Since the two chemicals are analogous, some spots of the genetic code that call for Thymidine will instead receive Brd U.
The special thing about Brd U is that it can be stained with an antibody using immunohistochemical methods, whereas Tritiated Thymidine is only detected by radiographic techniques.
When a scientist injects Brd U into the bloodstream of a test animal, the chemical becomes available to all cells – most importantly, those which are proliferating.
As the cells build their new genetic strand, they pull either Thymidine nucleotides or Brd U analogues from their environment.
These microorganisms will then incorporate Brd U into their DNA as they grow.