Assuming that the collision factor remains the same, by how much must an enzyme lower the activation energy of the reaction in order to achieve a 5×105-fold increase in the reaction rate?

By the same amount

Explanation:

The activation energy and the rate of a reaction rate is closely related to each other. The higher is the activation energy, the lower will the rate of chemical reaction.

Thus, if rate is to be increased by 5×105-fold then the activation energy must be lowered by the same factor as these two entities are inversely proportional to each other

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.