What would be the proportions of light and hybrid density DNA molecules, respectively, if Meselson and Stahl's

Question

What would be the proportions of light and hybrid density DNA molecules, respectively, if Meselson and Stahl's experiment was continued for 60 minutes?

Accepted Answer

According to the text, *E. coli* divides every 20 minutes. 
- At 0 min: DNA is 100% heavy .
- At 20 min (Generation I): DNA is 100% hybrid  because the N strands separate and act as templates for new N strands .
- At 40 min (Generation II): The hybrid DNA replicates in N medium. 50% of the progeny are hybrid  and 50% are light  .
- At 60 min (Generation III): The cells divide again. The hybrid molecules produce equal numbers of hybrid and light DNA. The light molecules produce only light DNA. 
   - Starting with 4 molecules from 40 min (2 Hybrid, 2 Light).
   - They double to 8 molecules: The 2 Hybrid yield 2 Hybrid + 2 Light. The 2 Light yield 4 Light.
   - Total: 2 Hybrid + 6 Light.
   - Ratio: 6 Light to 2 Hybrid, or 75% Light and 25% Hybrid.

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