Question

What will be the ratio of ${}^{15}N{/}^{15}N{:}^{15}N{/}^{14}N{:}^{14}N{/}^{14}N$ containing $DNAs$, if Meselson and Stahl experiment is continued for four generations in $E.coil$?

Answer 1

Messelson and Stahl in 1953 performed the experiment to find the semiconservative nature of DNA. They have taken Escherichia coli and grown into the nutrient medium. First, they have added heavy nitrogen N${}^1$${}^5$ in the medium for the growth of bacteria and then transferred it to light nitrogen N${}^1$${}^4$ and allowed to grow. They were grown for several generations in both the mediums. During the stages of DNA duplication they have taken the strain of bacteria and make it undergo centrifugation, the heavier ones settle down while lighter ones at the top. There were four generations that were observed as described in the figure.

• In generation 0 only N${}^1$${}^5$ is present as it was before the transportation of bacteria to light nitrogen.
  
• Generation 1 has intermediates only (N${}^1$${}^5$ N${}^1$${}^4$), It shows that due to the interaction of both the isotopes a hybrid is formed.
• Generation 2 produces two strands one intermediate and other lighter band (N${}^1$${}^4$) in equal proportion, so the ratio is 1:1.
• Generation 3 forms two bands one thin hybrid or an intermediate band and other thick light bands, so the ratio is 1:3.
• In generation 4 two bands are produced, a thinner intermediate band and thicker lighter band in the ratio 1:7

so, the total ration of N${}^1$${}^5$ / N${}^5$ : N${}^1$${}^5$ / N${}^1$${}^4$ : N${}^1$${}^4$ / N${}^1$${}^4$ is 0 : 1 : 7