Why do you have to spit out your gum before the extraction of dna?

The bacteria on the gum is the reason why. In the process of making gum, there are so many things that touch it that could mess up the results of the extraction. Even if it was machines that touched it, the ingredients from gum had to be touched by humans to create it.

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.