Question

QUESTION 27 The Meselson and Stahl experiment was used to compare the conservative, semiconservative and dispersive models of DNA replication. They cultured E. coli cells for several generations in a medium containing heavy nitrogen, 15 N. They then transferred the bacteria to a medium containing a lighter nitrogen isotope, 14 N. Samples were taken after the first round of DNA replication; another sample was taken after the second round of replication. They extracted DNA and centrifuged each sample. Explain (in your own words) how the results from the first and second rounds were used to determine the correct model and rule out the other two models of DNA replication.

Answer 1

the DNA structure which is thicker and bold- N15

Ist replication be 1 second replication f

the DNA structure which is lighter and thinner- N14.

This is the structure of DNA after 2 round of replication.
OD 115/N45 First replication N15/N14 OD-OD second replication NIS/N14 IN 14/N14 Figure' Density gradicut centrifugation 201
this is the result after density gradient centrifugation after each replication step.
Hf DNA suplication is consumative then after centrifugation the result will be- N15/N15 N15/N14 (not present) EN 14/N16 In this type of replication, no intermediate gradient (NIS/N14) would have formed.
If the DNA replication is conservative then this would have been the result.

But in the actual result of centrifugation after 2 round of replication intermediate gradient of N15/N14 also was present. So it can be concluded that dDNA replication is not conservative​​​​​​If the DNA replication is dispersive then this would have been the result but in actual result there were two prominent gradient observed after two round of replication,so it can be concluded that DNA replication is not dispersive.

If the DNA replication is dispersive then the centrifugation result would have been -