Question

In an experiment, E.coli with ${}^{15}$N DNA were taken. They were transferred to ${}^{14}$N medium to grow. What percentage of DNA will have ${}^{15}$N${}^{15}$N, ${}^{15}$N${}^{14}$N and ${}^{14}$N${}^{14}$N after 60 minutes?

• A. 0%, 50%, 50%
• B. 50%, 0%, 50%
• C. 0%, 25%, 75%
• D. 75%, 25%, 0%

Answer 1

The correct option is C 0%, 25%, 75%

DNA being semi-conservative in nature while replicating one strand is the parental strand and the complementary strand synthesised is the newly synthesised strand.

The generation time of E.coli is 20 minutes. As the bacteria is present at medium ${}^{15}$N, it will get completely incorporated in the DNA. When they are transferred in a medium containing ${}^{14}$N they will multiply and incorporate nitrogen in DNA.

So, at 20 mins : The bacteria will have two hybrid DNA synthesised from heavy DNA.
At 40 mins : There will be two hybrid DNA and two normal DNA.
At 60 mins : There will be two hybrid DNA and six normal DNA.
The red-coloured strands signify that they contain ${}^{15}$N while the blue-coloured strands signify that they contain ${}^{14}$N.

After 3 generations or 60 mins there will be no heavy DNA .
Number of DNA molecules with ${}^{15}$N = 0
⇒ Percentage of DNA with ${}^{15}$N${}^{15}$N = 0%

Number of DNA molecules with ${}^{15}$N in one strand and ${}^{14}$N
⇒ Percentage of DNA with ${}^{15}$N${}^{14}$N = (2/8) x 100% = 25%

Number of DNA molecules with ${}^{14}$N in both the strands = 6
⇒ Percentage of DNA with ${}^{14}$N${}^{14}$N = (6/8) x 100% = 75%

Therefore, starting with ${}^{15}$N${}^{15}$N DNA in E.coli, after 60 minutes, the percentage of DNA with ${}^{15}$N${}^{15}$N, ${}^{15}$N${}^{14}$N and ${}^{14}$N${}^{14}$N will be 0%, 25% and 75% respectively.