Editing Repeated sequence (DNA) (section)

=== Tandem repeats ===
[[Tandem repeat]]s are repeated sequences which are directly adjacent to each other in the genome.<ref>{{Cite web |title=Tandem Repeat |url=https://www.genome.gov/genetics-glossary/Tandem-Repeat |access-date=2022-09-30 |website=Genome.gov |language=en}}</ref> Tandem repeats may vary in the number of nucleotides comprising the repeated sequence, as well as the number of times the sequence repeats. When the repeating sequence is only 2–10 nucleotides long, the repeat is referred to as a short tandem repeat (STR) or [[microsatellite]].<ref>{{cite journal | vauthors = Sznajder ŁJ, Swanson MS | title = Short Tandem Repeat Expansions and RNA-Mediated Pathogenesis in Myotonic Dystrophy | journal = International Journal of Molecular Sciences | volume = 20 | issue = 13 | pages = 3365 | date = July 2019 | pmid = 31323950 | pmc = 6651174 | doi = 10.3390/ijms20133365 | doi-access = free }}</ref> When the repeating sequence is 10–60 nucleotides long, the repeat is referred to as a [[minisatellite]].<ref>{{Cite web |title=Minisatellite Repeats (MeSH Descriptor Data 2024) |id=D018598 |url=https://meshb.nlm.nih.gov/record/ui?name=Minisatellite |work=Medical Subject Headings |publisher=National Library of Medicine }}</ref> For minisatellites and microsatellites, the number of times the sequence repeats at a single locus can range from twice to hundreds of times.

Tandem repeats have a wide variety of biological functions in the genome. For example, minisatellites are often hotspots of meiotic [[homologous recombination]] in eukaryotic organisms.<ref name=Wahls98>{{cite journal | vauthors = Wahls WP | title = Meiotic recombination hotspots: shaping the genome and insights into hypervariable minisatellite DNA change | journal = Current Topics in Developmental Biology | volume = 37 | pages = 37–75 | date = 1998 | pmid = 9352183 | pmc = 3151733 | doi = 10.1016/s0070-2153(08)60171-4 | isbn = 9780121531379 }}</ref> Recombination is when two homologous chromosomes align, break, and rejoin to swap pieces. Recombination is important as a source of genetic diversity, as a mechanism for repairing damaged DNA, and a necessary step in the appropriate segregation of chromosomes in meiosis.<ref name=Wahls98 /> The presence of repeated sequence DNA makes it easier for areas of homology to align, thereby controlling when and where recombination occurs.

In addition to playing an important role in recombination, tandem repeats also play important structural roles in the genome. For example, [[telomere]]s are composed mainly of tandem TTAGGG repeats.<ref>{{cite journal | vauthors = Janssen A, Colmenares SU, Karpen GH | title = Heterochromatin: Guardian of the Genome | journal = Annual Review of Cell and Developmental Biology | volume = 34 | issue = 1 | pages = 265–288 | date = October 2018 | pmid = 30044650 | doi = 10.1146/annurev-cellbio-100617-062653 | s2cid = 51718804 | url = http://www.escholarship.org/uc/item/7294g81k | doi-access = free }}</ref> These repeats fold into highly organized [[G-quadruplex|G quadruplex]] structures which protect the ends of chromosomal DNA from degradation.<ref name=Qi05>{{cite journal | vauthors = Qi J, Shafer RH | title = Covalent ligation studies on the human telomere quadruplex | journal = Nucleic Acids Research | volume = 33 | issue = 10 | pages = 3185–92 | date = 2005-06-02 | pmid = 15933211 | pmc = 1142406 | doi = 10.1093/nar/gki632 }}</ref> Repetitive elements are enriched in the middle of chromosomes as well. [[Centromere]]s are the highly compact regions of chromosomes which join sister chromatids together and also allow the mitotic spindle to attach and separate sister chromatids during cell division.<ref>{{Cite web |title=Centromere |url=https://www.genome.gov/genetics-glossary/Centromere |access-date=2022-09-30 |website=Genome.gov |language=en}}</ref> Centromeres are composed of a 177 base pair tandem repeat named the α-satellite repeat.<ref name=Qi05 /> Pericentromeric heterochromatin, the DNA which surrounds the centromere and is important for structural maintenance, is composed of a mixture of different satellite subfamilies including the α-, β- and γ-satellites as well as HSATII, HSATIII, and sn5 repeats.<ref>{{cite journal | vauthors = Miga KH | title = Completing the human genome: the progress and challenge of satellite DNA assembly | journal = Chromosome Research | volume = 23 | issue = 3 | pages = 421–6 | date = September 2015 | pmid = 26363799 | doi = 10.1007/s10577-015-9488-2 | s2cid = 15229421 }}</ref>
[[File:Tandem_and_interspersed_repeat_schematic.png|thumb|350x350px|Tandem and interspersed repeat]]
Some repetitive sequences, such as those with structural roles discussed above, play roles necessary for proper biological functioning. Other tandem repeats have deleterious roles which drive diseases. Many other tandem repeats, however, have unknown or poorly understood functions.<ref>{{cite journal | vauthors = Padeken J, Zeller P, Gasser SM | title = Repeat DNA in genome organization and stability | journal = Current Opinion in Genetics & Development | volume = 31 | pages = 12–19 | date = April 2015 | pmid = 25917896 | doi = 10.1016/j.gde.2015.03.009 | series = Genome architecture and expression }}</ref>