Question

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

Answer 1

Hint: DNA replication is the process in which a new strand of DNA is formed from an existing DNA strand. E. coli takes 20 minutes to replicate. The proportion of the hybrid DNA gets halved as every replication cycle occurs.

Complete answer: E. coli stands for Escherichia coli. The replication time for the E. coli bacteria is 20 minutes. It means that in twenty minutes, a new generation of the E. coli bacteria is produced from the old one. So, if the E. coli is allowed to grow for 80 minutes, then we can observe four generations of the bacteria because the total time taken to replicate divided by the replication time of one bacterium, i.e., 80 divided by 20, is 4. Now, in the first generation of the bacteria, all the DNA will be hybrid because the heavy isotope will be incorporated in all the newly synthesized strands of the DNA making all the strands hybrid. In the second generation of the bacteria, half of the total DNA will be hybrid because old strands will act as the template strands for the new DNA strands. Due to this reason, the heavy isotope will be incorporated in half newly synthesized strands of the DNA making 50 % of strands hybrid. In the third generation of the bacteria, half of the total hybrid DNA from the second generation will be hybrid. This is because old strands will act as the template strand for the new DNA strands. Due to this reason, the heavy isotope will be incorporated in half of the newly synthesized strands of the DNA from the hybrid DNA of the second generation, making 25 % of strands hybrid. In the fourth generation of the bacteria, half of the total hybrid DNA from the third generation will be hybrid because the old strand will act as the template strand for the new DNA strands. Due to this reason, the heavy isotope will be incorporated in half of the newly synthesized strands of the DNA from the hybrid DNA of the third generation, making 12.5 % strands hybrid. Hence, after 80 minutes, 87.5 % of the strand will light density DNA molecules and 12.5 % of the strand will be heavy density DNA molecules.

Note: The above experiment was performed by Matthew Meselson and Franklin Stahl in 1958 to prove the semiconservative nature of DNA replication. They used the heavy isotope of Nitrogen-15. They grew the E. coli bacteria in the ammonium chloride culture having Nitrogen-15 isotope. They observed that the DNA after the first generation was completely hybrid, and after every replication, the amount halved. Thus, they were able to prove the semiconservative mode of DNA replication.