View source for Gene

{{Short description|Sequence of DNA that determines traits in an organism}}
{{About|sequences of DNA or RNA that code for functional molecules|other uses|Gene (disambiguation)}}
{{Pp-move|small=yes}}
{{Use dmy dates|date=December 2024}}
{{Good article}}
{{Chromosome}}
{{Genetics sidebar}}
In [[biology]], the word '''gene''' has two meanings. The Mendelian gene is a basic unit of [[heredity]]. The molecular gene is a sequence of [[nucleotide]]s in [[DNA]] that is transcribed to produce a functional [[RNA]]. There are two types of molecular genes: protein-coding genes and non-coding genes.<ref name="mendgene2"/><ref name="MedlinePlus 2020">{{cite web |title=What is a gene?: MedlinePlus Genetics |website=MedlinePlus |date=17 September 2020 |url=https://medlineplus.gov/genetics/understanding/basics/gene/ |access-date=4 January 2021 |archive-date=16 May 2020 |archive-url=https://web.archive.org/web/20200516195548/http://ghr.nlm.nih.gov/handbook/basics/gene |url-status=live }}</ref><ref name="Hirsch 2002 p.">{{cite book |vauthors=Hirsch ED |title=The new dictionary of cultural literacy |publisher=Houghton Mifflin |publication-place=Boston |year=2002 |isbn=0-618-22647-8 |oclc=50166721}}</ref> During [[gene expression]] (the synthesis of [[Gene product|RNA or protein]] from a gene), DNA is first [[transcription (biology)|copied into RNA]]. RNA can be [[non-coding RNA|directly functional]] or be the intermediate [[protein biosynthesis|template]] for the synthesis of a protein.

The transmission of genes to an organism's [[offspring]], is the basis of the inheritance of [[phenotypic trait]]s from one generation to the next. These genes make up different DNA sequences, together called a [[genotype]], that is specific to every given individual, within the [[gene pool]] of the [[population (biology)|population]] of a given [[species]]. The genotype, along with environmental and developmental factors, ultimately determines the [[phenotype]] of the individual.

Most biological traits occur under the combined influence of [[polygene]]s (a set of different genes) and [[gene–environment interaction]]s. Some genetic traits are instantly visible, such as [[eye color]] or the number of limbs, others are not, such as [[blood type]], the risk for specific diseases, or the thousands of basic [[biochemistry|biochemical]] processes that constitute [[life]]. A gene can acquire [[mutation]]s in its [[gene sequence|sequence]], leading to different variants, known as [[allele]]s, in the [[population]]. These alleles encode slightly different versions of a gene, which may cause different [[phenotypical]] traits.<ref>{{cite book |vauthors=Elston RC, Satagopan JM, Sun S |title=Statistical Human Genetics |chapter=Genetic terminology |series=Methods in Molecular Biology |volume=850 |pages=1–9 |date=2012 |isbn=978-1-61779-554-1 |publisher=Humana Press |doi=10.1007/978-1-61779-555-8_1 |pmc=4450815 |pmid=22307690}}</ref> Genes [[evolution|evolve]] due to [[natural selection]] or [[survival of the fittest]] and [[genetic drift]] of the alleles.

==Definitions==
There are many different ways to use the term "gene" based on different aspects of their inheritance, selection, biological function, or molecular structure but most of these definitions fall into two categories, the Mendelian gene or the molecular gene.<ref name="mendgene2">{{cite journal |vauthors=Orgogozo V, Peluffo AE, Morizot B |title=The 'Mendelian Gene' and the 'Molecular Gene': Two Relevant Concepts of Genetic Units |journal=Current Topics in Developmental Biology |volume=119 |pages=1–26 |date=2016 |doi=10.1016/bs.ctdb.2016.03.002 |pmid=27282022 |url=https://hal.archives-ouvertes.fr/hal-01354346/file/Orgogozo2016-gene.pdf }}</ref><ref name=Kampourakis/><ref name="Gericke">{{cite journal |vauthors=Gericke N, Hagberg M |date=5 December 2006 |title=Definition of historical models of gene function and their relation to students' understanding of genetics |journal=Science & Education |volume=16 |issue=7–8 |pages=849–881 |bibcode=2007Sc&Ed..16..849G |doi=10.1007/s11191-006-9064-4 }}</ref><ref>{{cite web |title=Stanford Encyclopedia of Philosophy: Gene |vauthors=Meunier R |date=2022 |website=Stanford Encyclopedia of Philosophy |url=https://plato.stanford.edu/entries/gene/ |access-date=28 February 2023}}</ref><ref>{{cite journal |display-authors=6 |vauthors=Kellis M, Wold B, Snyder MP, Bernstein BE, Kundaje A, Marinov GK, Ward LD, Birney E, Crawford GE, Dekker J, Dunham I, Elnitski LL, Farnham PJ, Feingold EA, Gerstein M, Giddings MC, Gilbert DM, Gingeras TR, Green ED, Guigo R, Hubbard T, Kent J, Lieb JD, Myers RM, Pazin MJ, Ren B, Stamatoyannopoulos JA, Weng Z, White KP, Hardison RC |date=April 2014 |title=Defining functional DNA elements in the human genome |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=111 |issue=17 |pages=6131–8 |bibcode=2014PNAS..111.6131K |doi=10.1073/pnas.1318948111 |doi-access=free |pmc=4035993 |pmid=24753594}}</ref>

The Mendelian gene is the classical gene of genetics and it refers to any heritable trait. This is the gene described in ''The Selfish Gene''.<ref name="simply2">{{cite book |title=The selfish gene |vauthors=Dawkins R |date=1976 |publisher=Oxford University Press |place=Oxford, UK}}</ref> More thorough discussions of this version of a gene can be found in the articles ''[[Genetics]]'' and ''[[Gene-centered view of evolution]]''.

The molecular gene definition is more commonly used across biochemistry, molecular biology, and most of genetics—the gene that is described in terms of DNA sequence.<ref name="mendgene2"/> There are many different definitions of this gene—some of which are misleading or incorrect.<ref name=Kampourakis/><ref>{{cite journal |vauthors=Stoltz K, Griffiths P |date=2004 |title=Genes: Philosophical Analyses Put to the Test |journal=History and Philosophy of the Life Sciences |volume=26 |issue=1 |pages=5–28 |doi=10.1080/03919710412331341621 |jstor=23333378 |pmid=15791804 }}</ref>

Very early work in the field that became [[molecular genetics]] suggested the concept that [[one gene–one enzyme hypothesis|one gene makes one protein]] (originally 'one gene – one enzyme').<ref name="pmid16588492">{{cite journal |vauthors=Beadle GW, Tatum EL |date=November 1941 |title=Genetic Control of Biochemical Reactions in Neurospora |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=27 |issue=11 |pages=499–506 |bibcode=1941PNAS...27..499B |doi=10.1073/pnas.27.11.499 |doi-access=free |pmc=1078370 |pmid=16588492}}</ref><ref name="pmid15020400">{{cite journal |vauthors=Horowitz NH, Berg P, Singer M, Lederberg J, Susman M, Doebley J, Crow JF |date=January 2004 |title=A centennial: George W. Beadle, 1903-1989 |journal=Genetics |volume=166 |issue=1 |pages=1–10 |doi=10.1534/genetics.166.1.1 |pmc=1470705 |pmid=15020400}}</ref> However, genes that produce repressor RNAs were proposed in the 1950s<ref>{{cite book |title=The Eight Day of Creation |vauthors=Judson HF |date=1996 |publisher=Cold Spring Harbor Laboratory Press |edition=Expanded |location=Plainview, NY (US)}}</ref> and by the 1960s, textbooks were using molecular gene definitions that included those that specified functional RNA molecules such as ribosomal RNA and tRNA (noncoding genes) as well as protein-coding genes.<ref name="Watson">{{cite book |vauthors=Watson JD |date=1965 |title=Molecular Biology of the Gene |publisher=W.A. Benjamin, Inc. |place=New York, NY, US}}</ref>

This idea of two kinds of genes is still part of the definition of a gene in most textbooks. For example,

{{blockquote|The primary function of the genome is to produce RNA molecules. Selected portions of the DNA nucleotide sequence are copied into a corresponding RNA nucleotide sequence, which either encodes a protein (if it is an mRNA) or forms a 'structural' RNA, such as a transfer RNA (tRNA) or ribosomal RNA (rRNA) molecule. Each region of the DNA helix that produces a functional RNA molecule constitutes a gene.<ref name=Alberts>{{cite book |vauthors=Alberts B, Bray D, Lewis J, Raff M, Roberts K, Watson JD |date=1994 |title=Molecular Biology of the Cell: Third Edition |publisher=Garland Publishing, Inc. |place=London, UK |isbn=0-8153-1619-4}}</ref>}}

{{blockquote|We define a gene as a DNA sequence that is transcribed. This definition includes genes that do not encode proteins (not all transcripts are messenger RNA). The definition normally excludes regions of the genome that control transcription but are not themselves transcribed. We will encounter some exceptions to our definition of a gene - surprisingly, there is no definition that is entirely satisfactory.<ref>{{cite book |vauthors=Moran LA, Horton HR, Scrimgeour KG, Perry MD |date=2012 |title=Principles of Biochemistry: Fifth Edition |publisher=Pearson |place=Upper Saddle River, NJ, US}}</ref>}}

{{blockquote|A gene is a DNA sequence that codes for a diffusible product. This product may be protein (as is the case in the majority of genes) or may be RNA (as is the case of genes that code for tRNA and rRNA). The crucial feature is that the product diffuses away from its site of synthesis to act elsewhere.<ref>{{cite book |vauthors=Lewin B |date=2004 |title=Genes VIII |publisher=Pearson/Prentice Hall |place=Upper Saddle River, NJ, US}}</ref>}}

The important parts of such definitions are: (1) that a gene corresponds to a transcription unit; (2) that genes produce both mRNA and noncoding RNAs; and (3) regulatory sequences control gene expression but are not part of the gene itself. However, there is one other important part of the definition and it is emphasized in Kostas Kampourakis' book ''Making Sense of Genes''.

{{blockquote|Therefore in this book I will consider genes as DNA sequences encoding information for functional products, be it proteins or RNA molecules. With 'encoding information', I mean that the DNA sequence is used as a template for the production of an RNA molecule or a protein that performs some function.<ref name=Kampourakis>{{cite book |vauthors=Kampourakis K |date=2017 |title=Making Sense of Genes |publisher=Cambridge University Press |place=Cambridge, UK}}</ref>}}

The emphasis on function is essential because there are stretches of DNA that produce non-functional transcripts and they do not qualify as genes. These include obvious examples such as transcribed pseudogenes as well as less obvious examples such as junk RNA produced as noise due to transcription errors. In order to qualify as a true gene, by this definition, one has to prove that the transcript has a biological function.<ref name=Kampourakis/>

Early speculations on the size of a typical gene were based on high-resolution genetic mapping and on the size of proteins and RNA molecules. A length of 1500 base pairs seemed reasonable at the time (1965).<ref name=Watson/> This was based on the idea that the gene was the DNA that was directly responsible for production of the functional product. The discovery of introns in the 1970s meant that many eukaryotic genes were much larger than the size of the functional product would imply. Typical mammalian protein-coding genes, for example, are about 62,000 base pairs in length (transcribed region) and since there are about 20,000 of them they occupy about 35–40% of the mammalian genome (including the human genome).<ref>{{cite journal |vauthors=Piovesan A, Pelleri MC, Antonaros F, Strippoli P, Caracausi M, and Vitale L |date=2019 |title=On the length, weight and GC content of the human genome |journal=BMC Research Notes |volume=12 |issue=1 |pages=106–173 |doi=10.1186/s13104-019-4137-z |pmid=30813969 |pmc=6391780 |doi-access=free}}</ref><ref>{{cite journal |vauthors=Hubé F, and Francastel C |date=2015 |title=Mammalian Introns: When the Junk Generates Molecular Diversity |journal=International Journal of Molecular Sciences |volume=16 |issue=3 |pages=4429–4452 |doi=10.3390/ijms16034429 |doi-access=free |pmc=4394429 |pmid=25710723}}</ref><ref>{{cite journal |vauthors=Francis WR, and Wörheide G |date=2017 |title=Similar ratios of introns to intergenic sequence across animal genomes |journal=Genome Biology and Evolution |volume=9 |issue=6 |pages=1582–1598 |doi=10.1093/gbe/evx103 |pmc=5534336 |pmid=28633296}}</ref>

In spite of the fact that both protein-coding genes and noncoding genes have been known for more than 50 years, there are still a number of textbooks, websites, and scientific publications that define a gene as a DNA sequence that specifies a protein. In other words, the definition is restricted to protein-coding genes. Here is an example from a 2021 article in American Scientist.

