Double telomeric signals on single chromatids revealed by FISH and PRINS

FISH probes for all human telomeres and specific telomeric probes that hybr idize to unique sequences on individual chromosomes have been used to chara cterize the telomeric hybridization pattern of human peripheral blood lymph ocytes and bone-marrow cells in interphase and metaphase chromosomes. We ha ve identified the existence of double hybridization signals on chromatids b oth with the (TTAGGG)(n) telomere repeat arrays and on non chromosome-speci fic subtelomeric regions as well as on chromosome-specific sequences locate d several kilobases from the end of chromosomes. Preliminary results using cosmid or YAC probes that hybridize to regions rich in GC sequences also re vealed double fluorescent spots on a single chromatid. Double spots were de tected by PRINS on terminal and interstitial telomeric sequences in avian c ells, The significance of this phenomenon is discussed based on some models of chromatid and DNA organization such as uninemy, looped chromatid organi zation and quartet DNA structures. The occurence of double spots should be taken into consideration for the clinical cytogenetic diagnosis of duplicat ions,