Question

Escherichia coli cells are grown for many generations in a medium in which the only available nitrogen is the heavy isotope 15N. They are then transferred to a medium containing 14N as the only source of nitrogen. (a) What distribution of 15N and 14N would be expected in the DNA molecules of cells that had grown for one generation in the 14N-containing medium assuming that DNA replication was (i) conservative, (ii) semiconservative, or (iii) dispersive? (b) What distribution would be expected after two generations of growth in the 14N-containing medium assuming (i) conservative, (ii) semiconservative, or (iii) dispersive replication?

          Escherichia coli cells are grown for many generations in a medium in which the only available nitrogen is the heavy isotope 15N. They are then transferred to a medium containing 14N as the only source of nitrogen.

(a) What distribution of 15N and 14N would be expected in the DNA molecules of cells that had grown for one generation in the 14N-containing medium assuming that DNA replication was (i) conservative, (ii) semiconservative, or (iii) dispersive?

(b) What distribution would be expected after two generations of growth in the 14N-containing medium assuming (i) conservative, (ii) semiconservative, or (iii) dispersive replication?

Added by Kenneth B.

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