{{blockquote|... to truly assess the potential significance of de novo genes, we relied on a strict definition of the word "gene" with which nearly every expert can agree. First, in order for a nucleotide sequence to be considered a true gene, an open reading frame (ORF) must be present. The ORF can be thought of as the "gene itself"; it begins with a starting mark common for every gene and ends with one of three possible finish line signals. One of the key enzymes in this process, the RNA polymerase, zips along the strand of DNA like a train on a monorail, transcribing it into its messenger RNA form. This point brings us to our second important criterion: A true gene is one that is both transcribed and translated. That is, a true gene is first used as a template to make transient messenger RNA, which is then translated into a protein.<ref>{{cite journal |vauthors=Mortola E, Long M |date=2021 |title=Turning Junk into Us: How Genes Are Born |journal=American Scientist |volume=109 |pages=174–182 |url=https://www.americanscientist.org/article/turning-junk-into-us-how-genes-are-born |access-date=27 February 2023 |archive-date=27 February 2023 |archive-url=https://web.archive.org/web/20230227065824/https://www.americanscientist.org/article/turning-junk-into-us-how-genes-are-born |url-status=live }}</ref>}}

This restricted definition is so common that it has spawned many recent articles that criticize this "standard definition" and call for a new expanded definition that includes noncoding genes. However, some modern writers still do not acknowledge noncoding genes although this so-called "new" definition has been recognised for more than half a century.<ref>{{cite journal |vauthors=Hopkin K |date=2009 |title=The Evolving Definition of a Gene: With the discovery that nearly all of the genome is transcribed, the definition of a "gene" needs another revision |journal=BioScience |volume=59 |pages=928–931 |doi=10.1525/bio.2009.59.11.3 |s2cid=88157272}}</ref><ref>{{cite journal |vauthors=Pearson H |date=2006 |title=What Is a Gene? |journal=Nature |volume=441 |issue=7092 |pages=399–401 |doi=10.1038/441398a |pmid=16724031 |bibcode=2006Natur.441..398P |s2cid=4420674 |doi-access=free}}</ref><ref>{{cite journal |vauthors=Pennisi E |date=2007 |title=DNA study forces rethink of what it means to be a gene |journal=Science |volume=316 |issue=5831 |pages=1556–1557 |doi=10.1126/science.316.5831.1556 |pmid=17569836 |s2cid=36463252 |doi-access=free}}</ref> 

Although some definitions can be more broadly applicable than others, the fundamental complexity of biology means that no definition of a gene can capture all aspects perfectly. Not all genomes are DNA (e.g. [[RNA virus]]es),<ref>{{cite journal |vauthors=Wolf YI, Kazlauskas D, Iranzo J, Lucía-Sanz A, Kuhn JH, Krupovic M, Dolja VV, Koonin EV |display-authors=6 |title=Origins and Evolution of the Global RNA Virome |journal=mBio |volume=9 |issue=6 |pages=e02329–18 |date=November 2018 |others=Eric Delwart, Luis Enjuanes |veditors=Racaniello VR |doi=10.1128/mBio.02329-18 |pmc=6282212 |pmid=30482837}}</ref> bacterial [[operon]]s are multiple protein-coding regions transcribed into single large mRNAs, [[alternative splicing]] enables a single genomic region to encode multiple district products and [[trans-splicing]] concatenates mRNAs from shorter coding sequence across the genome.<ref name="Rethink"/><ref>{{cite journal |vauthors=Marande W, Burger G |date=October 2007 |title=Mitochondrial DNA as a genomic jigsaw puzzle |journal=Science |publisher=AAAS |volume=318 |issue=5849 |pages=415 |bibcode=2007Sci...318..415M |doi=10.1126/science.1148033 |pmid=17947575 |s2cid=30948765}}</ref><ref>{{cite journal |display-authors=6 |vauthors=Parra G, Reymond A, Dabbouseh N, Dermitzakis ET, Castelo R, Thomson TM, Antonarakis SE, Guigó R |date=January 2006 |title=Tandem chimerism as a means to increase protein complexity in the human genome |journal=Genome Research |volume=16 |issue=1 |pages=37–44 |doi=10.1101/gr.4145906 |pmc=1356127 |pmid=16344564}}</ref> Since molecular definitions exclude elements such as introns, promotors, and other [[regulatory sequence|regulatory regions]], these are instead thought of as "associated" with the gene and affect its function.

An even broader operational definition is sometimes used to encompass the complexity of these diverse phenomena, where a gene is defined as a union of genomic sequences encoding a coherent set of potentially overlapping functional products.<ref name="Gerstein">{{cite journal |display-authors=6 |vauthors=Gerstein MB, Bruce C, Rozowsky JS, Zheng D, Du J, Korbel JO, Emanuelsson O, Zhang ZD, Weissman S, Snyder M |date=June 2007 |title=What is a gene, post-ENCODE? History and updated definition |journal=Genome Research |volume=17 |issue=6 |pages=669–81 |doi=10.1101/gr.6339607 |doi-access=free |pmid=17567988}}</ref> This definition categorizes genes by their functional products (proteins or RNA) rather than their specific DNA loci, with regulatory elements classified as ''gene-associated'' regions.<ref name="Gerstein"/>

==History==
{{Main|History of genetics}}

===Discovery of discrete inherited units===
[[File:Gregor Mendel - American Breeders Magazine, 1910.jpg|thumb|upright=0.8|Gregor Mendel|alt=Photograph of Gregor Mendel]]

The existence of discrete inheritable units was first suggested by [[Gregor Mendel]] (1822–1884).<ref>{{cite journal |vauthors=Noble D |title=Genes and causation |journal=Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences |volume=366 |issue=1878 |pages=3001–15 |date=September 2008 |bibcode=2008RSPTA.366.3001N |doi=10.1098/rsta.2008.0086 |doi-access=free |pmid=18559318}}</ref> From 1857 to 1864, in [[Brno]], [[Austrian Empire]] (today's Czech Republic), he studied inheritance patterns in 8000 common edible [[pea|pea plant]]s, tracking distinct traits from parent to offspring. He described these mathematically as 2<sup>n</sup>&nbsp;combinations where n is the number of differing characteristics in the original peas. Although he did not use the term ''gene'', he explained his results in terms of discrete inherited units that give rise to observable physical characteristics. This description prefigured [[Wilhelm Johannsen]]'s distinction between [[genotype]] (the genetic material of an organism) and [[phenotype]] (the observable traits of that organism). Mendel was also the first to demonstrate [[independent assortment]], the distinction between [[dominant gene|dominant]] and [[recessive]] traits, the distinction between a [[heterozygote]] and [[homozygote]], and the phenomenon of discontinuous inheritance.

Prior to Mendel's work, the dominant theory of heredity was one of [[blending inheritance]],<ref>{{cite book |doi=10.1016/B978-0-12-374984-0.00060-7 |chapter=Galton's Law of Ancestral Heredity |title=Brenner's Encyclopedia of Genetics |date=2013 |last1=Magnello |first1=M.E. |pages=143–146 |isbn=978-0-08-096156-9 }}</ref> which suggested that each parent contributed fluids to the fertilization process and that the traits of the parents blended and mixed to produce the offspring. [[Charles Darwin]] developed a theory of inheritance he termed [[pangenesis]], from [[ancient Greek|Greek]] pan ("all, whole") and genesis ("birth") / genos ("origin").<ref>{{Cite OED|genesis}}</ref><ref>{{cite book |vauthors=Magner LN |title=A History of the Life Sciences |edition=Third |publisher=[[Marcel Dekker]], [[CRC Press]] |year=2002 |isbn=978-0-203-91100-6 |page=371 |url=https://books.google.com/books?id=YKJ6gVYbrGwC}}</ref> Darwin used the term ''[[gemmule (pangenesis)|gemmule]]'' to describe hypothetical particles that would mix during reproduction.

Mendel's work went largely unnoticed after its first publication in 1866, but was rediscovered in the late 19th century by [[Hugo de Vries]], [[Carl Correns]], and [[Erich von Tschermak]], who (claimed to have) reached similar conclusions in their own research.<ref>{{cite book |vauthors=Henig RM |title=The Monk in the Garden: The Lost and Found Genius of Gregor Mendel, the Father of Genetics |publisher=Houghton Mifflin |location=Boston |year=2000 |isbn=978-0395-97765-1 |pages=[https://archive.org/details/monkingardenlost00heni/page/n14 1]–9 |url=https://archive.org/details/monkingardenlost00heni |url-access=registration}}</ref> Specifically, in 1889, Hugo de Vries published his book ''Intracellular Pangenesis'',<ref name="de_Vries_1889">{{cite book |vauthors=de Vries H |author-link1=Hugo de Vries |title=Intracellulare Pangenese |trans-title=Intracellular Pangenesis |language=de |publisher=Verlag von Gustav Fischer |location=Jena |date=1889 |translator-last1=Gager |translator-first1=C. Stuart |translator-link1=C. Stuart Gager |name-list-style=vanc |url=http://www.esp.org/books/devries/pangenesis/facsimile/ |access-date=6 May 2007 |archive-date=2 June 2023 |archive-url=https://web.archive.org/web/20230602053529/http://www.esp.org/books/devries/pangenesis/facsimile/ |url-status=live }} Translated in 1908 from German to English by Open Court Publishing Co., Chicago, 1910</ref> in which he postulated that different characters have individual hereditary carriers and that inheritance of specific traits in organisms comes in particles. De Vries called these units "pangenes" (''Pangens'' in German), after Darwin's 1868 pangenesis theory.

