Question

In the Meselson and Stahl experiment, the starting Escherichia coli culture was grown in heavy (15N) medium; the DNA was 15N15N. After TWO generations of growth in light (14N) medium, the DNA of the E. coli cells contained
A) 25% 15N15N DNA, 50% 15N14N DNA, and 25% 14N14N DNA.
B) 25% 15N14N DNA and 75% 14N14N DNA.
C) 50% 15N14N DNA and 50% 14N14N DNA.
D) 50% 15N15N DNA and 50% 14N14N DNA.
E) 50% 15N15N DNA and 50% 15N14N DNA.

Answer 1

Answer:

C) 50% 15N14N DNA and 50% 14N14N DNA.

Explanation:

Meselson and Stahl proved that DNA replication follows a semi conservative model. The strands separate from each other and new strands are synthesized so that the new DNA molecule has one parental and one new strand.

Here, originally E.coli was grown in 15N medium. So all the strands had 15N. After first generation new strands were synthesized so each DNA molecule had one 15N and one 14N strand. After second generation, the 15N strand lead to synthesis of 14N strand and the 14N strands also lead to synthesis of 14N strands. So half of the DNA molecules had 15N14N composition and other half had 14N14N composition. Thus, after two generations of growth in 14N medium, DNA of E.coli cells contained 50% 15N14N DNA and 50% 14N14N DNA.