Question

If you grew bacteria in heavy nitrogen and then switched them to light nitrogen, how many generations after switching would you have some light/light DNA?

• A. Never, because replication is semiconservative
• B. The first generation
• C. The second generation
• D. Only the third generation

Answer 1

The correct option is C The second generation

Meselson and Stahl performed the experiment to study the replication of DNA. They have taken bacterial cells and cultured them in 15N heavy isotope of nitrogen first and then shifted to the 14N light isotope of nitrogen. DNA was extracted at different replication cycles. The results at different replication cycles obtained are as follows:

• At the first replication cycle, the DNA was found to be intermediate, this shows the semiconservative nature of DNA that one strand should be of 15N heavy density and other should be of 14N light density no intermediate densities should be present.
• At the second replication cycles, Two bands of DNA were observed one was having light density while other were having an intermediate density which also proves the semiconservative nature of DNA replication.

It was predicted that the density of the DNA will become lower after each replication cycle.

So, the correct option is 'The second generation'.