Question

If Meselson and Stahl's experiment is continued for 4 generations in E. coli, then the ratio of ${}^{15}N$/${}^{15}N$:${}^{15}N$/${}^{14}N$:${}^{14}N$/${}^{14}N$ in the end would be:

• A. 0:1:3
• B. 0:1:7
• C. 0:1:15
• D. 0:1:31

Answer 1

The correct option is B 0:1:7

Matthew Meselson and Franklin Stahl performed an experiment in order to demonstrate the semiconservative replication of DNA.

E. coli was grown in a medium containing ${}^{15}{\mathrm{NH}}_4\mathrm{Cl}$ for many generations. DNA was extracted from this sample of cells and density was recorded. The heavy isotope of nitrogen, ${}^{15}N$ was found in the newly synthesised strands of the DNA as well as in the parent strands of the DNA.

These E. coli cells were now transferred into a medium containing ${}^{14}{\mathrm{NH}}_4\mathrm{Cl}$ and were allowed to grow for four generations ( E. coli cells take 20 minutes to divide). Samples were collected from each of the four generations. DNA was extracted from each of the four samples and density was recorded using the method of centrifugation in a cesium chloride (CsCl) density gradient.

In the sample which was extracted from the medium containing the heavy isotope of nitrogen, ${}^{15}N$, both the strands (the newly synthesised strand as well as the parent strand) were found to have the heavy isotope. Hence, heavy density of DNA was obtained.

In the samples which were isolated from the medium containing the normal isotope of nitrogen,
${}^{14}N$,

• In the first generation (after 20 minutes):

The newly synthesised strands did not contain the heavy isotope of nitrogen, ${}^{15}N$. Hence, only the parent strands contained the heavy isotope. Therefore, an intermediate or hybrid density of the DNA was obtained.

• In the second generation (after 40 minutes):

The DNA density was obtained in the ratio 1:1, where, 50 percent of the DNA was of the hybrid nature (${}^{15}N$${}^{14}N$ type) having intermediate density and the remaining 50 percent of the DNA was of the type ${}^{14}N$${}^{14}N$ having light density.

• In the third generation (after 60 minutes):

The density of DNA was in the ratio 1:3, where 25 percent was of hybrid nature (${}^{15}N$${}^{14}N$) having intermediate density and the remaining 75 percent was of the type ${}^{14}N$${}^{14}N$ having light density.

• In the fourth generation (after 80 minutes):

The DNA density was in the ratio 1:7 where 12.5 percent was of the hybrid nature (${}^{15}N$${}^{14}N$) having intermediate density and the remaining 87.5 percent was of the type ${}^{14}N$${}^{14}N$ having light density.

This experiment was used by Matthew Meselson and Franklin Stahl to prove the semiconservative nature of DNA replication.

Hence, if Meselson and Stahl's experiment is continued for 4 generations in E. coli, then the ratio of ${}^{15}N$/${}^{15}N$:${}^{15}N$/${}^{14}N$:${}^{14}N$/${}^{14}N$ in the end would be: 0:1:7.