Abstract
Telomeres are DNA-protein complexes that cap and protect the ends of linear chromosomes. In almost all species, telomeric DNA has a G/C strand bias, and the short tandem repeats of the G-rich strand have the capacity to form into secondary structures in vitro, such as four-stranded G-quadruplexes. This has long prompted speculation that G-quadruplexes play a positive role in telomere biology, resulting in selection for G-rich tandem telomere repeats during evolution. There is some evidence that G-quadruplexes at telomeres may play a protective capping role, at least in yeast, and that they may positively affect telomere maintenance by either the enzyme telomerase or by recombination-based mechanisms. On the other hand, G-quadruplex formation in telomeric DNA, as elsewhere in the genome, can form an impediment to DNA replication and a source of genome instability. This review summarizes recent evidence for the in vivo existence of G-quadruplexes at telomeres, with a focus on human telomeres, and highlights some of the many unanswered questions regarding the location, form, and functions of these structures.
Highlights
Telomeres are the DNA-protein complexes that cap the ends of linear eukaryotic chromosomes.In many species, telomeric DNA consists of tandem copies of a short guanine-rich repeat, containing a run of two to four consecutive guanines [1]
The two-guanine repeats of B. mori and the nematode Ascaris lumbricoides (GGTTA and GGCTTA, respectively) fold into G-quadruplexes that are in equilibrium with hairpin-duplexes [24], and those of C. elegans appear to only form hairpins [139,140]
This has led to the proposition that is the ability of G-rich telomere repeats to form stable secondary structures that confers an evolutionary advantage, regardless of whether those structures are G-quadruplexes or something else [139]
Summary
Telomeres are the DNA-protein complexes that cap the ends of linear eukaryotic chromosomes. The only exceptions were two-guanine repeats from the yeasts Schizosaccharomyces pombe and Candida guillermondii, but these organisms have irregular telomere repeat sequences, and other S. pombe repeat permutations containing 3–4 guanines do form G-quadruplexes [28]. It is more than 30 years since telomeric sequences were shown to form into secondary structures in vitro, yet many questions regarding the biological implications of this observation remain. This short review will highlight some of the many outstanding questions and areas for further research, relating to the existence and functions of telomeric G-quadruplexes in human cells
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