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?

Options

• Never, because replication is semiconservative.

• The first generation.

• The second generation.

• Only the third generation.

Answer 1

The second generation.

Explanation:

This is based on the semiconservative model of replication. In the first generation after the switch, every DNA molecule is a hybrid, containing one original heavy strand and one new light strand. When these hybrids replicate a second time in light nitrogen, the “light” strands from the first generation act as templates to create the very first light/light DNA molecules. Therefore, while the first generation only produces hybrids, the second generation is when the fully light DNA finally appears.