Question

You decide to repeat the Meselson-Stahl experiment, except this time you plan to grow the E. coli cells on light 14N medium for many generations and then transfer them to heavy 15N medium and allow them to grow for 2 additional generations (2 rounds of DNA replication). If the semiconservative model of DNA replication is correct, what is the expected distribution of DNA in the density gradient after two rounds of replication? A) One band of intermediate density and one band of heavy density. B) One band of intermediate density and one band of light density. C) One band of intermediate density. One band of heavy density. D) One band of heavy density and one band of light density.

Answer 1

A) One band of intermediate density and one band of heavy density.

Step-by-step explanation:

According to semi conservative model of replication the two DNA strands separate during replication such that the new DNA molecule has one parental strand and one new strand. In this experiment, originally E.Coli cells were grown in 14N medium so all the DNA strands had 14N. When the medium was changed to 15N, only 15N would be available to the cells for replication.

After first round of replication all DNA molecules would be of intermediate density as one strand would be old 14N strand and another would be new 15N strand. After second round of replication 15N strand would be formed from both 14N strand and 15N strand. So, one band will be formed of intermediate density (14N-15N) and another would be of heavier density (15N-15N).