Editing Genetics (section)

=== DNA and chromosomes ===
{{Main|DNA|Chromosome}}

[[File:DNA chemical structure.svg|thumb|right|The [[molecular structure]] of DNA. Bases pair through the arrangement of [[hydrogen bonding]] between the strands.]]The [[Molecule|molecular]] basis for genes is [[deoxyribonucleic acid]] (DNA). DNA is composed of [[deoxyribose]] (sugar molecule), a phosphate group, and a base (amine group). There are four types of bases: [[adenine]] (A), [[cytosine]] (C), [[guanine]] (G), and [[thymine]] (T). The phosphates make phosphodiester bonds with the sugars to make long phosphate-sugar backbones. Bases specifically pair together (T&A, C&G) between two backbones and make like rungs on a ladder. The bases, phosphates, and sugars together make a [[nucleotide]] that connects to make long chains of DNA.<ref>{{Cite web | vauthors = Urry L, Cain M, Wasserman S, Minorsky P, Reece J, Campbell N |title=Campbell Biology |url=https://plus.pearson.com/courses/gregg91165/products/GTP1DPWIL20/pages/ac865b14db19976dfd6054de245cd8d8e65000756?locale=&key=2790626781132109428282022&iesCode=5VEW6xrTXI |access-date=2022-09-28 |website=plus.pearson.com}}</ref> Genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the DNA chain.<ref name=Pearson_2006>{{cite journal | vauthors = Pearson H | title = Genetics: what is a gene? | journal = Nature | volume = 441 | issue = 7092 | pages = 398–401 | date = May 2006 | pmid = 16724031 | doi = 10.1038/441398a | s2cid = 4420674 | doi-access = free | bibcode = 2006Natur.441..398P }}</ref> These chains coil into a double a-helix structure and wrap around proteins called [[Histone]]s which provide the structural support. DNA wrapped around these histones are called chromosomes.<ref>{{Cite web |title=Histone |url=https://www.genome.gov/genetics-glossary/histone |access-date=2022-09-28 |website=Genome.gov |language=en}}</ref> [[Virus]]es sometimes use the similar molecule [[RNA]] instead of DNA as their genetic material.<ref>{{cite book |title=Microbiology |vauthors=Prescott LM, Harley JP, Klein DA |year=1996 |publisher=Wm. C. Brown |edition=3rd |isbn=0-697-21865-1 |url=https://archive.org/details/microbiology0000pres/page/342/mode/2up |page=343}}</ref>

DNA normally exists as a double-stranded molecule, coiled into the shape of a [[double helix]]. Each nucleotide in DNA preferentially pairs with its partner nucleotide on the opposite strand: A pairs with T, and C pairs with G. Thus, in its two-stranded form, each strand effectively contains all necessary information, redundant with its partner strand. This structure of DNA is the physical basis for inheritance: DNA replication duplicates the genetic information by splitting the strands and using each strand as a template for synthesis of a new partner strand.<ref name=griffiths2000sect1523>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.1523 |chapter=Mechanism of DNA Replication}}</ref>

