Interactive Figure

Interactive Figure 1

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Fig. 1. The end-protection problem. When a mammalian chromosome breaks (top), the exposed DNA ends can activate two signaling pathways (the ATM and ATR kinase pathways) that arrest the cell division cycle and can induce cell death. The broken chromosome is usually repaired by one of two different DNA repair pathways (NHEJ and HDR), allowing cells to continue their divisions with an intact genome. The presence of these DNA damage response pathways poses a problem for the ends of linear chromosomes (telomeres, bottom) because activation of DNA damage signaling or DNA repair at telomeres would be disastrous. Mammalian telomeres solve this end-protection problem through the use of a telomere-specific protein complex (shelterin) and an altered structure (the t-loop) that together ensure that all four pathways remain blocked.