Question
E. coli cells grown on 15N
medium are transferred to 14N
medium and allowed to grow for two more generations (two rounds of DNA
replication). DNA
extracted from these cells is centrifuged. What density distribution of DNA
would you expect in this experiment?
one low-density and one intermediate-density band
one high-density and one low-density band
one intermediate-density band
one high-density and one intermediate-density band
medium are transferred to 14N
medium and allowed to grow for two more generations (two rounds of DNA
replication). DNA
extracted from these cells is centrifuged. What density distribution of DNA
would you expect in this experiment?
one low-density and one intermediate-density band
one high-density and one low-density band
one intermediate-density band
one high-density and one intermediate-density band
Answers
GPT-4o mini
In this experiment, E. coli cells are initially grown in a medium containing the heavy isotope \(^{15}\)N (nitrogen). When these cells are transferred to a medium containing the lighter isotope \(^{14}\)N, the following DNA replication process can be analyzed:
1. **First Generation**: When the cells grow in \(^{14}\)N medium after being in \(^{15}\)N, all the newly synthesized DNA will incorporate \(^{14}\)N. The original \(^{15}\)N DNA will still be present, but the new strands will be lighter. Therefore, after one generation, you would have hybrid DNA molecules that contain one \(^{15}\)N strand and one \(^{14}\)N strand. The density of this hybrid DNA will be intermediate between the heavy and light densities.
2. **Second Generation**: In the second generation, the hybrid DNA produced in the first generation serves as the template for replication. The two strands of the hybrid DNA will separate, and each strand will be replicated, using the \(^{14}\)N medium. This results in:
- The hybrid DNA (one \(^{15}\)N and one \(^{14}\)N) will produce one \(^{15}\)N DNA molecule (that is, one heavy strand and one light strand, resulting in hybrid DNA) and two new \(^{14}\)N DNA molecules (that is, two light strands).
After the second generation, you would expect to see:
- One population of molecules with all \(^{14}\)N (light density).
- Another population of molecules that are hybrid \(^{15}\)N/ \(^{14}\)N (intermediate density).
Putting this all together, when you centrifuge the extracted DNA, you will observe:
- **One low-density band** (representing DNA made purely with \(^{14}\)N after the second generation).
- **One intermediate-density band** (representing the hybrid DNA from the first generation).
Therefore, the correct answer is **one low-density and one intermediate-density band**.
1. **First Generation**: When the cells grow in \(^{14}\)N medium after being in \(^{15}\)N, all the newly synthesized DNA will incorporate \(^{14}\)N. The original \(^{15}\)N DNA will still be present, but the new strands will be lighter. Therefore, after one generation, you would have hybrid DNA molecules that contain one \(^{15}\)N strand and one \(^{14}\)N strand. The density of this hybrid DNA will be intermediate between the heavy and light densities.
2. **Second Generation**: In the second generation, the hybrid DNA produced in the first generation serves as the template for replication. The two strands of the hybrid DNA will separate, and each strand will be replicated, using the \(^{14}\)N medium. This results in:
- The hybrid DNA (one \(^{15}\)N and one \(^{14}\)N) will produce one \(^{15}\)N DNA molecule (that is, one heavy strand and one light strand, resulting in hybrid DNA) and two new \(^{14}\)N DNA molecules (that is, two light strands).
After the second generation, you would expect to see:
- One population of molecules with all \(^{14}\)N (light density).
- Another population of molecules that are hybrid \(^{15}\)N/ \(^{14}\)N (intermediate density).
Putting this all together, when you centrifuge the extracted DNA, you will observe:
- **One low-density band** (representing DNA made purely with \(^{14}\)N after the second generation).
- **One intermediate-density band** (representing the hybrid DNA from the first generation).
Therefore, the correct answer is **one low-density and one intermediate-density band**.