Twenty years later, in 1909, [[Wilhelm Johannsen]] introduced the term "gene" (inspired by the [[ancient Greek]]: γόνος, ''gonos'', meaning offspring and procreation)<ref name="Johannsen">{{cite book |vauthors=Johannsen W |title=Elemente der exakten Erblichkeitslehre |trans-title=Elements of the exact theory of heredity |date=1909 |publisher=Gustav Fischer |location=Jena, Germany |page=124 |language=German |url=https://www.biodiversitylibrary.org/item/15717#page/134/mode/1up |access-date=16 June 2021 |archive-date=24 June 2021 |archive-url=https://web.archive.org/web/20210624195335/https://www.biodiversitylibrary.org/item/15717#page/134/mode/1up |url-status=live }} From p. 124: ''"Dieses "etwas" in den Gameten bezw. in der Zygote, ... – kurz, was wir eben Gene nennen wollen – bedingt sind."'' (This "something" in the gametes or in the zygote, which has crucial importance for the character of the organism, is usually called by the quite ambiguous term ''Anlagen'' [primordium, from the German word ''Anlage'' for "plan, arrangement; rough sketch"]. Many other terms have been suggested, mostly unfortunately in closer connection with certain hypothetical opinions. The word "pangene", which was introduced by Darwin, is perhaps used most frequently in place of ''Anlagen''. However, the word "pangene" was not well chosen, as it is a compound word containing the roots ''pan'' (the neuter form of Πας all, every) and ''gen'' (from γί-γ(ε)ν-ομαι, to become). Only the meaning of this latter [i.e., ''gen''] comes into consideration here; just the basic idea – [namely,] that a trait in the developing organism can be determined or is influenced by "something" in the gametes – should find expression. No hypothesis about the nature of this "something" should be postulated or supported by it. For that reason it seems simplest to use in isolation the last syllable ''gen'' from Darwin's well-known word, which alone is of interest to us, in order to replace, with it, the poor, ambiguous word ''Anlage''. Thus we will say simply "gene" and "genes" for "pangene" and "pangenes". The word gene is completely free of any hypothesis; it expresses only the established fact that in any case many traits of the organism are determined by specific, separable, and thus independent "conditions", "foundations", "plans" – in short, precisely what we want to call genes.)</ref> and, in 1906, [[William Bateson]], that of "[[genetics]]"<ref>{{cite book |vauthors=Bateson W |date=1906 |chapter-url=https://www.biodiversitylibrary.org/item/206746#page/129/mode/1up |chapter=The progress of genetic research |title=Report of the Third International Conference 1906 on Genetics |veditors=Wilks W |location=London, England |publisher=Royal Horticultural Society |pages=90–97 |quote=... the science itself [i.e. the study of the breeding and hybridisation of plants] is still nameless, and we can only describe our pursuit by cumbrous and often misleading periphrasis. To meet this difficulty I suggest for the consideration of this Congress the term ''Genetics'', which sufficiently indicates that our labors are devoted to the elucidation of the phenomena of heredity and variation: in other words, to the physiology of Descent, with implied bearing on the theoretical problems of the evolutionist and the systematist, and application to the practical problems of breeders, whether of animals or plants. |access-date=17 June 2021 |archive-date=27 June 2023 |archive-url=https://web.archive.org/web/20230627221528/https://www.biodiversitylibrary.org/item/206746#page/129/mode/1up |url-status=live }}</ref><ref name="Gerstein"/> while [[Eduard Strasburger]], among others, still used the term "pangene" for the fundamental physical and functional unit of heredity.<ref name="de_Vries_1889"/>{{rp|Translator's preface, viii}}

===Discovery of DNA===

Advances in understanding genes and inheritance continued throughout the 20th century. [[Deoxyribonucleic acid]] (DNA) was shown to be the molecular repository of genetic information by experiments in the 1940s to 1950s.<ref name=Avery_et_al>{{cite journal |vauthors=Avery OT, Macleod CM, McCarty M |title=Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types: Induction of Transformation by a Desoxyribonucleic Acid Fraction Isolated From Pneumococcus Type III |journal=The Journal of Experimental Medicine |volume=79 |issue=2 |pages=137–58 |date=February 1944 |doi=10.1084/jem.79.2.137 |pmc=2135445 |pmid=19871359}} Reprint: {{cite journal |vauthors=Avery OT, MacLeod CM, McCarty M |title=Studies on the chemical nature of the substance inducing transformation of pneumococcal types. Inductions of transformation by a desoxyribonucleic acid fraction isolated from pneumococcus type III |journal=The Journal of Experimental Medicine |volume=149 |issue=2 |pages=297–326 |date=February 1979 |doi=10.1084/jem.149.2.297 |pmc=2184805 |pmid=33226}}</ref><ref>{{cite journal |vauthors=Hershey AD, Chase M |title=Independent functions of viral protein and nucleic acid in growth of bacteriophage |journal=The Journal of General Physiology |volume=36 |issue=1 |pages=39–56 |date=May 1952 |doi=10.1085/jgp.36.1.39 |pmc=2147348 |pmid=12981234}}</ref> The structure of DNA was studied by [[Rosalind Franklin]] and [[Maurice Wilkins]] using [[X-ray crystallography]], which led [[James D. Watson]] and [[Francis Crick]] to publish a model of the double-stranded DNA molecule whose paired [[nucleotide base]]s indicated a compelling hypothesis for the mechanism of genetic replication.<ref>{{cite book |title=The Eighth Day of Creation: Makers of the Revolution in Biology |vauthors=Judson H |author-link=Horace Freeland Judson |year=1979 |publisher=Cold Spring Harbor Laboratory Press |isbn=978-0-87969-477-7 |pages=51–169}}</ref><ref name=watsoncrick_1953a>{{cite journal |vauthors=Watson JD, Crick FH |title=Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid |journal=Nature |volume=171 |issue=4356 |pages=737–8 |date=April 1953 |bibcode=1953Natur.171..737W |doi=10.1038/171737a0 |pmid=13054692 }}</ref>

In the early 1950s the prevailing view was that the genes in a chromosome acted like discrete entities arranged like beads on a string. The experiments of [[Seymour Benzer|Benzer]] using [[mutant]]s defective in the [[T4 rII system|rII region of bacteriophage T4]] (1955–1959) showed that individual genes have a simple linear structure and are likely to be equivalent to a linear section of DNA.<ref name="pmid16589677">{{cite journal |vauthors=Benzer S |title=Fine Structure of a Genetic Region in Bacteriophage |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=41 |issue=6 |pages=344–54 |date=June 1955 |bibcode=1955PNAS...41..344B |doi=10.1073/pnas.41.6.344 |doi-access=free |pmc=528093 |pmid=16589677}}</ref><ref name="pmid16590553">{{cite journal |vauthors=Benzer S |title=On the Topology of the Genetic Fine Structure |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=45 |issue=11 |pages=1607–20 |date=November 1959 |bibcode=1959PNAS...45.1607B |doi=10.1073/pnas.45.11.1607 |doi-access=free |pmc=222769 |pmid=16590553}}</ref>

Collectively, this body of research established the [[central dogma of molecular biology]], which states that [[protein]]s are translated from [[RNA]], which is transcribed from [[DNA]]. This dogma has since been shown to have exceptions, such as [[reverse transcription]] in [[retrovirus]]es. The modern study of [[genetics]] at the level of DNA is known as [[molecular genetics]].

In 1972, [[Walter Fiers]] and his team were the first to determine the sequence of a gene: that of [[bacteriophage MS2]] coat protein.<ref name=Min_1972>{{cite journal |vauthors=Min Jou W, Haegeman G, Ysebaert M, Fiers W |title=Nucleotide sequence of the gene coding for the bacteriophage MS2 coat protein |journal=Nature |volume=237 |issue=5350 |pages=82–8 |date=May 1972 |bibcode=1972Natur.237...82J |doi=10.1038/237082a0 |pmid=4555447 |s2cid=4153893}}</ref> The subsequent development of [[Sanger sequencing|chain-termination]] [[DNA sequencing]] in 1977 by [[Frederick Sanger]] improved the efficiency of sequencing and turned it into a routine laboratory tool.<ref name=sanger_et_al>{{cite journal |vauthors=Sanger F, Nicklen S, Coulson AR |title=DNA sequencing with chain-terminating inhibitors |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=74 |issue=12 |pages=5463–7 |date=December 1977 |bibcode=1977PNAS...74.5463S |doi=10.1073/pnas.74.12.5463 |doi-access=free |pmc=431765 |pmid=271968}}</ref> An automated version of the Sanger method was used in early phases of the [[Human Genome Project]].<ref name=scitable_adams>{{cite journal |vauthors=Adams JU |title=DNA Sequencing Technologies |journal=Nature Education Knowledge |date=2008 |volume=1 |issue=1 |page=193 |series=SciTable |publisher=Nature Publishing Group |url=https://www.nature.com/scitable/topicpage/dna-sequencing-technologies-690/ |access-date=2 May 2024 |archive-date=19 October 2008 |archive-url=https://web.archive.org/web/20081019192918/https://www.nature.com/scitable/topicpage/dna-sequencing-technologies-690/ |url-status=live }}</ref>

===Modern synthesis and its successors===
{{Main|Modern synthesis (20th century)}}

The theories developed in the early 20th century to integrate [[Mendelian genetics]] with [[Darwinian evolution]] are called the [[modern synthesis (20th century)|modern synthesis]], a term introduced by [[Julian Huxley]].<ref>{{cite book |vauthors=Huxley J |title=Evolution: the Modern Synthesis |date=1942 |publisher=MIT Press |location=Cambridge, Massachusetts |isbn=978-0262513661}}</ref>

This view of evolution was emphasized by [[George C. Williams (biologist)|George C. Williams]]' [[gene-centered view of evolution|gene-centric view of evolution]]. He proposed that the Mendelian gene is a [[unit of selection|unit]] of [[natural selection]] with the definition: "that which segregates and recombines with appreciable frequency."<ref>{{cite book |vauthors=Williams GC |title=Adaptation and Natural Selection a Critique of Some Current Evolutionary Thought |date=2001 |publisher=Princeton University Press |location=Princeton |isbn=9781400820108 |edition=Online}}</ref>{{rp|24}} Related ideas emphasizing the centrality of Mendelian genes and the importance of natural selection in evolution were popularized by [[Richard Dawkins]].<ref name="simply2"/><ref>{{cite book |vauthors=Dawkins R |title=The extended phenotype |date=1989 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-286088-0 |edition=Paperback}}</ref>

The development of the [[neutral theory of molecular evolution|neutral theory of evolution]] in the late 1960s led to the recognition that random genetic drift is a major player in evolution and that neutral theory should be the null hypothesis of molecular evolution.<ref>{{cite journal |vauthors=Duret L |date=2008 |title=Neutral Theory: The Null Hypothesis of Molecular Evolution |journal=Nature Education |volume=1 |pages=218 |url=https://www.nature.com/scitable/topicpage/neutral-theory-the-null-hypothesis-of-molecular-839/}}</ref> This led to the construction of [[phylogenetic tree]]s and the development of the [[molecular clock]], which is the basis of all dating techniques using DNA sequences. These techniques are not confined to molecular gene sequences but can be used on all DNA segments in the genome.

==Molecular basis==
{{Main|DNA}}
[[File:DNA chemical structure 2.svg|class=skin-invert-image|thumb|upright=1.5|The chemical structure of a four base pair fragment of a [[DNA]] [[double helix]]. The [[deoxyribose|sugar]]-[[phosphate]] backbone chains run in opposite directions with the [[nucleobase|bases]] pointing inward, [[base-pair]]ing [[adenine|A]] to [[thymine|T]] and [[cytosine|C]] to [[guanine|G]] with [[hydrogen bond]]s. |alt=DNA chemical structure diagram showing how the double helix consists of two chains of sugar-phosphate backbone with bases pointing inward and specifically base pairing A to T and C to G with hydrogen bonds.]]

