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. Author manuscript; available in PMC: 2013 Oct 22.
Published in final edited form as: Nat Rev Mol Cell Biol. 2013 Jan 9;14(2):69–82. doi: 10.1038/nrm3505

Figure 1.

Figure 1

Chromosome end-protection versus telomerase recruitment and action. Deprotected, protein-free telomeric DNA (G/C strand duplex at top) could be a substrate for illicit DNA end-joining and cell cycle checkpoint activation events at the telomere. By specifically binding telomeric DNA, telomeric proteins protect chromosome ends from such deleterious processes. DNA sequences are lost at the ends of chromosomes due to incomplete replication by DNA polymerases. Telomerase is recruited to telomeres via interaction with specific telomeric proteins to extend chromosome ends and thereby counter DNA lost from incomplete replication. In the absence of telomerase recruitment or action, telomeric DNA shortens, ultimately leading to cell senescence. Furthermore, shorter telomeric DNA results in a loss of bound telomeric proteins, which could result in deprotected chromosome ends.