 20 minutes i.e., after first generation in the experiment had a hybrid density i.e., it was intermediate between N¹⁴ and N¹⁵ type of DNA. The DNA extracted after 40 minutes i.e., after second generation had equal amounts of hybrid and light densities. 

Conclusion : This implies that the newly synthesized DNA obtained one of its strands is old (N¹⁵ type) and the other strand is new (N¹⁴ type). Thus, replication was semi-conservative.