Question

A culture of E. coli growing in a medium containing a heavy isotope of nitrogen (15N) as the sole source of nitrogen is switched to a medium containing the normal, lighter isotope of nitrogen (14N) for 2 generations. What is the molar ratio of the hybrid DNA (15N-14N) to light DNA (14N-14N) at this point? Report your answer as a decimal to the nearest hundredths. For example, if you felt that the answer was 5/6 (which it is not) you would enter 0.83 for the answer.

Answer 1

This experiment is an extension of the Meselson-Stahl experiment, which demon-strates that replication in

E. coli is semiconservative. After three generations the molar ratioof

15N–14N DNA to14N–14N DNA = 2/6

                                                            = 0.33