Fine-tuning the chromosome ends: the last base of human telomeres

Telomeres protect chromosomes from degradation and loss of vital sequence, block end-end fusion, and allow the cell to distinguish between broken ends and chromosome ends. Mammalian telomeres end in single-stranded (TTAGGG)-rich 3'-overhangs that are tucked back into the preceding double stranded region to form a T-loop. The end structure of mammalian telomeres has just started to be elucidated and through this extra views we highlight one aspect of that structure. We have recently identified the terminal nucleotides of both the C-rich and G-rich telomere strands in human cells and showed that approximately 80% of the C-rich strands terminate precisely in ATC-5', while the last base of the G-strand is less precise. This finding has important implications for the processing events that act on the telomere ends post-replication. While the mechanism behind this phenotype is yet to be unraveled, we discuss potential models that could explain the last base specificity.