Question

Ten E. coli cells with 15N- dsDNA are incubated in a medium containing 14N nucleotides. After 60 minutes, how many E. coli cells will have DNA totally free from 15N?

• A. 20 cells
• B. 40 cells
• C. 60 cells
• D. 80 cells

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
DNA replication is semi-conservative, as proved by Meselson and Stahl. In each round of replication, the two parent strands separate, and each serves as a template for a new complementary strand.
Key Formula or Approach:
The generation time of \(E.\ coli\) is 20 minutes.
In 60 minutes, there will be \(60 / 20 = 3\) generations.
Total cells after \(n\) generations = \(\text{Initial cells} \times 2^{n}\).
Step 2: Detailed Explanation:
Initial: 10 cells (all strands are \(^{15}N\), let's call them heavy-heavy, HH).
- Gen 1 (20 min): 10 HH cells divide to form 20 cells. Each cell gets one \(^{15}N\) parent strand and one new \(^{14}N\) strand. (20 Hybrid cells, HL). Cells free from \(^{15}N = 0\).
- Gen 2 (40 min): 20 HL cells divide to form 40 cells. The 20 parent \(^{15}N\) strands result in 20 HL cells. The 20 parent \(^{14}N\) strands result in 20 LL cells (\(^{14}N\)-\(^{14}N\)). Cells free from \(^{15}N = 20\).
- Gen 3 (60 min): 40 cells divide to form 80 cells. The 20 \(^{15}N\) strands from the HL cells will always stay in 20 hybrid HL cells. All other strands are \(^{14}N\).
Total cells = 80.
Hybrid cells (containing \(^{15}N\)) = 20.
Cells totally free from \(^{15}N = 80 - 20 = 60\).
Step 3: Final Answer:
After 60 minutes, 60 cells will have only \(^{14}N\) DNA.