===DNA===
The vast majority of organisms encode their genes in long strands of [[DNA]] (deoxyribonucleic acid). DNA consists of a [[polymer|chain]] made from four types of [[nucleotide]] subunits, each composed of: a five-carbon sugar ([[deoxyribose|2-deoxyribose]]), a [[phosphate]] group, and one of the four [[nucleobase|bases]] [[adenine]], [[cytosine]], [[guanine]], and [[thymine]].<ref name="MBOC">{{cite book |last1=Alberts |first1=Bruce |last2=Johnson |first2=Alexander |last3=Lewis |first3=Julian |last4=Raff |first4=Martin |last5=Roberts |first5=Keith |last6=Walter |first6=Peter |title=Molecular Biology of the Cell |date=2002 |publisher=Garland Science |isbn=978-0-8153-3218-3 |edition=4th |url=https://www.ncbi.nlm.nih.gov/books/NBK21054/ }}</ref>{{rp|2.1}}

Two chains of DNA twist around each other to form a DNA [[double helix]] with the phosphate–sugar backbone spiralling around the outside, and the bases pointing inward with adenine [[base pair]]ing to thymine and guanine to cytosine. The specificity of base pairing occurs because adenine and thymine align to form two [[hydrogen bond]]s, whereas cytosine and guanine form three hydrogen bonds. The two strands in a double helix must, therefore, be [[complementarity (molecular biology)|complementary]], with their sequence of bases matching such that the adenines of one strand are paired with the thymines of the other strand, and so on.<ref name=MBOC/>{{rp|4.1}}

Due to the chemical composition of the [[pentose]] residues of the bases, DNA strands have directionality. One end of a DNA [[polymer]] contains an exposed [[hydroxyl]] group on the [[deoxyribose]]; this is known as the [[3'&nbsp;end]] of the molecule. The other end contains an exposed [[phosphate]] group; this is the [[5'&nbsp;end]]. The two strands of a double-helix run in opposite directions. Nucleic acid synthesis, including [[DNA replication]] and [[transcription (genetics)|transcription]] occurs in the 5'→3'&nbsp;direction, because new nucleotides are added via a [[dehydration reaction]] that uses the exposed 3'&nbsp;hydroxyl as a [[nucleophile]].<ref name="Stryer_2002">{{cite book |vauthors=Stryer L, Berg JM, Tymoczko JL |title=Biochemistry |publisher=W.H. Freeman |location=San Francisco |year=2002 |edition=5th |isbn=978-0-7167-4955-4 |url=https://www.ncbi.nlm.nih.gov/books/NBK21154/ |access-date=31 August 2017 |archive-date=11 March 2014 |archive-url=https://web.archive.org/web/20140311010455/http://www.ncbi.nlm.nih.gov/books/NBK21154/ |url-status=live }}</ref>{{rp|27.2}}

The [[gene expression|expression]] of genes encoded in DNA begins by [[transcription (genetics)|transcribing]] the gene into [[RNA]], a second type of nucleic acid that is very similar to DNA, but whose monomers contain the sugar [[ribose]] rather than [[deoxyribose]]. RNA also contains the base [[uracil]] in place of [[thymine]]. RNA molecules are less stable than DNA and are typically single-stranded. Genes that encode proteins are composed of a series of three-[[nucleotide]] sequences called [[codon]]s, which serve as the "words" in the genetic "language". The [[genetic code]] specifies the correspondence during [[translation (genetics)|protein translation]] between codons and [[amino acid]]s. The genetic code is nearly the same for all known organisms.<ref name=MBOC/>{{rp|4.1}}

===Chromosomes===
[[File:NHGRI human male karyotype.png|class=skin-invert-image|thumb|[[Micrograph|Micrographic]] [[karyotype|karyogram]] of human male, showing 23 pairs of chromosomes. The largest [[chromosome]]s are around 10 times the size of the smallest.<ref>{{cite journal |vauthors=Bolzer A, Kreth G, Solovei I, Koehler D, Saracoglu K, Fauth C, Müller S, Eils R, Cremer C, Speicher MR, Cremer T |display-authors=6 |title=Three-dimensional maps of all chromosomes in human male fibroblast nuclei and prometaphase rosettes |journal=PLOS Biology |volume=3 |issue=5 |pages=e157 |date=May 2005 |doi=10.1371/journal.pbio.0030157 |doi-access=free |pmc=1084335 |pmid=15839726}} {{open access}}</ref>]]
[[File:Human karyotype with bands and sub-bands.png|thumb|Schematic [[karyotype|karyogram]] of a human, with annotated [[locus (genetics)|bands and sub-bands]]. It shows dark and white regions on [[G banding]]. It shows 22 [[homologous chromosome]]s, both the male (XY) and female (XX) versions of the [[sex chromosome]] (bottom right), as well as the [[human mitochondrial genetics|mitochondrial genome]] (at bottom left). {{further|Karyotype}}]]
The total complement of genes in an organism or cell is known as its [[genome]], which may be stored on one or more [[chromosome]]s. A chromosome consists of a single, very long DNA helix on which thousands of genes are encoded.<ref name=MBOC/>{{rp|4.2}} The region of the chromosome at which a particular gene is located is called its [[locus (genetics)|locus]]. Each locus contains one [[allele]] of a gene; however, members of a population may have different alleles at the locus, each with a slightly different gene sequence.

The majority of [[eukaryotic]] genes are stored on a set of large, linear chromosomes. The chromosomes are packed within the [[cell nucleus|nucleus]] in complex with storage proteins called [[histone]]s to form a unit called a [[nucleosome]]. DNA packaged and condensed in this way is called [[chromatin]].<ref name=MBOC/>{{rp|4.2}} The manner in which DNA is stored on the histones, as well as chemical modifications of the histone itself, regulate whether a particular region of DNA is accessible for [[gene expression]]. In addition to genes, eukaryotic chromosomes contain sequences involved in ensuring that the DNA is copied without degradation of end regions and sorted into daughter cells during cell division: [[replication origin]]s, [[telomere]]s, and the [[centromere]].<ref name=MBOC/>{{rp|4.2}} Replication origins are the sequence regions where [[DNA replication]] is initiated to make two copies of the chromosome. Telomeres are long stretches of repetitive sequences that cap the ends of the linear chromosomes and prevent degradation of coding and regulatory regions during [[DNA replication]]. The length of the telomeres decreases each time the genome is replicated and has been implicated in the [[aging]] process.<ref name="Braig">{{cite journal |vauthors=Braig M, Schmitt CA |title=Oncogene-induced senescence: putting the brakes on tumor development |journal=Cancer Research |volume=66 |issue=6 |pages=2881–4 |date=March 2006 |doi=10.1158/0008-5472.CAN-05-4006 |doi-access=free |pmid=16540631}}</ref> The centromere is required for binding [[spindle fibre]]s to separate sister chromatids into daughter cells during [[cell division]].<ref name=MBOC/>{{rp|18.2}}

[[Prokaryote]]s ([[bacteria]] and [[archaea]]) typically store their genomes on a single, large, [[circular chromosome]]. Similarly, some eukaryotic [[organelle]]s contain a remnant circular chromosome with a small number of genes.<ref name="MBOC"/>{{rp|14.4}} Prokaryotes sometimes supplement their chromosome with additional small circles of DNA called [[plasmid]]s, which usually encode only a few genes and are transferable between individuals. For example, the genes for [[antibiotic resistance]] are usually encoded on bacterial plasmids and can be passed between individual cells, even those of different species, via [[horizontal gene transfer]].<ref name=bennett>{{cite journal |vauthors=Bennett PM |title=Plasmid encoded antibiotic resistance: acquisition and transfer of antibiotic resistance genes in bacteria |journal=British Journal of Pharmacology |volume=153 |issue=Suppl 1 |pages=S347-57 |date=March 2008 |doi=10.1038/sj.bjp.0707607 |pmc=2268074 |pmid=18193080}}</ref>
<!-- this would be a good place for a table of % 'junk' under different definitions or similar - the 80% expressed doesn't give a reader much context on biological relevance -->

Whereas the chromosomes of prokaryotes are relatively gene-dense, those of eukaryotes often contain regions of DNA that serve no obvious function. Simple single-celled eukaryotes have relatively small amounts of such DNA, whereas the genomes of complex [[multicellular organism]]s, including humans, contain an absolute majority of DNA without an identified function.<ref name="IHSGC2004">{{cite journal |author=International Human Genome Sequencing Consortium |title=Finishing the euchromatic sequence of the human genome |journal=Nature |volume=431 |issue=7011 |pages=931–45 |date=October 2004 |bibcode=2004Natur.431..931H |doi=10.1038/nature03001 |doi-access=free |pmid=15496913}}</ref> This DNA has often been referred to as "[[junk DNA]]". However, more recent analyses suggest that, although protein-coding DNA makes up barely 2% of the [[human genome]], about 80% of the bases in the genome may be expressed, so the term "junk DNA" may be a misnomer.<ref name="Rethink">{{cite journal |vauthors=Pennisi E |author-link=Elizabeth Pennisi |title=Genomics. DNA study forces rethink of what it means to be a gene |journal=Science |volume=316 |issue=5831 |pages=1556–7 |date=June 2007 |doi=10.1126/science.316.5831.1556 |doi-access=free |pmid=17569836 |s2cid=36463252}}</ref>

==Structure and function==<!-- NOTE: The gene structure image templates below are in a 1x1 table so that they scroll properly on mobile and tablet screens (as of 2015) -->

===Structure===
{| style="clear:right; float: right;"
|-
|{{Eukaryote gene structure}}
{{Prokaryote gene structure}}
|}

The [[gene structure|structure of a protein-coding gene]] consists of many elements of which the actual [[coding region|protein coding sequence]] is often only a small part. These include introns and untranslated regions of the mature mRNA. Noncoding genes can also contain introns that are removed during processing to produce the mature functional RNA.

All genes are associated with [[regulatory sequence]]s that are required for their expression. First, genes require a [[promoter (genetics)|promoter]] sequence. The promoter is recognized and bound by [[transcription factor]]s that recruit and help [[RNA polymerase]] bind to the region to initiate transcription.<ref name="MBOC"/>{{rp|7.1}} The recognition typically occurs as a [[consensus sequence]] like the [[TATA box]]. A gene can have more than one promoter, resulting in messenger RNAs ([[mRNA]]) that differ in how far they extend in the 5'&nbsp;end.<ref>{{cite journal |vauthors=Mortazavi A, Williams BA, McCue K, Schaeffer L, Wold B |title=Mapping and quantifying mammalian transcriptomes by RNA-Seq |journal=Nature Methods |volume=5 |issue=7 |pages=621–8 |date=July 2008 |doi=10.1038/nmeth.1226 |pmid=18516045 |s2cid=205418589}}</ref> Highly transcribed genes have "strong" promoter sequences that form strong associations with transcription factors, thereby initiating transcription at a high rate. Others genes have "weak" promoters that form weak associations with transcription factors and initiate transcription less frequently.<ref name="MBOC"/>{{rp|7.2}} [[Eukaryote|Eukaryotic]] [[promoter (genetics)|promoter]] regions are much more complex and difficult to identify than [[prokaryote|prokaryotic]] promoters.<ref name="MBOC"/>{{rp|7.3}}

Additionally, genes can have regulatory regions many kilobases upstream or downstream of the gene that alter expression. These act by [[DNA binding site|binding]] to transcription factors which then cause the DNA to loop so that the regulatory sequence (and bound transcription factor) become close to the RNA polymerase binding site.<ref name="5questions">{{cite journal |vauthors=Pennacchio LA, Bickmore W, Dean A, Nobrega MA, Bejerano G |title=Enhancers: five essential questions |journal=Nature Reviews. Genetics |volume=14 |issue=4 |pages=288–95 |date=April 2013 |doi=10.1038/nrg3458 |pmc=4445073 |pmid=23503198}}</ref> For example, [[enhancer (genetics)|enhancers]] increase transcription by binding an [[activator (genetics)|activator]] protein which then helps to recruit the RNA polymerase to the promoter; conversely [[silencer (DNA)|silencers]] bind [[repressor]] proteins and make the DNA less available for RNA polymerase.<ref>{{cite journal |vauthors=Maston GA, Evans SK, Green MR |title=Transcriptional regulatory elements in the human genome |journal=Annual Review of Genomics and Human Genetics |volume=7 |pages=29–59 |year=2006 |doi=10.1146/annurev.genom.7.080505.115623 |doi-access=free |pmid=16719718}}</ref>

The mature messenger RNA produced from protein-coding genes contains [[untranslated region]]s at both ends which contain binding sites for [[ribosome binding site|ribosomes]], [[RNA-binding protein]]s, [[microRNA|miRNA]], as well as [[terminator (genetics)|terminator]], and [[start codon|start]] and [[stop codons]].<ref>{{cite journal |vauthors=Mignone F, Gissi C, Liuni S, Pesole G |title=Untranslated regions of mRNAs |journal=Genome Biology |volume=3 |issue=3 |pages=REVIEWS0004 |date=28 February 2002 |doi=10.1186/gb-2002-3-3-reviews0004 |doi-access=free |pmc=139023 |pmid=11897027}}</ref> In addition, most eukaryotic [[open reading frame]]s contain untranslated [[intron]]s, which are removed and [[exon]]s, which are connected together in a process known as [[RNA splicing]]. Finally, the ends of gene transcripts are defined by [[polyadenylation|cleavage and polyadenylation (CPA) sites]], where newly produced pre-mRNA gets cleaved and a string of ~200 adenosine monophosphates is added at the 3' end. The [[polyadenylation|poly(A)]] tail protects mature mRNA from degradation and has other functions, affecting translation, localization, and transport of the transcript from the nucleus. Splicing, followed by CPA, generate the final [[mature messenger RNA|mature mRNA]], which encodes the protein or RNA product.<ref>{{cite journal |vauthors=Bicknell AA, Cenik C, Chua HN, Roth FP, Moore MJ |title=Introns in UTRs: why we should stop ignoring them |journal=BioEssays |volume=34 |issue=12 |pages=1025–34 |date=December 2012 |doi=10.1002/bies.201200073 |doi-access=free |pmid=23108796 |s2cid=5808466}}</ref>

Many noncoding genes in eukaryotes have different transcription termination mechanisms and they do not have poly(A) tails.

