Editing Non-coding DNA (section)

===Highly repetitive DNA===

Highly repetitive DNA consists of short stretches of DNA that are repeated many times in [[Tandem repeat|tandem]] (one after the other). The repeat segments are usually between 2 bp and 10 bp but longer ones are known. Highly repetitive DNA is rare in prokaryotes but common in eukaryotes, especially those with large genomes. It is sometimes called [[satellite DNA]].

Most of the highly repetitive DNA is found in centromeres and telomeres (see above) and most of it is functional although some might be redundant. The other significant fraction resides in short tandem repeats (STRs; also called [[microsatellite]]s) consisting  of short stretches of a simple repeat such as ATC. There are about 350,000 STRs in the human genome and they are scattered throughout the genome with an average length of about 25 repeats.<ref>{{ cite journal | vauthors = Gymrek M, Willems T, Guilmatre A, Zeng H, Markus B, Georgiev S, Daly MJ, Price AL, Pritchard JK, Sharp AJ, Erlich Y | date = 2016 | title = Abundant contribution of short tandem repeats to gene expression variation in humans | journal = Nature Genetics | volume = 48 | issue = 1 | pages = 22–29 | doi = 10.1038/ng.3461| pmid = 26642241 | pmc = 4909355 }}</ref><ref>{{ cite journal | vauthors = Kronenberg ZN, Fiddes IT, Gordon D, Murali S, Cantsilieris S, Meyerson OS, Underwood JG, Nelson BJ, Chaisson MJ, Dougherty ML | date = 2018 | title=  High-resolution comparative analysis of great ape genomes | journal = Science | volume = 360 | issue = 6393 | pages = 1085 | doi = 10.1126/science.aar6343| pmid = 29880660 | pmc = 6178954 }}</ref>

Variations in the number of STR repeats can cause genetic diseases when they lie within a gene but most of these regions appear to be non-functional junk DNA where the number of repeats can vary considerably from individual to individual. This is why these length differences are used extensively in [[DNA profiling|DNA fingerprinting]].