Question

You grow E. coli for several generations in media containing 15N. The cells are then transferred to media containing 14N and grown for two additional generations. What proportion of dsDNA isolated from these cells will be heavy, light or intermediate in density?

A. Heavy = 100%; Light = 0%; Intermediate = 0%
B. Heavy = 0%; Light = 100%; Intermediate = 0%
C. Heavy = 50%; Light = 0%; Intermediate = 50%
D. Heavy = 33%; Light = 33%; Intermediate = 33%
E. Heavy = 0%; Light = 50%; Intermediate = 50%

Answer 1

When E. coli cells are grown in media containing 15N and then transferred to media containing 14N, the proportion of dsDNA isolated from these cells will be heavy, light, and intermediate in density.

Step-by-step explanation:

When E. coli cells are grown in media containing 15N and then transferred to media containing 14N and allowed to grow for two generations, the proportion of dsDNA isolated from these cells will be heavy, light, and intermediate in density. After the first generation in 14N media, the dsDNA will sediment halfway between the 15N and 14N levels, indicating that it contains fifty percent 14N. After two generations in 14N media, there will be two bands of DNA - one at the intermediate position between 15N and 14N, representing DNA with a mixture of 15N and 14N, and the other corresponding to fully 14N DNA. Therefore, the correct answer is E. Heavy = 0%; Light = 50%; Intermediate = 50%.