Editing Telomere (section)

=== Telomere ends and shelterin ===
[[File:Telosome.png|thumb|300x300px|Shelterin co-ordinates the T-loop formation of telomeres.]]
{{Main|Shelterin}}

At the very 3'-end of the telomere there is a 300 base pair overhang which can invade the double-stranded portion of the telomere forming a structure known as a T-loop. This loop is analogous to a knot, which stabilizes the telomere, and prevents the telomere ends from being recognized as breakpoints by the DNA repair machinery. Should non-homologous end joining occur at the telomeric ends, chromosomal fusion would result. The T-loop is maintained by several proteins, collectively referred to as the shelterin complex. In humans, the shelterin complex consists of six proteins identified as [[TERF1|TRF1]], [[TERF2|TRF2]], [[TINF2|TIN2]], [[POT1]], [[ACD (gene)|TPP1]], and [[TERF2IP|RAP1]].<ref name="pmid20569239">{{cite journal | vauthors = Martínez P, Blasco MA | title = Role of shelterin in cancer and aging | journal = Aging Cell | volume = 9 | issue = 5 | pages = 653–66 | date = October 2010 | pmid = 20569239 | doi = 10.1111/j.1474-9726.2010.00596.x | doi-access = free }}</ref> In many species, the sequence repeats are enriched in [[guanine]], e.g. TTAGGG in [[vertebrate]]s,<ref name=pmid2780561>{{cite journal | vauthors = Meyne J, Ratliff RL, Moyzis RK | title = Conservation of the human telomere sequence (TTAGGG)n among vertebrates | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 86 | issue = 18 | pages = 7049–53 | date = September 1989 | pmid = 2780561 | pmc = 297991 | doi = 10.1073/pnas.86.18.7049 | bibcode = 1989PNAS...86.7049M | doi-access = free }}</ref> which allows the formation of [[G-quadruplex]]es, a special conformation of DNA involving non-Watson-Crick base pairing. There are different subtypes depending on the involvement of single- or double-stranded DNA, among other things. There is evidence for the 3'-overhang in [[ciliate]]s (that possess telomere repeats similar to those found in [[vertebrate]]s) to form such G-quadruplexes that accommodate it, rather than a T-loop. G-quadruplexes present an obstacle for enzymes such as DNA-polymerases and are thus thought to be involved in the regulation of replication and transcription.<ref>{{cite journal | vauthors = Lipps HJ, Rhodes D | title = G-quadruplex structures: in vivo evidence and function | journal = Trends in Cell Biology | volume = 19 | issue = 8 | pages = 414–22 | date = August 2009 | pmid = 19589679 | doi = 10.1016/j.tcb.2009.05.002 }}</ref>