Many prokaryotic genes are organized into [[operon]]s, with multiple protein-coding sequences that are transcribed as a unit.<ref>{{cite journal |vauthors=Salgado H, Moreno-Hagelsieb G, Smith TF, Collado-Vides J |title=Operons in Escherichia coli: genomic analyses and predictions |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=97 |issue=12 |pages=6652–7 |date=June 2000 |bibcode=2000PNAS...97.6652S |doi=10.1073/pnas.110147297 |doi-access=free |pmc=18690 |pmid=10823905}}</ref><ref>{{cite journal |vauthors=Blumenthal T |title=Operons in eukaryotes |journal=Briefings in Functional Genomics & Proteomics |volume=3 |issue=3 |pages=199–211 |date=November 2004 |doi=10.1093/bfgp/3.3.199 |doi-access=free |pmid=15642184}}</ref> The genes in an [[operon]] are transcribed as a continuous [[messenger RNA]], referred to as a [[messenger RNA#Monocistronic versus polycistronic mRNA|polycistronic mRNA]]. The term [[cistron]] in this context is equivalent to gene. The transcription of an operon's mRNA is often controlled by a [[repressor]] that can occur in an active or inactive state depending on the presence of specific metabolites.<ref name="pmid13718526">{{cite journal |vauthors=Jacob F, Monod J |title=Genetic regulatory mechanisms in the synthesis of proteins |journal=Journal of Molecular Biology |volume=3 |issue=3 |pages=318–56 |date=June 1961 |doi=10.1016/S0022-2836(61)80072-7 |pmid=13718526 |s2cid=19804795}}</ref> When active, the repressor binds to a DNA sequence at the beginning of the operon, called the [[operon#Operator|operator region]], and represses [[transcription (genetics)|transcription]] of the [[operon]]; when the repressor is inactive transcription of the operon can occur (see e.g. [[Lac operon]]). The products of operon genes typically have related functions and are involved in the same [[gene regulatory network|regulatory network]].<ref name="MBOC"/>{{rp|7.3}}

===Complexity===
Though many genes have simple structures, as with much of biology, others can be quite complex or represent unusual edge-cases. Eukaryotic genes often have introns that are much larger than their exons,<ref>{{cite journal |vauthors=Pozzoli U, Menozzi G, Comi GP, Cagliani R, Bresolin N, Sironi M |title=Intron size in mammals: complexity comes to terms with economy |journal=Trends in Genetics |volume=23 |issue=1 |pages=20–24 |date=January 2007 |doi=10.1016/j.tig.2006.10.003 |pmid=17070957}}</ref><ref>{{cite journal |vauthors=Marais G, Nouvellet P, Keightley PD, Charlesworth B |title=Intron size and exon evolution in Drosophila |journal=Genetics |volume=170 |issue=1 |pages=481–485 |date=May 2005 |doi=10.1534/genetics.104.037333 |pmc=1449718 |pmid=15781704}}</ref> and those introns can even have other genes [[nested gene|nested inside them]].<ref name="Kumar">{{cite journal |vauthors=Kumar A |title=An overview of nested genes in eukaryotic genomes |journal=Eukaryotic Cell |volume=8 |issue=9 |pages=1321–1329 |date=September 2009 |doi=10.1128/EC.00143-09 |pmc=2747821 |pmid=19542305}}.</ref> Associated enhancers may be many kilobase away, or even on entirely different chromosomes operating via physical contact between two chromosomes.<ref>{{cite journal |vauthors=Spilianakis CG, Lalioti MD, Town T, Lee GR, Flavell RA |title=Interchromosomal associations between alternatively expressed loci |journal=Nature |volume=435 |issue=7042 |pages=637–645 |date=June 2005 |bibcode=2005Natur.435..637S |doi=10.1038/nature03574 |pmid=15880101 |s2cid=1755326}}</ref><ref>{{cite journal |vauthors=Williams A, Spilianakis CG, Flavell RA |title=Interchromosomal association and gene regulation in trans |journal=Trends in Genetics |volume=26 |issue=4 |pages=188–197 |date=April 2010 |doi=10.1016/j.tig.2010.01.007 |pmc=2865229 |pmid=20236724}}</ref> A single gene can encode multiple different functional products by [[alternative splicing]], and conversely a gene may be split across chromosomes but those transcripts are concatenated back together into a functional sequence by [[trans-splicing]].<ref name="Lei2016">{{cite journal |vauthors=Lei Q, Li C, Zuo Z, Huang C, Cheng H, Zhou R |title=Evolutionary Insights into RNA trans-Splicing in Vertebrates |journal=Genome Biology and Evolution |volume=8 |issue=3 |pages=562–577 |date=March 2016 |doi=10.1093/gbe/evw025 |pmc=4824033 |pmid=26966239}}</ref> It is also possible for [[overlapping gene]]s to share some of their DNA sequence, either on opposite strands or the same strand (in a different reading frame, or even the same reading frame).<ref name="Wright_2021">{{cite journal |vauthors=Wright BW, Molloy MP, Jaschke PR |title=Overlapping genes in natural and engineered genomes |journal=Nature Reviews. Genetics |volume=23 |issue=3 |pages=154–168 |date=March 2022 |doi=10.1038/s41576-021-00417-w |pmc=8490965 |pmid=34611352}}</ref>

==Gene expression==
{{Main|Gene expression}}

In all organisms, two steps are required to read the information encoded in a gene's DNA and produce the protein it specifies. First, the gene's DNA is ''[[transcription (genetics)|transcribed]]'' to messenger RNA ([[mRNA]]).<ref name="MBOC"/>{{rp|6.1}} Second, that mRNA is ''[[translation (genetics)|translated]]'' to protein.<ref name="MBOC"/>{{rp|6.2}} RNA-coding genes must still go through the first step, but are not translated into protein.<ref name="Edd01"/> The process of producing a biologically functional molecule of either RNA or protein is called [[gene expression]], and the resulting molecule is called a [[gene product]].

===Genetic code===
[[File:RNA-codons-aminoacids.svg|class=skin-invert-image|thumb|upright=1.5|Schematic of a single-stranded RNA molecule illustrating a series of three-base [[codon]]s. Each three-[[nucleotide]] codon corresponds to an [[amino acid]] when translated to protein.|alt=An RNA molecule consisting of nucleotides. Groups of three nucleotides are indicated as codons, with each corresponding to a specific amino acid.]]

The nucleotide sequence of a gene's DNA specifies the amino acid sequence of a protein through the [[genetic code]]. Sets of three nucleotides, known as [[codon]]s, each correspond to a specific amino acid.<ref name="MBOC"/>{{Rp|6}} The principle that three sequential bases of DNA code for each amino acid was demonstrated in 1961 using frameshift mutations in the rIIB gene of bacteriophage T4<ref name="pmid13882203">{{cite journal |vauthors=Crick FH, Barnett L, Brenner S, Watts-Tobin RJ |title=General nature of the genetic code for proteins |journal=Nature |volume=192 |issue=4809 |pages=1227–32 |date=December 1961 |bibcode=1961Natur.192.1227C |doi=10.1038/1921227a0 |pmid=13882203 |s2cid=4276146}}</ref> (see [[Crick, Brenner et al. experiment]]).

Additionally, a "[[start codon]]", and three "[[stop codon]]s" indicate the beginning and end of the [[coding region|protein coding region]]. There are 64&nbsp;possible codons (four possible nucleotides at each of three positions, hence 4<sup>3</sup>&nbsp;possible codons) and only 20&nbsp;standard amino acids; hence the code is redundant and multiple codons can specify the same amino acid. The correspondence between codons and amino acids is nearly universal among all known living organisms.<ref>{{cite journal |vauthors=Crick FH |title=The genetic code |journal=Scientific American |volume=207 |issue=4 |pages=66–74 |date=October 1962 |publisher=WH Freeman and Company |bibcode=1962SciAm.207d..66C |doi=10.1038/scientificamerican1062-66 |pmid=13882204 }}</ref>

===Transcription===
[[Transcription (genetics)|Transcription]] produces a single-stranded [[RNA]] molecule known as [[messenger RNA]], whose nucleotide sequence is complementary to the DNA from which it was transcribed.<ref name="MBOC"/>{{rp|6.1}} The mRNA acts as an intermediate between the DNA gene and its final protein product. The gene's DNA is used as a template to generate a [[base pair|complementary]] mRNA. The mRNA matches the sequence of the gene's DNA [[coding strand]] because it is synthesised as the complement of the [[template strand]]. Transcription is performed by an [[enzyme]] called an [[RNA polymerase]], which reads the template strand in the [[3' end|3']] to [[5' end|5']]&nbsp;direction and synthesizes the RNA from [[5' end|5']] to [[3' end|3']]. To initiate transcription, the polymerase first recognizes and binds a [[promoter (biology)|promoter]] region of the gene. Thus, a major mechanism of [[gene regulation]] is the blocking or sequestering the promoter region, either by tight binding by [[repressor]] molecules that physically block the polymerase or by organizing the DNA so that the promoter region is not accessible.<ref name="MBOC"/>{{rp|7}}

In [[prokaryote]]s, transcription occurs in the [[cytoplasm]]; for very long transcripts, translation may begin at the 5'&nbsp;end of the RNA while the 3'&nbsp;end is still being transcribed. In [[eukaryote]]s, transcription occurs in the nucleus, where the cell's DNA is stored. The RNA molecule produced by the polymerase is known as the [[primary transcript]] and undergoes [[post-transcriptional modification]]s before being exported to the cytoplasm for translation. One of the modifications performed is the [[splicing (genetics)|splicing]] of [[intron]]s which are sequences in the transcribed region that do not encode a protein. [[Alternative splicing]] mechanisms can result in mature transcripts from the same gene having different sequences and thus coding for different proteins. This is a major form of regulation in eukaryotic cells and also occurs in some prokaryotes.<ref name="MBOC"/>{{rp|7.5}}<ref>{{cite journal |vauthors=Woodson SA |title=Ironing out the kinks: splicing and translation in bacteria |journal=Genes & Development |volume=12 |issue=9 |pages=1243–7 |date=May 1998 |doi=10.1101/gad.12.9.1243 |doi-access=free |pmid=9573040}}</ref>

===Translation===
[[File:DNA to protein or ncRNA.svg|thumb|upright=1.5|Protein coding genes are transcribed to an [[mRNA]] intermediate, then translated to a functional [[protein]]. RNA-coding genes are transcribed to a functional [[non-coding RNA]] ({{PDB|3BSE|1OBB|3TRA}}).|alt=A protein-coding gene in DNA being transcribed and translated to a functional protein or a non-protein-coding gene being transcribed to a functional RNA]]
[[Translation (genetics)|Translation]] is the process by which a [[mature messenger RNA|mature mRNA]] molecule is used as a template for synthesizing a new [[protein]].<ref name="MBOC"/>{{rp|6.2}} Translation is carried out by [[ribosome]]s, large complexes of RNA and protein responsible for carrying out the chemical reactions to add new [[amino acid]]s to a growing [[polypeptide chain]] by the formation of [[peptide bond]]s. The genetic code is read three nucleotides at a time, in units called [[codon]]s, via interactions with specialized RNA molecules called [[transfer RNA]] (tRNA). Each tRNA has three unpaired bases known as the [[anticodon]] that are complementary to the codon it reads on the mRNA. The tRNA is also [[covalent]]ly attached to the [[amino acid]] specified by the complementary codon. When the tRNA binds to its complementary codon in an mRNA strand, the ribosome attaches its amino acid cargo to the new polypeptide chain, which is synthesized from [[N-terminus|amino terminus]] to [[C-terminus|carboxyl terminus]]. During and after synthesis, most new proteins must [[protein folding|fold]] to their active [[tertiary structure|three-dimensional structure]] before they can carry out their cellular functions.<ref name="MBOC"/>{{rp|3}}

