Telomeric 3′-overhang length is associated with the size of telomeres

Telomeres are specialized DNA/protein complexes that cap eukaryotic chromosome ends as T-loop structures and maintain genomic integrity. Vertebrate telomeric DNA consists of tandem double-strand repeats which terminate in a 3′ single-strand G-rich overhang. The telomeric 3′-overhang is important for the formation of the T-loop. In mammalian mortal somatic cells, telomeres shorten with each successive division and contribute to the onset of replicative senescence. The exact molecular mechanism underlying replicative senescence remains unclear: whether telomere shortening is the only trigger or loss of telomeric 3′-overhang plays a causal role. To further address this issue, we investigated telomeric 3′-overhang and telomere changes during cell proliferation toward replicative senescence. We demonstrate here that telomeric 3′-overhang, similar to telomeres, exhibits progressive attrition with each cell division in primary sheep fibroblasts and that telomeric 3′-overhang size does not determine the rate of telomere shortening. Furthermore, the sizes of telomeric 3′-overhangs are associated with telomere lengths. Our results suggest that alteration of the 3′-overhang and the telomere during cellular proliferation are associated. Together they may contribute to maintain chromosomal stability and to regulate replicative senescence.