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 medium are transferred to 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?

Answer 1

The density distribution that one will expect from this experiment is ONE LOW DENSITY AND ONE INTERMEDIATE DENSITY BANDS.
The 14N  and the 15N media are refereed to as light and heavy medium respectively, they are composed of isotopes of nitrogen. When growing on the 15N medium, the E coli takes part of the nitrogen isotope and use it to make new biological molecules including DNA. The same thing happens when the E coli was put in the 14N media. Density gradient centrifugation is then used to measure the density of the DNA, thus, indirectly measuring the 15N and the 14N components of the DNA.