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

To solve this problem, we need to understand the principles of bacterial DNA replication using the classic Meselson-Stahl experiment framework. The E. coli cells initially contain DNA labeled with ¹⁵N, a heavy isotope of nitrogen. They are then incubated in a medium containing ¹⁴N, a lighter isotope.

1. Binary Fission Process: E. coli cells replicate their DNA through a semi-conservative mechanism during binary fission. Each round of cell division takes approximately 20 minutes.

2. Replicative Timeline: Given that we start with 10 E. coli cells and the incubation period lasts for 60 minutes:

• After 20 minutes, the cells go through one division creating 20 cells. Each of these cells has DNA where one strand is ¹⁵N (heavy) and one strand is ¹⁴N (light).
• After 40 minutes, another division occurs, resulting in 40 cells. Half of these cells (20) will contain fully ¹⁴N DNA (light strands only) since the ¹⁴N strand serves as a template for entirely new strands.
• After 60 minutes, a third division transpires, producing 80 cells in total. Half of these cells (40) still contain hybrid DNA, but the remaining 40 cells have completely ¹⁴N DNA.

3. Conclusion: After 60 minutes, 60 cells will contain DNA wholly free from ¹⁵N. This is because the subsequent division results in half of the new cells containing exclusively ¹⁴N strands.

Therefore, the correct answer is 60 cells.