What is added in order to detect sister chromatid exchange (SCE)?

To detect sister chromatid exchange (SCE), BrdU (bromodeoxyuridine) is incorporated into the DNA of dividing cells during the S-phase of the cell cycle. BrdU mimics thymidine, a normal component of DNA, allowing it to replace thymidine in the newly synthesized DNA strands. When cells undergo replication, if a sister chromatid exchange occurs, the BrdU will be incorporated in a non-identical and alternating manner across the sister chromatids. This allows for the visualization of SCE under a fluorescent microscope, typically after the application of specific staining techniques that differentiate between BrdU-labeled and unlabeled DNA.

Colcemid is a mitotic inhibitor that disrupts the spindle fibers during cell division, preventing proper separation of chromosomes but does not facilitate the visualization of SCE directly. Bleomycin is a chemotherapeutic agent that induces breaks in DNA but is not used specifically to identify SCE. Giemsa stain is used for chromosomal staining but does not have the specificity required to visualize exchanges between sister chromatids. Therefore, the use of BrdU is essential in SCE detection, making it the correct choice.