The essential function of the telomere is to facilitate the complete replication of the chromosome terminus and to prevent the terminus from eliciting a DNA damage response that would cause genome instability. Telomere failure can occur from the loss of telomere capping proteins or the prolonged absence of telomerase. Because of its extraordinary tolerance to genome instability, Arabidopsis thaliana is a powerful model for telomere biology. Telomerase expression is highly regulated and in both plants and animals is confined to cells with long-term proliferation capacity. Unlike vertebrate telomeres, plant telomeres are asymmetric with one end of the chromosome terminating in a 30 single-stranded overhang and the other in a blunt end. The protein composition of plant telomeres reveals an evolutionary bridge with yeast and vertebrates; some factors are more like yeast, others more like vertebrates, and still others unique to plants. The identification and characterization of the CST complex in Arabidopsis paved the way for understanding the molecular basis of human stem cell disease. Gene duplication and diversification have had made a significant impact on the composition and regulation of Arabidopsis telomerase. The telomerase RNA component is evolving very rapidly in plants and is giving rise to novel regulatory mechanisms. De novo telomere formation by telomerase at internal double-strand breaks in the DNA causes loss of chromosomal DNA and must be strictly regulated to ensure faithful repair of DNA damage.
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