Question

A heavy DNA (${}^{15}N{-}^{15}N$) E.coli is allowed to replicate in (${}^{14}N$) medium for 60 minutes. What would be the proportion of light (${}^{14}N{-}^{14}N$) and hybrid density of DNA (${}^{14}N{-}^{15}N$) molecules?

Answer 1

As we can see from the diagram with every 20 minutes two strands of DNA get separated and a new DNA strand is synthesized using ${}^1$${}^4$N medium.

After 40 minutes, we have 2 hybrid density DNA and 2 light density DNA. After another 20 minutes i.e. 60 minutes, each of these four DNA will synthesise new DNA strand. First of all two strands of DNA will get separated resulting in 8 strands of DNA ( out of which 2 are going to be ${}^1$${}^5$N strand ). All newly synthesised DNA strands are going to be ${}^1$${}^4$N type.

Hence, after 60 minutes we have 2/8 DNA of hybrid density ( ${}^1$${}^5$N- ${}^1$${}^4$N ) and rest 6/8 DNA of light density ( ${}^1$${}^4$N - ${}^1$${}^4$N )