FISH—Determination of Telomere Length (Q-FISH/CO-FISH)

Telomeres are specialized nucleoprotein structures at the ends of linear chromosomes. They consist of a shelterin protein complex and a long assembly of hexameric repeats oriented in 5' to 3' direction, ending as a single strand 3' overhang. By possessing at least two active domains in each subunit, shelterins directly inhibit homologous recombination (HR) and non-homologous end-joining. Chromosome orientation FISH method is used to identify the direction of a synthetized DNA chains, or particular DNA sequence. Telomere length can be determined in human and other vertebrate cells and should be done in metaphases. The method is particularly convenient for species that contain interstitial telomeric sites in their genome, as well as for species that have ultra-long and heterogeneous telomeres, such as mice. Activity of homologous replication attempting to extend telomeres will be seen as sister-chromatid exchanges; i.e. both chromatid ends will display the telomeric signal.