===Regulation===
[[Regulation of gene expression|Genes are regulated]] so that they are [[gene expression|expressed]] only when the product is needed, since expression draws on limited resources.<ref name="MBOC"/>{{rp|7}} A cell regulates its gene expression depending on its [[environment (biophysical)|external environment]] (e.g. [[nutrient|available nutrients]], [[heat shock protein|temperature]] and other [[cellular stress response|stresses]]), its internal environment (e.g. [[cell division cycle]], [[metabolism]], [[infection|infection status]]), and its [[Cellular differentiation|specific role]] if in a [[multicellular]] organism. Gene expression can be regulated at any step: from [[transcriptional regulation|transcriptional initiation]], to [[RNA processing]], to [[post-translational modification]] of the protein. The regulation of [[lactose]] metabolism genes in ''[[E. coli]]'' ([[lac operon|''lac'' operon]]) was the first such mechanism to be described in 1961.<ref>{{cite journal |vauthors=Jacob F, Monod J |title=Genetic regulatory mechanisms in the synthesis of proteins |journal=Journal of Molecular Biology |volume=3 |issue=3 |pages=318–56 |date=June 1961 |author-link=François Jacob |author-link2=Jacques Monod |doi=10.1016/S0022-2836(61)80072-7 |pmid=13718526 |s2cid=19804795}}</ref>

===RNA genes===
A typical protein-coding gene is first copied into [[RNA]] as an intermediate in the manufacture of the final protein product.<ref name="MBOC"/>{{rp|6.1}} In other cases, the RNA molecules are the actual functional products, as in the synthesis of [[ribosomal RNA]] and [[transfer RNA]]. Some RNAs known as [[ribozyme]]s are capable of [[enzyme|enzymatic function]], while others such as [[microRNA]]s and [[riboswitch]]es have regulatory roles. The [[DNA]] sequences from which such RNAs are transcribed are known as [[non-coding RNA|non-coding RNA genes]].<ref name="Edd01">{{cite journal |vauthors=Eddy SR |title=Non-coding RNA genes and the modern RNA world |journal=Nature Reviews. Genetics |volume=2 |issue=12 |pages=919–29 |date=December 2001 |doi=10.1038/35103511 |pmid=11733745 |s2cid=18347629}}</ref>

Some [[virus]]es store their entire genomes in the form of [[RNA]], and contain no DNA at all.<ref>{{cite journal |vauthors=Koonin EV, Dolja VV |title=Evolution and taxonomy of positive-strand RNA viruses: implications of comparative analysis of amino acid sequences |journal=Critical Reviews in Biochemistry and Molecular Biology |volume=28 |issue=5 |pages=375–430 |date=January 1993 |doi=10.3109/10409239309078440 |pmid=8269709}}</ref><ref>{{cite journal |vauthors=Domingo E |title=RNA Virus Genomes |journal=eLS |date=2001 |isbn=978-0470016176 |doi=10.1002/9780470015902.a0001488.pub2}}</ref> Because they use RNA to store genes, their [[cell (biology)|cellular]] [[host (biology)|hosts]] may synthesize their proteins as soon as they are [[infection|infected]] and without the delay in waiting for transcription.<ref>{{cite journal |vauthors=Domingo E, Escarmís C, Sevilla N, Moya A, Elena SF, Quer J, Novella IS, Holland JJ |display-authors=6 |title=Basic concepts in RNA virus evolution |journal=FASEB Journal |volume=10 |issue=8 |pages=859–64 |date=June 1996 |doi=10.1096/fasebj.10.8.8666162 |doi-access=free |pmid=8666162 |s2cid=20865732}}</ref> On the other hand, RNA [[retrovirus]]es, such as [[HIV]], require the [[reverse transcription]] of their [[genome]] from RNA into DNA before their proteins can be synthesized.

==Inheritance==
[[File: Autosomal recessive - mini.svg|class=skin-invert-image|thumb|Inheritance of a gene that has two different [[allele]]s (blue and white). The gene is located on an [[autosomal chromosome]]. The white allele is [[recessive]] to the blue allele. The probability of each outcome in the children's generation is one quarter, or 25 percent.|alt=Illustration of autosomal recessive inheritance. Each parent has one blue allele and one white allele. Each of their 4 children inherit one allele from each parent such that one child ends up with two blue alleles, one child has two white alleles and two children have one of each allele. Only the child with both blue alleles shows the trait because the trait is recessive.]]{{Main|Mendelian inheritance|Heredity}}Organisms inherit their genes from their parents. [[Asexual reproduction|Asexual]] organisms simply inherit a complete copy of their parent's genome. [[Sexual reproduction|Sexual]] organisms have two copies of each chromosome because they inherit one complete set from each parent.<ref name="MBOC"/>{{rp|1}}

===Mendelian inheritance===
According to [[Mendelian inheritance]], variations in an organism's [[phenotype]] (observable physical and behavioral characteristics) are due in part to variations in its [[genotype]] (particular set of genes). Each gene specifies a particular trait with a different sequence of a gene ([[allele]]s) giving rise to different phenotypes. Most eukaryotic organisms (such as the pea plants Mendel worked on) have two alleles for each trait, one inherited from each parent.<ref name="MBOC"/>{{rp|20}}

Alleles at a locus may be [[dominant gene|dominant]] or [[recessive gene|recessive]]; dominant alleles give rise to their corresponding phenotypes when paired with any other allele for the same trait, whereas recessive alleles give rise to their corresponding phenotype only when paired with another copy of the same allele. If you know the genotypes of the organisms, you can determine which alleles are dominant and which are recessive. For example, if the allele specifying tall stems in pea plants is dominant over the allele specifying short stems, then pea plants that inherit one tall allele from one parent and one short allele from the other parent will also have tall stems. Mendel's work demonstrated that alleles assort independently in the production of [[gamete]]s, or [[germ cell]]s, ensuring variation in the next generation. Although Mendelian inheritance remains a good model for many traits determined by single genes (including a number of well-known [[genetic disorder#Single-gene disorder|genetic disorders]]) it does not include the physical processes of DNA replication and cell division.<ref name=scitable_miko>{{cite journal |vauthors=Miko I |title=Gregor Mendel and the Principles of Inheritance |journal=Nature Education Knowledge |date=2008 |volume=1 |issue=1 |page=134 |series=SciTable |publisher=Nature Publishing Group |url=https://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593/ |access-date=2 May 2024 |archive-date=19 July 2019 |archive-url=https://web.archive.org/web/20190719224056/http://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593 |url-status=live }}</ref><ref name=scitable_chial>{{cite journal |vauthors=Chial H |title=Mendelian Genetics: Patterns of Inheritance and Single-Gene Disorders |journal=Nature Education Knowledge |date=2008 |volume=1 |issue=1 |page=63 |series=SciTable |publisher=Nature Publishing Group |url=https://www.nature.com/scitable/topicpage/mendelian-genetics-patterns-of-inheritance-and-single-966/ |access-date=2 May 2024 |archive-date=31 March 2019 |archive-url=https://web.archive.org/web/20190331174639/http://www.nature.com/scitable/topicpage/mendelian-genetics-patterns-of-inheritance-and-single-966 |url-status=live }}</ref>

===DNA replication and cell division===
The growth, development, and reproduction of organisms relies on [[cell division]]; the process by which a single [[cell (biology)|cell]] divides into two usually identical [[daughter cell]]s. This requires first making a duplicate copy of every gene in the [[genome]] in a process called [[DNA replication]].<ref name="MBOC"/>{{rp|5.2}} The copies are made by specialized [[enzyme]]s known as [[DNA polymerase]]s, which "read" one strand of the double-helical DNA, known as the template strand, and synthesize a new complementary strand. Because the DNA double helix is held together by [[base pair]]ing, the sequence of one strand completely specifies the sequence of its complement; hence only one strand needs to be read by the enzyme to produce a faithful copy. The process of DNA replication is [[semiconservative replication|semiconservative]]; that is, the copy of the genome inherited by each daughter cell contains one original and one newly synthesized strand of DNA.<ref name="MBOC"/>{{rp|5.2}}

The rate of DNA replication in living cells was first measured as the rate of phage T4 DNA elongation in phage-infected ''E. coli'' and found to be impressively rapid.<ref name="pmid789903">{{cite journal |vauthors=McCarthy D, Minner C, Bernstein H, Bernstein C |title=DNA elongation rates and growing point distributions of wild-type phage T4 and a DNA-delay amber mutant |journal=Journal of Molecular Biology |volume=106 |issue=4 |pages=963–81 |date=October 1976 |doi=10.1016/0022-2836(76)90346-6 |pmid=789903}}</ref> During the period of exponential DNA increase at 37&nbsp;°C, the rate of elongation was 749 nucleotides per second.

After DNA replication, the cell must physically separate the two genome copies and divide into two distinct membrane-bound cells.<ref name="MBOC"/>{{rp|18.2}} In [[prokaryote]]s&nbsp;([[bacteria]] and [[archaea]]) this usually occurs via a relatively simple process called [[binary fission]], in which each circular genome attaches to the [[cell membrane]] and is separated into the daughter cells as the membrane [[invagination|invaginates]] to split the [[cytoplasm]] into two membrane-bound portions. Binary fission is extremely fast compared to the rates of cell division in [[eukaryote]]s. Eukaryotic cell division is a more complex process known as the [[cell cycle]]; DNA replication occurs during a phase of this cycle known as [[S phase]], whereas the process of segregating [[chromosome]]s and splitting the [[cytoplasm]] occurs during [[M phase]].<ref name="MBOC"/>{{rp|18.1}}

===Molecular inheritance===
The duplication and transmission of genetic material from one generation of cells to the next is the basis for molecular inheritance and the link between the classical and molecular pictures of genes. Organisms inherit the characteristics of their parents because the cells of the offspring contain copies of the genes in their parents' cells. In [[asexual reproduction|asexually reproducing]] organisms, the offspring will be a genetic copy or [[clone (genetics)|clone]] of the parent organism. In [[sexual reproduction|sexually reproducing]] organisms, a specialized form of cell division called [[meiosis]] produces cells called [[gamete]]s or [[germ cell]]s that are [[haploid]], or contain only one copy of each gene.<ref name="MBOC"/>{{rp|20.2}} The gametes produced by females are called [[egg (biology)|eggs]] or ova, and those produced by males are called [[sperm]]. Two gametes fuse to form a [[diploid]] [[fertilized egg]], a single cell that has two sets of genes, with one copy of each gene from the mother and one from the father.<ref name="MBOC"/>{{rp|20}}

During the process of meiotic cell division, an event called [[genetic recombination]] or ''crossing-over'' can sometimes occur, in which a length of DNA on one [[chromatid]] is swapped with a length of DNA on the corresponding homologous non-sister chromatid. This can result in reassortment of otherwise linked alleles.<ref name="MBOC"/>{{rp|5.5}} The Mendelian principle of independent assortment asserts that each of a parent's two genes for each trait will sort independently into gametes; which allele an organism inherits for one trait is unrelated to which allele it inherits for another trait. This is in fact only true for genes that do not reside on the same chromosome or are located very far from one another on the same chromosome. The closer two genes lie on the same chromosome, the more closely they will be associated in gametes and the more often they will appear together (known as [[genetic linkage]]).<ref name="scitable_lobo">{{cite journal |vauthors=Lobo I, Shaw K |date=2008 |title=Discovery and Types of Genetic Linkage |journal=Nature Education Knowledge |series=SciTable |publisher=Nature Publishing Group |volume=1 |issue=1 |page=139 |url=https://www.nature.com/scitable/topicpage/discovery-and-types-of-genetic-linkage-500/ |access-date=2 May 2024 |archive-date=12 May 2020 |archive-url=https://web.archive.org/web/20200512071347/https://www.nature.com/scitable/topicpage/discovery-and-types-of-genetic-linkage-500/ |url-status=live }}</ref> Genes that are very close are essentially never separated because it is extremely unlikely that a crossover point will occur between them.<ref name="scitable_lobo"/>

