Question

Starting with ${}^{15}N{}^{15}N$ DNA in E.coli, 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%

The generation time of E.coli = 20 minutes.
Hence, the number of replication cycles after 60 minutes = 60/20 = 3
The red-colored strand signifies that it contains ${}^{15}N$ while the blue-colored strand signifies that it contains ${}^{14}N$. After three cycles of replication:
Total number of DNA molecules formed = 8
Number of DNA molecules with ${}^{15}N$ in both the strands = 0
⇒ Percentage of DNA with ${}^{15}N{}^{15}N=0\%$
Number of DNA molecules with ${}^{15}N$ in one strand and ${}^{14}N$ in the other strand = 2
⇒ Percentage of DNA with ${}^{15}N{}^{14}N=\left(2/8\right)\times 100\%=25\%$
Number of DNA molecules with ${}^{14}N$ in both the strands = 6
⇒ Percentage of DNA with ${}^{14}N{}^{14}N=\left(6/8\right)\times 100\%=75\%$
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.