Editing DNA (section)

=== Genes and genomes ===
{{further|Cell nucleus|Chromatin|Chromosome|Gene|Noncoding DNA}}
Genomic DNA is tightly and orderly packed in the process called [[DNA condensation]], to fit the small available volumes of the cell. In eukaryotes, DNA is located in the [[cell nucleus]], with small amounts in [[mitochondrion|mitochondria]] and [[chloroplast]]s. In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the [[nucleoid]].<ref>{{cite journal | vauthors = Thanbichler M, Wang SC, Shapiro L | title = The bacterial nucleoid: a highly organized and dynamic structure | journal = Journal of Cellular Biochemistry | volume = 96 | issue = 3 | pages = 506–21 | date = October 2005 | pmid = 15988757 | doi = 10.1002/jcb.20519 | doi-access = free }}</ref> The genetic information in a genome is held within genes, and the complete set of this information in an organism is called its [[genotype]]. A gene is a unit of [[heredity]] and is a region of DNA that influences a particular characteristic in an organism. Genes contain an [[open reading frame]] that can be transcribed, and [[regulatory sequence]]s such as [[promoter (biology)|promoters]] and [[enhancer (genetics)|enhancers]], which control transcription of the open reading frame.

In many [[species]], only a small fraction of the total sequence of the [[genome]] encodes protein. For example, only about 1.5% of the human genome consists of protein-coding [[exon]]s, with over 50% of human DNA consisting of non-coding [[repeated sequence (DNA)|repetitive sequences]].<ref>{{cite journal | vauthors = Wolfsberg TG, McEntyre J, Schuler GD | title = Guide to the draft human genome | journal = Nature | volume = 409 | issue = 6822 | pages = 824–26 | date = February 2001 | pmid = 11236998 | doi = 10.1038/35057000 | bibcode = 2001Natur.409..824W | url = https://zenodo.org/record/1233093 | doi-access = free }}</ref> The reasons for the presence of so much [[noncoding DNA]] in eukaryotic genomes and the extraordinary differences in [[genome size]], or ''[[C-value]]'', among species, represent a long-standing puzzle known as the "[[C-value enigma]]".<ref>{{cite journal | vauthors = Gregory TR | title = The C-value enigma in plants and animals: a review of parallels and an appeal for partnership | journal = Annals of Botany | volume = 95 | issue = 1 | pages = 133–46 | date = January 2005 | pmid = 15596463 | doi = 10.1093/aob/mci009 | pmc = 4246714 }}</ref> However, some DNA sequences that do not code protein may still encode functional [[non-coding RNA]] molecules, which are involved in the [[regulation of gene expression]].<ref name="Birney_2007" />
[[File:T7 RNA polymerase.jpg|thumb|[[T7 RNA polymerase]] (blue) producing an [[Messenger RNA|mRNA]] (green) from a DNA template (orange)<ref>{{Cite web| vauthors = Yin YW, Steitz TA |title=RCSB PDB – 1MSW: Structural basis for the transition from initiation to elongation transcription in T7 RNA polymerase|url=https://www.rcsb.org/structure/1MSW|access-date=2023-03-27|website=www.rcsb.org|language=en-US}}</ref>]]

Some noncoding DNA sequences play structural roles in chromosomes. [[Telomere]]s and [[centromere]]s typically contain few genes but are important for the function and stability of chromosomes.<ref name=Nugent /><ref>{{cite journal | vauthors = Pidoux AL, Allshire RC | title = The role of heterochromatin in centromere function | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 360 | issue = 1455 | pages = 569–79 | date = March 2005 | pmid = 15905142 | pmc = 1569473 | doi = 10.1098/rstb.2004.1611 }}</ref> An abundant form of noncoding DNA in humans are [[pseudogene]]s, which are copies of genes that have been disabled by mutation.<ref>{{cite journal | vauthors = Harrison PM, Hegyi H, Balasubramanian S, Luscombe NM, Bertone P, Echols N, Johnson T, Gerstein M | title = Molecular fossils in the human genome: identification and analysis of the pseudogenes in chromosomes 21 and 22 | journal = Genome Research | volume = 12 | issue = 2 | pages = 272–80 | date = February 2002 | pmid = 11827946 | pmc = 155275 | doi = 10.1101/gr.207102 }}</ref> These sequences are usually just molecular [[fossil]]s, although they can occasionally serve as raw [[Genome|genetic material]] for the creation of new genes through the process of [[gene duplication]] and [[divergent evolution|divergence]].<ref>{{cite journal | vauthors = Harrison PM, Gerstein M | title = Studying genomes through the aeons: protein families, pseudogenes and proteome evolution | journal = Journal of Molecular Biology | volume = 318 | issue = 5 | pages = 1155–74 | date = May 2002 | pmid = 12083509 | doi = 10.1016/S0022-2836(02)00109-2 }}</ref>