Editing Non-coding DNA (section)

===Centromeres===
{{Main|Centromere}}
[[File:Human karyotype with bands and sub-bands.png|thumb|Schematic [[karyotype|karyogram]] of a human, showing an overview of the [[human genome]] on [[G banding]], wherein non-coding DNA is present at the centromeres (shown as narrow segment of each chromosome), and also occurs to a greater extent in darker ([[GC-content|GC poor]]) regions.<ref name=Romiguier2017>{{cite journal | vauthors = Romiguier J, Roux C | title = Analytical Biases Associated with GC-Content in Molecular Evolution | journal = Frontiers in Genetics | volume = 8 | issue =  | pages = 16 | year = 2017 | pmid = 28261263 | pmc = 5309256 | doi = 10.3389/fgene.2017.00016 | doi-access = free }} </ref>]]

Centromeres are the sites where spindle fibers attach to newly replicated chromosomes in order to segregate them into daughter cells when the cell divides. Each eukaryotic chromosome has a single functional centromere that is seen as a constricted region in a condensed metaphase chromosome. Centromeric DNA consists of a number of repetitive DNA sequences that often take up a significant fraction of the genome because each centromere can be millions of base pairs in length. In humans, for example, the sequences of all 24 centromeres have been determined<ref>{{ cite journal | vauthors = Altemose N, Logsdon GA, Bzikadze AV, Sidhwani P, Langley SA, Caldas GV, et al. | title = Complete genomic and epigenetic maps of human centromeres | journal = Science | volume = 376 | pages = 56 | date = 2021 | issue = 6588 | doi = 10.1126/science.abl4178| pmid = 35357911 | pmc = 9233505 | s2cid = 247853627 }}</ref> and they account for about 6% of the genome. However, it is unlikely that all of this noncoding DNA is essential since there is considerable variation in the total amount of centromeric DNA in different individuals.<ref>{{cite journal | vauthors = Miga KH | title = Centromeric satellite DNAs: hidden sequence variation in the human population | journal = Genes | volume = 10 | pages = 353 | date = 2019 | issue = 5 | doi = 10.3390/genes10050352| pmid = 31072070 | pmc = 6562703 | doi-access = free }}</ref> Centromeres are another example of functional noncoding DNA sequences that have been known for almost half a century and it is likely that they are more abundant than coding DNA.