==Genome==
The [[genome]] is the total genetic material of an organism and includes both the genes and [[non-coding sequence]]s.<ref>Ridley, M. (2006). ''Genome''. New York, NY: Harper Perennial. {{ISBN|0-06-019497-9}}</ref> Eukaryotic genes can be annotated using FINDER.<ref name="pmid33879057">{{cite journal |vauthors=Banerjee S, Bhandary P, Woodhouse M, Sen TZ, Wise RP, Andorf CM |title=FINDER: an automated software package to annotate eukaryotic genes from RNA-Seq data and associated protein sequences |journal=BMC Bioinformatics |volume=44 |issue=9 |pages=e89 |date=Apr 2021 |doi=10.1186/s12859-021-04120-9 |doi-access=free |pmc=8056616 |pmid=33879057}}</ref>

===Number of genes===
[[File:Gene numbers.svg|class=skin-invert-image|thumb|upright=2.55|Depiction of numbers of genes for representative [[plant]]s (green), [[vertebrate]]s (blue), [[invertebrate]]s (orange), [[fungi]] (yellow), [[bacteria]] (purple), and [[virus]]es (grey). An inset on the right shows the smaller genomes expanded 100-fold area-wise.<ref name="WatsonJD">Watson, JD, Baker TA, Bell SP, Gann A, Levine M, Losick R. (2004). "Ch9-10", Molecular Biology of the Gene, 5th ed., Peason Benjamin Cummings; CSHL Press.</ref><ref>{{cite web |title=Integr8 – A.thaliana Genome Statistics |url=http://www.ebi.ac.uk/integr8/OrganismStatsAction.do;jsessionid=08E9058B5B688A4F7FF7D161CB9E36A4?orgProteomeId=3 |access-date=26 April 2015 |archive-date=23 May 2013 |archive-url=https://web.archive.org/web/20130523210340/http://www.ebi.ac.uk/integr8/OrganismStatsAction.do;jsessionid=08E9058B5B688A4F7FF7D161CB9E36A4?orgProteomeId=3 |url-status=live }}</ref><ref>{{cite web |title=Understanding the Basics |website=The Human Genome Project |url=http://web.ornl.gov/sci/techresources/Human_Genome/project/info.shtml |access-date=26 April 2015}}</ref><ref>{{cite web |date=10 August 2011 |title=WS227 Release Letter |publisher=WormBase |url=http://www.wormbase.org/wiki/index.php/WS227 |access-date=19 November 2013 |url-status=dead |archive-url=https://archive.today/20131128221823/http://www.wormbase.org/wiki/index.php/WS227 |archive-date=28 November 2013}}</ref><ref name="pmid11935017"/><ref name=mitochondrial_genome>{{cite journal |vauthors=Anderson S, Bankier AT, Barrell BG, de Bruijn MH, Coulson AR, Drouin J, Eperon IC, Nierlich DP, Roe BA, Sanger F, Schreier PH, Smith AJ, Staden R, Young IG |display-authors=6 |title=Sequence and organization of the human mitochondrial genome |journal=Nature |volume=290 |issue=5806 |pages=457–65 |date=April 1981 |bibcode=1981Natur.290..457A |doi=10.1038/290457a0 |pmid=7219534 |s2cid=4355527}}</ref><ref>{{cite journal |vauthors=Adams MD, Celniker SE, Holt RA, Evans CA, Gocayne JD, Amanatides PG, Scherer SE, Li PW, Hoskins RA, Galle RF, George RA, Lewis SE, Richards S, Ashburner M, Henderson SN, Sutton GG, Wortman JR, Yandell MD, Zhang Q, Chen LX, Brandon RC, Rogers YH, Blazej RG, Champe M, Pfeiffer BD, Wan KH, Doyle C, Baxter EG, Helt G, Nelson CR, Gabor GL, Abril JF, Agbayani A, An HJ, Andrews-Pfannkoch C, Baldwin D, Ballew RM, Basu A, Baxendale J, Bayraktaroglu L, Beasley EM, Beeson KY, Benos PV, Berman BP, Bhandari D, Bolshakov S, Borkova D, Botchan MR, Bouck J, Brokstein P, Brottier P, Burtis KC, Busam DA, Butler H, Cadieu E, Center A, Chandra I, Cherry JM, Cawley S, Dahlke C, Davenport LB, Davies P, de Pablos B, Delcher A, Deng Z, Mays AD, Dew I, Dietz SM, Dodson K, Doup LE, Downes M, Dugan-Rocha S, Dunkov BC, Dunn P, Durbin KJ, Evangelista CC, Ferraz C, Ferriera S, Fleischmann W, Fosler C, Gabrielian AE, Garg NS, Gelbart WM, Glasser K, Glodek A, Gong F, Gorrell JH, Gu Z, Guan P, Harris M, Harris NL, Harvey D, Heiman TJ, Hernandez JR, Houck J, Hostin D, Houston KA, Howland TJ, Wei MH, Ibegwam C, Jalali M, Kalush F, Karpen GH, Ke Z, Kennison JA, Ketchum KA, Kimmel BE, Kodira CD, Kraft C, Kravitz S, Kulp D, Lai Z, Lasko P, Lei Y, Levitsky AA, Li J, Li Z, Liang Y, Lin X, Liu X, Mattei B, McIntosh TC, McLeod MP, McPherson D, Merkulov G, Milshina NV, Mobarry C, Morris J, Moshrefi A, Mount SM, Moy M, Murphy B, Murphy L, Muzny DM, Nelson DL, Nelson DR, Nelson KA, Nixon K, Nusskern DR, Pacleb JM, Palazzolo M, Pittman GS, Pan S, Pollard J, Puri V, Reese MG, Reinert K, Remington K, Saunders RD, Scheeler F, Shen H, Shue BC, Sidén-Kiamos I, Simpson M, Skupski MP, Smith T, Spier E, Spradling AC, Stapleton M, Strong R, Sun E, Svirskas R, Tector C, Turner R, Venter E, Wang AH, Wang X, Wang ZY, Wassarman DA, Weinstock GM, Weissenbach J, Williams SM, Worley KC, Wu D, Yang S, Yao QA, Ye J, Yeh RF, Zaveri JS, Zhan M, Zhang G, Zhao Q, Zheng L, Zheng XH, Zhong FN, Zhong W, Zhou X, Zhu S, Zhu X, Smith HO, Gibbs RA, Myers EW, Rubin GM, Venter JC |display-authors=6 |title=The genome sequence of Drosophila melanogaster |journal=Science |volume=287 |issue=5461 |pages=2185–95 |date=March 2000 |citeseerx=10.1.1.549.8639 |bibcode=2000Sci...287.2185. |doi=10.1126/science.287.5461.2185 |pmid=10731132}}</ref><ref name="ReferenceB">{{cite journal |vauthors=Pertea M, Salzberg SL |title=Between a chicken and a grape: estimating the number of human genes |journal=Genome Biology |volume=11 |issue=5 |pages=206 |date=2010 |doi=10.1186/gb-2010-11-5-206 |doi-access=free |pmc=2898077 |pmid=20441615}}</ref>]]

The [[genome size]], and the number of genes it encodes varies widely between organisms. The smallest genomes occur in [[virus]]es,<ref>{{cite journal |vauthors=Belyi VA, Levine AJ, Skalka AM |title=Sequences from ancestral single-stranded DNA viruses in vertebrate genomes: the parvoviridae and circoviridae are more than 40 to 50 million years old |journal=Journal of Virology |volume=84 |issue=23 |pages=12458–62 |date=December 2010 |doi=10.1128/JVI.01789-10 |pmc=2976387 |pmid=20861255}}</ref> and [[viroid]]s (which act as a single non-coding RNA gene).<ref>{{cite journal |vauthors=Flores R, Di Serio F, Hernández C |title=Viroids: The Noncoding Genomes |journal=Seminars in Virology |date=February 1997 |volume=8 |issue=1 |pages=65–73 |doi=10.1006/smvy.1997.0107}}</ref> Conversely, plants can have extremely large genomes,<ref>{{cite journal |vauthors=Zonneveld BJ |title=New Record Holders for Maximum Genome Size in Eudicots and Monocots |journal=Journal of Botany |date=2010 |volume=2010 |pages=1–4 |doi=10.1155/2010/527357 |doi-access=free}}</ref> with [[Oryza sativa|rice]] containing >46,000 protein-coding genes.<ref name="pmid11935017">{{cite journal |vauthors=Yu J, Hu S, Wang J, Wong GK, Li S, Liu B, Deng Y, Dai L, Zhou Y, Zhang X, Cao M, Liu J, Sun J, Tang J, Chen Y, Huang X, Lin W, Ye C, Tong W, Cong L, Geng J, Han Y, Li L, Li W, Hu G, Huang X, Li W, Li J, Liu Z, Li L, Liu J, Qi Q, Liu J, Li L, Li T, Wang X, Lu H, Wu T, Zhu M, Ni P, Han H, Dong W, Ren X, Feng X, Cui P, Li X, Wang H, Xu X, Zhai W, Xu Z, Zhang J, He S, Zhang J, Xu J, Zhang K, Zheng X, Dong J, Zeng W, Tao L, Ye J, Tan J, Ren X, Chen X, He J, Liu D, Tian W, Tian C, Xia H, Bao Q, Li G, Gao H, Cao T, Wang J, Zhao W, Li P, Chen W, Wang X, Zhang Y, Hu J, Wang J, Liu S, Yang J, Zhang G, Xiong Y, Li Z, Mao L, Zhou C, Zhu Z, Chen R, Hao B, Zheng W, Chen S, Guo W, Li G, Liu S, Tao M, Wang J, Zhu L, Yuan L, Yang H |display-authors=6 |title=A draft sequence of the rice genome (Oryza sativa L. ssp. indica) |journal=Science |volume=296 |issue=5565 |pages=79–92 |date=April 2002 |bibcode=2002Sci...296...79Y |doi=10.1126/science.1068037 |pmid=11935017 |s2cid=208529258}}</ref> The total number of protein-coding genes (the Earth's [[proteome]]) is estimated to be 5&nbsp;million sequences.<ref>{{cite journal |vauthors=Perez-Iratxeta C, Palidwor G, Andrade-Navarro MA |title=Towards completion of the Earth's proteome |journal=EMBO Reports |volume=8 |issue=12 |pages=1135–41 |date=December 2007 |doi=10.1038/sj.embor.7401117 |pmc=2267224 |pmid=18059312}}</ref>

