Telomere dynamics and end processing in mammalian cells

by Sfeir, Agnel J.

Mammalian telomeres end in single-stranded, G-rich 3’-overhangs resulting from both the “end-replication problem” (the inability of DNA polymerase to replicate the very end of the telomeres) and post replication processing. Telomeric G-rich overhangs are precisely defined in ciliates; the length and the terminal nucleotides are fixed. Human telomeres have very long overhangs that are heterogeneous in size (35-600nt), indicating that their processing must differ in some respects from model organisms. We developed telomere-end ligation protocols that allowed us to identify the terminal nucleotides of both the C-rich and G-rich telomere strands. Up to approximately 80 % of the C-rich strands terminate in CCAATC-5’, suggesting that following replication a nuclease with high specificity or constrained action acts on the C-strand. In contrast, the G-terminal nucleotide was less precise than Tetrahymena and Euplotes but still had a bias that changed as a function of telomerase expression.