Telomeres are the specialized protein-DNA complexes that cap linear eukaryotic chromosomes and are essential for the stable maintenance of chromosomes. The finding, about 20 years ago, that telomeric DNA consists of repeated sequence motifs opened the way for identifying both the specific proteins associated with telomeric DNA and the replication mechanism unique to telomeres. Telomeric DNA consists of species-specific short sequence motifs that typically contain clusters of three or four G-residues (e. g. TTGGGG in ciliates and TTAGGG in vertebrates). These sequence motifs are repeated in tandem, forming long double-stranded regions of telomeric DNA (reviewed in 1). The sequence and organization of telomeric DNA arises from the de novo addition of telomeric repeats by the telomerase, a reverse transcriptase containing an RNA template complementary to the G-rich strand. Since the telomerase is switched off in human somatic cells but is reactivated in cancer cells, telomere length has been implicated in both ageing and cancer (reviewed in 2).
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