89.Ten E.coli cells with 15N-dsDNA are incubated in medium containing 14N Nucleotide. After 60 minutes, how many E.coli cells will have DNA totally free from 15N? A. 40 cells B. 60 cells C. 80 cells D. 20 cells
Option B DNA being semi-conservative in nature while replicating one strand is the parental strand and the complementary strand synthesised is the newly synthesised strand. The generation time of E.coli is 20 minutes. As the bacteria is present at medium 15N, it will get completely incorporated in the DNA. When they are transferred in a medium containing 14N they will multiply and incorporate nitrogen in DNA. So, at 20 mins : The bacteria will have two hybrid DNA synthesised from heavy DNA. At 40 mins : There will be two hybrid DNA and two normal DNA. At 60 mins : There will be two hybrid DNA and six normal DNA.
