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?
A) one high-density and one low-density band
B) one intermediate-density band
C) one high-density and one intermediate-density band
D) one low-density and one intermediate-density band

Question

Quizplus · Accepted Answer

After the first round of DNA replication in the 14N medium, each DNA molecule consists of one strand with 15N (heavy) and one with 14N (light), resulting in intermediate-density DNA. After the second round of replication, there will be DNA molecules with both strands containing 14N (low-density) and those with one strand of 15N and one of 14N (intermediate-density), hence the two bands.

E Coli Cells Grown on 15N Medium Are Transferred to 14N