The bacterium E. coli can divide in every 20 min . In other words, after every 20 minutes, the number of cells becomes doubled. When is incubated in medium containing nucleotide, the first generation has intermediates only ( ). Second generation (after 20 minutes) will produce two hybrid strands, i.e., in equal proportion, so the ratio is . Third generation (after 20 minutes) will produce 2 hybrid strands and 2 light strands containing only. Fourth generation (after 20 minutes) will produce 2 hybrid strands and 6 light strands. So, after 60 minutes there will be 6 light strands totally free from .
Total number of E.coli cells cells cells.

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