Question

E. coli cells grown on 15N medium are transferred to 14N medium and allowed to grow for two more generations (two rounds of DNA replication). DNA extracted from these cells is centrifuged. What density distribution of DNA would you expect in this experiment? E. coli cells grown on 15N medium are transferred to 14N medium and allowed to grow for two more generations (two rounds of DNA replication). DNA extracted from these cells is centrifuged. What density distribution of DNA would you expect in this experiment?
one high-density and one intermediate-density band
one low-density and one intermediate-density band
one high-density and one low-density band
one intermediate-density band

Answer 1

Answer:

The correct answer is "one low-density and one intermediate-density band".

Explanation:

The 15N medium contains 15N, a heavy isotope of nitrogen. DNA synthesized by E. coli growing in 15N medium will be of high-density. If the E. coli culture is  transferred to a 14N medium, after two rounds of DNA replication, one low-density and one intermediate-density band will be observed in an experiment of centrifugation. This result supports the semiconservative model of DNA replication, where half of the DNA population will be a mixture of 15N-14N DNA corresponding to a intermediate-density band, while the other half is a 14N-14N light density DNA.