[[File:Human karyotype with bands and sub-bands.png|thumb|Schematic [[karyotype|karyogram]] of a human, showing 22 [[homologous chromosome]] pairs, both the female (XX) and male (XY) versions of the [[sex chromosome]] (bottom right), as well as the [[human mitochondrial genetics|mitochondrial genome]] (at bottom left) {{further|Karyotype}}]]
Genes are arranged linearly along long chains of DNA base-pair sequences. In [[bacteria]], each cell usually contains a single circular [[Nucleoid|genophore]], while [[Eukaryote|eukaryotic]] organisms (such as plants and animals) have their DNA arranged in multiple linear chromosomes. These DNA strands are often extremely long; the largest human chromosome, for example, is about 247 million [[base pair]]s in length.<ref>{{cite journal | vauthors = Gregory SG, Barlow KF, McLay KE, Kaul R, Swarbreck D, Dunham A, Scott CE, Howe KL, Woodfine K, Spencer CC, Jones MC, Gillson C, Searle S, Zhou Y, Kokocinski F, McDonald L, Evans R, Phillips K, Atkinson A, Cooper R, Jones C, Hall RE, Andrews TD, Lloyd C, Ainscough R, Almeida JP, Ambrose KD, Anderson F, Andrew RW, Ashwell RI, Aubin K, Babbage AK, Bagguley CL, Bailey J, Beasley H, Bethel G, Bird CP, Bray-Allen S, Brown JY, Brown AJ, Buckley D, Burton J, Bye J, Carder C, Chapman JC, Clark SY, Clarke G, Clee C, Cobley V, Collier RE, Corby N, Coville GJ, Davies J, Deadman R, Dunn M, Earthrowl M, Ellington AG, Errington H, Frankish A, Frankland J, French L, Garner P, Garnett J, Gay L, Ghori MR, Gibson R, Gilby LM, Gillett W, Glithero RJ, Grafham DV, Griffiths C, Griffiths-Jones S, Grocock R, Hammond S, Harrison ES, Hart E, Haugen E, Heath PD, Holmes S, Holt K, Howden PJ, Hunt AR, Hunt SE, Hunter G, Isherwood J, James R, Johnson C, Johnson D, Joy A, Kay M, Kershaw JK, Kibukawa M, Kimberley AM, King A, Knights AJ, Lad H, Laird G, Lawlor S, Leongamornlert DA, Lloyd DM, Loveland J, Lovell J, Lush MJ, Lyne R, Martin S, Mashreghi-Mohammadi M, Matthews L, Matthews NS, McLaren S, Milne S, Mistry S, Moore MJ, Nickerson T, O'Dell CN, Oliver K, Palmeiri A, Palmer SA, Parker A, Patel D, Pearce AV, Peck AI, Pelan S, Phelps K, Phillimore BJ, Plumb R, Rajan J, Raymond C, Rouse G, Saenphimmachak C, Sehra HK, Sheridan E, Shownkeen R, Sims S, Skuce CD, Smith M, Steward C, Subramanian S, Sycamore N, Tracey A, Tromans A, Van Helmond Z, Wall M, Wallis JM, White S, Whitehead SL, Wilkinson JE, Willey DL, Williams H, Wilming L, Wray PW, Wu Z, Coulson A, Vaudin M, Sulston JE, Durbin R, Hubbard T, Wooster R, Dunham I, Carter NP, McVean G, Ross MT, Harrow J, Olson MV, Beck S, Rogers J, Bentley DR, Banerjee R, Bryant SP, Burford DC, Burrill WD, Clegg SM, Dhami P, Dovey O, Faulkner LM, Gribble SM, Langford CF, Pandian RD, Porter KM, Prigmore E | title = The DNA sequence and biological annotation of human chromosome 1 | journal = Nature | volume = 441 | issue = 7091 | pages = 315–321 | date = May 2006 | pmid = 16710414 | doi = 10.1038/nature04727 | doi-access = free | bibcode = 2006Natur.441..315G }}</ref> The DNA of a chromosome is associated with structural proteins that organize, compact, and control access to the DNA, forming a material called [[chromatin]]; in eukaryotes, chromatin is usually composed of [[nucleosome]]s, segments of DNA wound around cores of [[histone]] proteins.<ref>Alberts et al. (2002), [https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.608 II.4. DNA and chromosomes: Chromosomal DNA and Its Packaging in the Chromatin Fiber] {{webarchive|url=https://web.archive.org/web/20071018075642/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.608 |date=18 October 2007 }}</ref> The full set of hereditary material in an organism (usually the combined DNA sequences of all chromosomes) is called the [[genome]].

DNA is most often found in the nucleus of cells, but Ruth Sager helped in the discovery of nonchromosomal genes found outside of the nucleus.<ref name="ruth">{{cite web |title=Ruth Sager |url=https://www.britannica.com/biography/Ruth-Sager |website=Encyclopaedia Britannica |access-date=8 June 2020}}</ref> In plants, these are often found in the chloroplasts and in other organisms, in the mitochondria.<ref name="ruth" /> These nonchromosomal genes can still be passed on by either partner in sexual reproduction and they control a variety of hereditary characteristics that replicate and remain active throughout generations.<ref name="ruth"/>

While [[haploid]] organisms have only one copy of each chromosome, most animals and many plants are [[diploid]], containing two of each chromosome and thus two copies of every gene. The two alleles for a gene are located on identical [[Locus (genetics)|loci]] of the two [[homologous chromosomes]], each allele inherited from a different parent.<ref name=griffiths2000sect484>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.484 |chapter=Mendelian genetics in eukaryotic life cycles}}</ref>

Many species have so-called [[sex chromosome]]s that determine the sex of each organism.<ref name="griffiths2000sect222">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.222 |chapter=Sex chromosomes and sex-linked inheritance}}</ref> In humans and many other animals, the [[Y chromosome]] contains the gene that triggers the development of the specifically male characteristics. In evolution, this chromosome has lost most of its content and also most of its genes, while the [[X chromosome]] is similar to the other chromosomes and contains many genes. This being said, Mary Frances Lyon discovered that there is X-chromosome inactivation during reproduction to avoid passing on twice as many genes to the offspring.<ref name="lyon">{{cite journal | vauthors = Rastan S | title = Mary F. Lyon (1925-2014) | journal = Nature | volume = 518 | issue = 7537 | pages = 36 | date = February 2015 | pmid = 25652989 | doi = 10.1038/518036a | publisher = Springer Nature Limited | s2cid = 4405984 | bibcode = 2015Natur.518...36R | doi-access = free }}</ref> Lyon's discovery led to the discovery of X-linked diseases.<ref name="lyon" />