If E.Coli was allowed to grow for 80 minutes then what would be the proportions of light and hybrid densities DNA molecules?


If we take the generation time of E.coli to be 20 minutes then in 80 minutes the bacteria would divide 4 times making the count 24 =16. Initially the bacteria is grown in a  medium containing N15 isotope. Then it is transferred to the medium containing  N14 isotope. Semiconservative replication of DNA takes place and after first division both the DNA would have one strand as N15   and the other as N14 ( hybrid DNA) . In the second generation one half contain light DNA while the other half contain hybrid DNA (2 hybrid, 2 light). In the third generation, 25% would be the hybrid DNA and 75% would be light DNA (2 hybrid, 6 light). In the fourth generation out of 16 copies of DNA two would be hybrid while 14 would be light making the proportion of light DNA to be 12.50% of the total DNA while light DNA makes up the other 87.5 %.

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