Editing DNA (section)

=== Quadruplex structures ===
{{further|G-quadruplex}}
[[File:Parallel telomere quadruple.png|thumb|right|DNA quadruplex formed by [[telomere]] repeats. The looped conformation of the DNA backbone is very different from the typical DNA helix. The green spheres in the center represent potassium ions.<ref>{{Cite web|title=Structure and packing of human telomeric DNA|url=http://ndbserver.rutgers.edu/service/ndb/atlas/summary?searchTarget=UD0017|access-date=2023-05-18|website=ndbserver.rutgers.edu}}</ref>]]

At the ends of the linear chromosomes are specialized regions of DNA called [[telomere]]s. The main function of these regions is to allow the cell to replicate chromosome ends using the enzyme [[telomerase]], as the enzymes that normally replicate DNA cannot copy the extreme 3′ ends of chromosomes.<ref name=Greider>{{cite journal | vauthors = Greider CW, Blackburn EH | title = Identification of a specific telomere terminal transferase activity in Tetrahymena extracts | journal = Cell | volume = 43 | issue = 2 Pt 1 | pages = 405–13 | date = December 1985 | pmid = 3907856 | doi = 10.1016/0092-8674(85)90170-9 | doi-access = free }}</ref> These specialized chromosome caps also help protect the DNA ends, and stop the [[DNA repair]] systems in the cell from treating them as damage to be corrected.<ref name=Nugent>{{cite journal | vauthors = Nugent CI, Lundblad V | title = The telomerase reverse transcriptase: components and regulation | journal = Genes & Development | volume = 12 | issue = 8 | pages = 1073–85 | date = April 1998 | pmid = 9553037 | doi = 10.1101/gad.12.8.1073 | doi-access = free }}</ref> In [[List of distinct cell types in the adult human body|human cells]], telomeres are usually lengths of single-stranded DNA containing several thousand repeats of a simple TTAGGG sequence.<ref>{{cite journal | vauthors = Wright WE, Tesmer VM, Huffman KE, Levene SD, Shay JW | title = Normal human chromosomes have long G-rich telomeric overhangs at one end | journal = Genes & Development | volume = 11 | issue = 21 | pages = 2801–09 | date = November 1997 | pmid = 9353250 | pmc = 316649 | doi = 10.1101/gad.11.21.2801 }}</ref>

These guanine-rich sequences may stabilize chromosome ends by forming structures of stacked sets of four-base units, rather than the usual base pairs found in other DNA molecules. Here, four guanine bases, known as a [[guanine tetrad]], form a flat plate. These flat four-base units then stack on top of each other to form a stable [[G-quadruplex]] structure.<ref name=Burge>{{cite journal | vauthors = Burge S, Parkinson GN, Hazel P, Todd AK, Neidle S | title = Quadruplex DNA: sequence, topology and structure | journal = Nucleic Acids Research | volume = 34 | issue = 19 | pages = 5402–15 | year = 2006 | pmid = 17012276 | pmc = 1636468 | doi = 10.1093/nar/gkl655 }}</ref> These structures are stabilized by hydrogen bonding between the edges of the bases and [[chelation]] of a metal ion in the centre of each four-base unit.<ref>{{cite journal | vauthors = Parkinson GN, Lee MP, Neidle S | title = Crystal structure of parallel quadruplexes from human telomeric DNA | journal = Nature | volume = 417 | issue = 6891 | pages = 876–80 | date = June 2002 | pmid = 12050675 | doi = 10.1038/nature755 | bibcode = 2002Natur.417..876P | s2cid = 4422211 }}</ref> Other structures can also be formed, with the central set of four bases coming from either a single strand folded around the bases, or several different parallel strands, each contributing one base to the central structure.

In addition to these stacked structures, telomeres also form large loop structures called telomere loops, or T-loops. Here, the single-stranded DNA curls around in a long circle stabilized by telomere-binding proteins.<ref>{{cite journal | vauthors = Griffith JD, Comeau L, Rosenfield S, Stansel RM, Bianchi A, Moss H, de Lange T | s2cid = 721901 | title = Mammalian telomeres end in a large duplex loop | journal = Cell | volume = 97 | issue = 4 | pages = 503–14 | date = May 1999 | pmid = 10338214 | doi = 10.1016/S0092-8674(00)80760-6 | citeseerx = 10.1.1.335.2649 }}</ref> At the very end of the T-loop, the single-stranded telomere DNA is held onto a region of double-stranded DNA by the telomere strand disrupting the double-helical DNA and base pairing to one of the two strands. This [[Triple-stranded DNA|triple-stranded]] structure is called a displacement loop or [[D-loop]].<ref name=Burge />