Explain the function of counterstain in a FISH procedure and how the most appropriate counterstain ...

Although bound probes emit fluorescent signal, it can be relatively weak as compared with the background. A
good counterstain provides enough contrast to enhance visualization of the probe. The common counterstains,
propidium iodide (PI, reddish orange fluorescence) and diamidino-2-phenylindole (DAPI, pale blue fluorescence),
intercalate in DNA, providing a good background color for a probe's fluorescence. The counterstain should be
chosen based on how much contrast it would provide to a probe. For example, if a probe is labeled with a green
fluorochrome, PI provides a better background than DAPI.