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. After one generation time, what does the DNA look like?

a. All of the DNA is made of 2 "light" strands.
b. All of the DNA is made of 2 "heavy" strands.
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. Half of the DNA is made of 2 "light" strands and half of the DNA is made of 2 "heavy" strands.

Answer 1

After one generation of E. coli growing in 14N, following growth in 15N, each DNA molecule consists of one heavy strand from the original DNA and one light strand, newly synthesized in the presence of 14N, supporting the semi-conservative model of DNA replication.

Step-by-step explanation:

The experiment conducted by Meselson and Stahl involved growing E. coli in a medium containing the heavy isotope of nitrogen (15N) before transferring them to a medium with the lighter isotope (14N). After one generation of growth in 14N, the DNA would have undergone one round of replication. According to the semi-conservative model of DNA replication supported by their findings, each DNA molecule consists of one old (heavy) strand and one new (light) strand, because during replication, each of the original strands serves as a template for a new complementary strand. Therefore, after one generation in 14N, all the DNA molecules will consist of one heavy strand and one light strand.