Ten E.coli cells with 15N - ds DNA are incubated in medium containing 14N nucleotide. After 60 minutes, how ma

Question

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

Accepted Answer

According to the text, E. coli divides every 20 minutes. In 60 minutes, the cells undergo 3 generations (60/20 = 3).

1.  Total Cells: Starting with 10 cells, the number increases to  cells.
2.  DNA Composition: The initial DNA is fully heavy , providing 20 heavy strands (10 cells  2 strands). In semi-conservative replication, these original heavy strands are conserved and serve as templates. 
3.  Distribution: In the presence of N, newly synthesized strands are light. The 20 original N strands will be part of 20 hybrid  DNA molecules, found in 20 cells. 
4.  Free from N: The remaining cells will contain only light DNA . 
   Calculation: Total cells (80) - Cells with N (20) = 60 cells totally free from N.

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