Question

Escherichia coli with completely radioactive DNA was allowed to replicate in nonradioactive medium for two generations. Percentage of bacteria with radioactive DNA is

• A. 100%
• B. 50%
• C. 25%
• D. 12.5%

Answer 1

Answer: (B)

The correct option is B 50%
DNA replication is semi-conservative which means that when a new double-stranded DNA molecule is formed one strand will be from the original template molecule and another strand will be newly synthesised.

When Escherichia coli with completely radioactive DNA is allowed to replicate in nonradioactive medium, with each generation one strand will remain as such radioactive while new one synthesised will be nonradioactive.

After the first generation, both the bacteria formed will have 50% radioactive and 50% radioactive DNA. In the second generation, these strands with mixed composition separate and again replicate in the same manner. The non-radioactive strand replicates and synthesises new radioactive strands. While radioactive strands replicate to form new bacterial cells with the mixed composition of DNA. Now, among 4 bacterial cells, 2 have mixed composition of DNA ( both radioactive and nonradioactive strands ) while 2 cells have purely nonradioactive DNA.

So, the correct option is '50%'.