Question

(a) What did Meselson and Stahl observe when
(i) They cultured E.coli in a medium containing ${}^{15}{\text{NH}}_{4}Cl$ for a few generations and centrifuged the content.
(ii) They transferred one such bacterium to the normal medium of $N{H}_{4}Cl$ and cultured for two generations.

Answer 1

Meselson and Stahl tested the hypothesis of DNA replication. They cultured bacteria in a 15N medium. 15N is a heavy isotope of nitrogen so the DNA synthesized is of heavy density. After two replication cycles, two bands of DNA were seen, one of intermediate density and one of light density this proved that DNA strands replicate in a semi-conservative way.

First they cultured bacteria in N${}^{1}5$ containing medium and allowed the DNA strand to replicate then after centrifugation they observed that all the DNA content centrifuged at the bottom. This shows that they all were heavy containing N${}^{1}5$.

Then they transfer some bacteria from N${}^{1}5$ medium to the N${}^{1}4$ containing medium then again they were allowed to replicate and after 20 minutes they were centrifuged. The DNA content this time was in the middle of the test tube. This observation states that the DNA content has both N${}^{1}5$ and N${}^{1}4$ as it’s components.

So the above observation states that the replication is the semi-conservative.