In the Meselson and Stahl experiment, E. coli was grown for many generations in a medium containing 15NH4Cl as the only nitrogen source and then transferred to a medium with normal 14NH4Cl. E. coli divides in 20 minutes. If the culture is allowed to grow for 80 minutes in the 14N medium, what would be the ratio of hybrid DNA molecules to light DNA molecules?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 1 : 7
DNA replicates semiconservatively: each new molecule keeps one old strand and makes one new strand. The two heavy (15N) strands of the starting molecule are never broken up. After the transfer, they sit in two separate molecules, each paired with a new light (14N) strand. So in every generation there are only two hybrid molecules; all the other molecules are light.
- Number of generations = 80 ÷ 20 = 4
- Number of DNA molecules from one molecule after 4 generations = 24 = 16
- Hybrid molecules (one 15N strand + one 14N strand) = 2
- Light molecules (both strands 14N) = 16 − 2 = 14
- Hybrid : light = 2 : 14 = 1 : 7
Option (1) 1 : 3 — Incorrect. This ratio is reached after 60 minutes (3 generations): 8 molecules, of which 2 are hybrid and 6 are light.
Option (2) 1 : 1 — Incorrect. This is the ratio after 40 minutes (2 generations): 4 molecules, 2 hybrid and 2 light.
Option (3) 1 : 15 — Incorrect. This would need 32 molecules, that is 5 generations or 100 minutes (2 hybrid : 30 light).
Option (4) 1 : 7 — Correct. At 80 minutes there are 16 molecules. Two still carry the original heavy strands, and 14 are fully light.
NCERT Extract:
The Experimental Proof – “… (iii) Thus, the DNA that was extracted from the culture one generation after the transfer from 15N to 14N medium [that is after 20 minutes; E. coli divides in 20 minutes] had a hybrid or intermediate density. DNA extracted from the culture after another generation [that is after 40 minutes, II generation] was composed of equal amounts of this hybrid DNA and of ‘light’ DNA.”