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 conducted an experiment to demonstrate the semi-conservative nature of DNA replication. They used two cultures of E. coli, one growing in medium with N${}^{14}$ isotope while other growing in medium with N${}^{15}$ isotopes. They were allowed to grow separately for several generations. When these were subjected to density gradient centrifugation the cells growing on N${}^{14}$ medium contained both the strand of N${}^{14}$ while the cells of N${}^{15}$ medium contained both the strands of N${}^{15}$, which settled down during centrifugation due to higher density.
In the second experiment, the N${}^{15}$ culture was transferred to N${}^{14}$ media. The cells were subjected to density gradient centrifugation after one generation. It was found that the daughter cells produced bands of DNA which was located at intermediate position between N${}^{14}$ and N${}^{15}$ which lead to the conclusion that the resultant daughter DNA contained two strands, one of N${}^{14}$ and the other is of N${}^{15}$, which act as an evidence for the semi-conservative nature of DNA replication.