Q5 / 79 · NEST (NISER) 2026 · Biology
In an experiment, bacteria were grown for $500$ generations in a medium containing $^{14}\text{N}$ (light isotope), then transferred to a medium with $^{15}\text{N}$ (heavy isotope) for one generation, and finally transferred back to the $^{14}\text{N}$ medium for one more generation.
Assuming all cells divide synchronously and replication is semi-conservative, the ratio of $^{14}\text{N}^{15}\text{N}$ to $^{14}\text{N}^{14}\text{N}$ double-stranded DNA at the end of the experiment is:
Assuming all cells divide synchronously and replication is semi-conservative, the ratio of $^{14}\text{N}^{15}\text{N}$ to $^{14}\text{N}^{14}\text{N}$ double-stranded DNA at the end of the experiment is:
- A$1:3$
- B$1:4$
- Correct answerC$1:1$
- D$1:2$
Solution
Initially, bacteria are grown in $^{14}\text{N}$ medium for $500$ generations, so all DNA molecules are $^{14}\text{N}^{14}\text{N}$ (light). Let the initial number of DNA molecules be $x$.
Upon transfer to $^{15}\text{N}$ medium for one generation, the DNA replicates semi-conservatively. The two $^{14}\text{N}$ strands of each molecule separate and act as templates to synthesize new $^{15}\text{N}$ strands. This results in $2x$ molecules of $^{14}\text{N}^{15}\text{N}$ (hybrid) DNA.
Upon transfer back to $^{14}\text{N}$ medium for one more generation, the $2x$ hybrid molecules separate into $2x$ strands of $^{14}\text{N}$ and $2x$ strands of $^{15}\text{N}$. Each strand synthesizes a new $^{14}\text{N}$ strand from the medium.
The $2x$ strands of $^{14}\text{N}$ form $2x$ molecules of $^{14}\text{N}^{14}\text{N}$ (light) DNA.
The $2x$ strands of $^{15}\text{N}$ form $2x$ molecules of $^{14}\text{N}^{15}\text{N}$ (hybrid) DNA.
The ratio of $^{14}\text{N}^{15}\text{N}$ to $^{14}\text{N}^{14}\text{N}$ double-stranded DNA at the end of the experiment is $2x : 2x = 1 : 1$.
Answer: $1:1$
Upon transfer to $^{15}\text{N}$ medium for one generation, the DNA replicates semi-conservatively. The two $^{14}\text{N}$ strands of each molecule separate and act as templates to synthesize new $^{15}\text{N}$ strands. This results in $2x$ molecules of $^{14}\text{N}^{15}\text{N}$ (hybrid) DNA.
Upon transfer back to $^{14}\text{N}$ medium for one more generation, the $2x$ hybrid molecules separate into $2x$ strands of $^{14}\text{N}$ and $2x$ strands of $^{15}\text{N}$. Each strand synthesizes a new $^{14}\text{N}$ strand from the medium.
The $2x$ strands of $^{14}\text{N}$ form $2x$ molecules of $^{14}\text{N}^{14}\text{N}$ (light) DNA.
The $2x$ strands of $^{15}\text{N}$ form $2x$ molecules of $^{14}\text{N}^{15}\text{N}$ (hybrid) DNA.
The ratio of $^{14}\text{N}^{15}\text{N}$ to $^{14}\text{N}^{14}\text{N}$ double-stranded DNA at the end of the experiment is $2x : 2x = 1 : 1$.
Answer: $1:1$
Hence, the correct answer is option: 'C'.