**2.2 Fluorescence in situ hybridization (FISH)**

Fluorescence in situ hybridization (FISH) is a technique for determining complex DNA sequences as well as the number and structure of chromosomes. The method is focused on the use of fluorescent probes that can recognize specific DNA sequences. FISH is a technique for detecting genetic defects in embryos that is fast and sensitive. Targeting and denaturing DNA fixed in cells, nuclei, or metaphase chromosomes on the surface of the slide is the basis of the FISH analysis. Next, after its denaturation, a complementary single-stranded DNA sequence probe will precisely re-anneal double-stranded DNA (hybrid) molecules during the hybridization reaction. Probe DNA molecules are labeled enzymatically with modified nucleotides. They are DNA molecules designated hapten-labeled (indirect FISH) and fluorescent-labeled (direct FISH). An antifade solution containing 4′,6-diamidino-2 phenylindole is added to the slide after the removal of unbound single-stranded DNA and nonspecifically bound DNA from the slide by posthybridization washing. Using epifluorescence microscopes with specialized filters for detecting fluorochromes, FISH signals are observed. The signal is captured by a charge-coupled system camera, and the fluorescent signals are then quantified [7, 8].
