Question

Bacteria are grown in a medium containing 15NH4Cl for a number of generations so that all of the DNA is made of fully "heavy" DNA. The bacteria are moved to a new medium and grown in 14NH4Cl so that all new DNA will be "light". If replication were conservative, what would the DNA look like after two generation times.

a. All of the DNA is made of 2 "light" strands.
b. Half of the DNA is made of 2 "light" strands and half of the DNA is made of 1 "light" strand and 1 "heavy" strand.
c. All of the DNA is made of 1 "heavy" strand and 1 "light" strand.
d. Each strand is made of a mixture of "heavy" and "light" DNA.
e. 75% of the DNA is made of 2 "light" strands and 25% of the DNA is made of 2 "heavy" strands.

Answer 1

In conservative replication after two generations in 14N, 75% of the DNA would have 'light' strands while 25% would retain the original 'heavy' strands, so the correct answer is (e).

Step-by-step explanation:

The Meselson and Stahl experiment is a classic in biology, particularly in the study of DNA replication, and your question pertains to what the DNA would look like after two generations if replication were conservative. Under conservative replication, after two generations in 14N medium, you would expect to see 75% of the DNA made of two 'light' strands (from the second generation) and 25% made of two 'heavy' strands (the original DNA from before the first generation in the 14N medium). Therefore, the correct answer would be (e) 75% of the DNA is made of 2 'light' strands and 25% of the DNA is made of 2 'heavy' strands.