Although the number of base-pairs of DNA in the human genome has been known since the 1950s, the estimated number of genes has changed over time as definitions of genes, and methods of detecting them have been refined. Initial theoretical predictions of the number of human genes in the 1960s and 1970s were based on mutation load estimates and the numbers of mRNAs and these estimates tended to be about 30,000 protein-coding genes.<ref>{{cite journal |vauthors=Muller HJ |title=The gene material as the initiator and the organizing basis of life |date=1966 |journal=American Naturalist |volume=100 |issue=915 |pages=493–517 |doi=10.1086/282445 |jstor=2459205 |bibcode=1966ANat..100..493M }}</ref><ref>{{cite journal |vauthors=Ohno S |date=1972 |title=So much "junk" DNA in our genome |journal=Brookhaven Symposia in Biology |volume=23 |pages=366–370 |pmid=5065367}}</ref><ref>{{cite journal |vauthors=Hatje K, Mühlhausen S, Simm D, Killmar M |title=The Protein-Coding Human Genome: Annotating High-Hanging Fruits. |year=2019 |journal=BioEssays |volume=41 |issue=11 |pages=1900066 |doi=10.1002/bies.201900066 |doi-access=free |pmid=31544971 |s2cid=202732556}}</ref> During the 1990s there were guesstimates of up to 100,000 genes and early data on detection of mRNAs ([[expressed sequence tag]]s) suggested more than the traditional value of 30,000 genes that had been reported in the textbooks during the 1980s.<ref>{{cite journal |vauthors=Schuler GD, [[Mark Boguski |Boguski MS]], Stewart EA, Stein LD, Gyapay G, Rice K, White RE, Rodriguez-Tomé P, Aggarwal A, Bajorek E, Bentolila S, Birren BB, Butler A, Castle AB, Chiannilkulchai N, Chu A, Clee C, Cowles S, Day PJ, Dibling T, Drouot N, Dunham I, Duprat S, East C, Edwards C, Fan JB, Fang N, Fizames C, Garrett C, Green L, Hadley D, Harris M, Harrison P, Brady S, Hicks A, Holloway E, Hui L, Hussain S, Louis-Dit-Sully C, Ma J, MacGilvery A, Mader C, Maratukulam A, Matise TC, McKusick KB, Morissette J, Mungall A, Muselet D, Nusbaum HC, Page DC, Peck A, Perkins S, Piercy M, Qin F, Quackenbush J, Ranby S, Reif T, Rozen S, Sanders C, She X, Silva J, Slonim DK, Soderlund C, Sun WL, Tabar P, Thangarajah T, Vega-Czarny N, Vollrath D, Voyticky S, Wilmer T, Wu X, Adams MD, Auffray C, Walter NA, Brandon R, Dehejia A, Goodfellow PN, Houlgatte R, Hudson JR, Ide SE, Iorio KR, Lee WY, Seki N, Nagase T, Ishikawa K, Nomura N, Phillips C, Polymeropoulos MH, Sandusky M, Schmitt K, Berry R, Swanson K, Torres R, Venter JC, Sikela JM, Beckmann JS, Weissenbach J, Myers RM, Cox DR, James MR, Bentley D, Deloukas P, Lander ES, Hudson TJ |display-authors=6 |title=A gene map of the human genome |journal=Science |volume=274 |issue=5287 |pages=540–6 |date=October 1996 |bibcode=1996Sci...274..540S |doi=10.1126/science.274.5287.540 |pmid=8849440 |s2cid=22619}}</ref>

The initial draft sequences of the human genome confirmed the earlier predictions of about 30,000 protein-coding genes however that estimate has fallen to about 19,000 with the ongoing [[GENCODE]] annotation project.<ref>{{cite journal |vauthors=Chi KR |title=The dark side of the human genome |language=En |journal=Nature |volume=538 |issue=7624 |pages=275–277 |date=October 2016 |bibcode=2016Natur.538..275C |doi=10.1038/538275a |doi-access=free |pmid=27734873}}</ref> The number of noncoding genes is not known with certainty but the latest estimates from Ensembl suggest 26,000 noncoding genes.<ref>{{cite web |title=Human assembly and gene annotation |date=2022 |website=Ensembl |url=https://useast.ensembl.org/Homo_sapiens/Info/Annotation |access-date=28 February 2023 |archive-date=3 November 2023 |archive-url=https://web.archive.org/web/20231103222302/https://useast.ensembl.org/Homo_sapiens/Info/Annotation |url-status=live }}</ref>

===Essential genes===
{{Main|Essential gene}}

[[File:Syn3 genome.svg|class=skin-invert-image|thumb|upright=1.35|Gene functions in the minimal [[genome]] of the [[synthetic biology|synthetic organism]], ''[[Syn 3]]''<ref name="Hutchison"/>]]

Essential genes are the set of genes thought to be critical for an organism's survival.<ref>{{cite journal |vauthors=Glass JI, Assad-Garcia N, Alperovich N, Yooseph S, Lewis MR, Maruf M, Hutchison CA, Smith HO, Venter JC |display-authors=6 |title=Essential genes of a minimal bacterium |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=103 |issue=2 |pages=425–30 |date=January 2006 |bibcode=2006PNAS..103..425G |doi=10.1073/pnas.0510013103 |doi-access=free |pmc=1324956 |pmid=16407165}}</ref> This definition assumes the abundant availability of all relevant [[nutrient]]s and the absence of environmental stress. Only a small portion of an organism's genes are essential. In bacteria, an estimated 250–400 genes are essential for ''[[Escherichia coli]]'' and ''[[Bacillus subtilis]]'', which is less than 10% of their genes.<ref>{{cite journal |vauthors=Gerdes SY, Scholle MD, Campbell JW, Balázsi G, Ravasz E, Daugherty MD, Somera AL, Kyrpides NC, Anderson I, Gelfand MS, Bhattacharya A, Kapatral V, D'Souza M, Baev MV, Grechkin Y, Mseeh F, Fonstein MY, Overbeek R, Barabási AL, Oltvai ZN, Osterman AL |display-authors=6 |title=Experimental determination and system level analysis of essential genes in Escherichia coli MG1655 |journal=Journal of Bacteriology |volume=185 |issue=19 |pages=5673–84 |date=October 2003 |doi=10.1128/jb.185.19.5673-5684.2003 |pmc=193955 |pmid=13129938}}</ref><ref>{{cite journal |vauthors=Baba T, Ara T, Hasegawa M, Takai Y, Okumura Y, Baba M, Datsenko KA, Tomita M, Wanner BL, Mori H |display-authors=6 |title=Construction of Escherichia coli K-12 in-frame, single-gene knockout mutants: the Keio collection |journal=Molecular Systems Biology |volume=2 |pages=2006.0008 |date=2006 |doi=10.1038/msb4100050 |pmc=1681482 |pmid=16738554}}</ref><ref name=":0">{{cite journal |vauthors=Juhas M, Reuß DR, Zhu B, Commichau FM |title=Bacillus subtilis and Escherichia coli essential genes and minimal cell factories after one decade of genome engineering |journal=Microbiology |volume=160 |issue=Pt 11 |pages=2341–2351 |date=November 2014 |doi=10.1099/mic.0.079376-0 |doi-access=free |pmid=25092907}}</ref> Half of these genes are [[ortholog]]s in both organisms and are largely involved in [[protein synthesis]].<ref name=":0"/> In the budding yeast ''[[Saccharomyces cerevisiae]]'' the number of essential genes is slightly higher, at 1000 genes (~20% of their genes).<ref name="ReferenceA">{{cite journal |vauthors=Tu Z, Wang L, Xu M, Zhou X, Chen T, Sun F |title=Further understanding human disease genes by comparing with housekeeping genes and other genes |journal=BMC Genomics |volume=7 |pages=31 |date=February 2006 |doi=10.1186/1471-2164-7-31 |doi-access=free |pmc=1397819 |pmid=16504025}} {{open access}}</ref> Although the number is more difficult to measure in higher eukaryotes, mice and humans are estimated to have around 2000 essential genes (~10% of their genes).<ref name="georgi">{{cite journal |vauthors=Georgi B, Voight BF, Bućan M |title=From mouse to human: evolutionary genomics analysis of human orthologs of essential genes |journal=PLOS Genetics |volume=9 |issue=5 |pages=e1003484 |date=May 2013 |doi=10.1371/journal.pgen.1003484 |doi-access=free |pmc=3649967 |pmid=23675308}} {{open access}}</ref> The synthetic organism, ''[[Syn 3]]'', has a minimal genome of 473 essential genes and quasi-essential genes (necessary for fast growth), although 149 have unknown function.<ref name="Hutchison">{{cite journal |vauthors=Hutchison CA, Chuang RY, Noskov VN, Assad-Garcia N, Deerinck TJ, Ellisman MH, Gill J, Kannan K, Karas BJ, Ma L, Pelletier JF, Qi ZQ, Richter RA, Strychalski EA, Sun L, Suzuki Y, Tsvetanova B, Wise KS, Smith HO, Glass JI, Merryman C, Gibson DG, Venter JC |display-authors=6 |title=Design and synthesis of a minimal bacterial genome |journal=Science |volume=351 |issue=6280 |pages=aad6253 |date=March 2016 |bibcode=2016Sci...351.....H |doi=10.1126/science.aad6253 |doi-access=free |pmid=27013737}}</ref>

Essential genes include [[housekeeping gene]]s (critical for basic cell functions)<ref>{{cite journal |vauthors=Eisenberg E, Levanon EY |title=Human housekeeping genes, revisited |journal=Trends in Genetics |volume=29 |issue=10 |pages=569–74 |date=October 2013 |doi=10.1016/j.tig.2013.05.010 |pmid=23810203}}</ref> as well as genes that are expressed at different times in the organisms [[developmental biology|development]] or [[biological life cycle|life cycle]].<ref>{{cite journal |vauthors=Amsterdam A, Hopkins N |title=Mutagenesis strategies in zebrafish for identifying genes involved in development and disease |journal=Trends in Genetics |volume=22 |issue=9 |pages=473–8 |date=September 2006 |doi=10.1016/j.tig.2006.06.011 |pmid=16844256}}</ref> Housekeeping genes are used as [[experimental control]]s when [[gene expression analysis|analysing gene expression]], since they are [[gene expression|constitutively expressed]] at a relatively constant level.

===Genetic and genomic nomenclature===
[[Gene nomenclature]] was established by the [[HUGO Gene Nomenclature Committee]] (HGNC), a committee of the [[Human Genome Organisation]], for each known human gene in the form of an approved gene name and [[symbol]] (short-form [[abbreviation]]), which can be accessed through a database maintained by HGNC. Symbols are chosen to be unique, and each gene has only one symbol (although approved symbols sometimes change). Symbols are preferably kept consistent with other members of a [[gene family]] and with homologs in other species, particularly the [[mouse]] due to its role as a common [[model organism]].<ref>{{cite web |title=About the HGNC |website=HGNC Database of Human Gene Names |publisher=HUGO Gene Nomenclature Committee |url=https://www.genenames.org/about/overview |access-date=14 May 2015 |url-status=dead |archive-url=https://web.archive.org/web/20230326032945/https://www.genenames.org/about/overview |archive-date=26 March 2023}}</ref>

==Genetic engineering==
[[File:Breeding transgenesis cisgenesis.svg|class=skin-invert-image|thumb|right|upright=1.35|Comparison of conventional plant breeding with transgenic and cisgenic genetic modification]]

{{Main|Genetic engineering}}

Genetic engineering is the modification of an organism's [[genome]] through [[biotechnology]]. Since the 1970s, a [[genetic engineering techniques|variety of techniques]] have been developed to specifically add, remove and edit genes in an organism.<ref>{{cite journal |vauthors=Cohen SN, Chang AC |title=Recircularization and autonomous replication of a sheared R-factor DNA segment in Escherichia coli transformants |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=70 |issue=5 |pages=1293–7 |date=May 1973 |bibcode=1973PNAS...70.1293C |doi=10.1073/pnas.70.5.1293 |doi-access=free |pmc=433482 |pmid=4576014}}</ref> Recently developed [[genome engineering]] techniques use engineered [[nuclease]] [[enzyme]]s to create targeted [[DNA repair]] in a [[chromosome]] to either disrupt or edit a gene when the break is repaired.<ref>{{cite journal |vauthors=Esvelt KM, Wang HH |title=Genome-scale engineering for systems and synthetic biology |journal=Molecular Systems Biology |volume=9 |issue=1 |pages=641 |year=2013 |doi=10.1038/msb.2012.66 |pmc=3564264 |pmid=23340847}}</ref><ref>{{cite book |vauthors=Tan WS, Carlson DF, Walton MW, Fahrenkrug SC, Hackett PB |title=Advances in Genetics Volume 80 |chapter=Precision editing of large animal genomes |volume=80 |pages=37–97 |year=2012 |isbn=9780124047426 |doi=10.1016/B978-0-12-404742-6.00002-8 |pmc=3683964 |pmid=23084873}}</ref><ref>{{cite journal |vauthors=Puchta H, Fauser F |title=Gene targeting in plants: 25 years later |journal=The International Journal of Developmental Biology |volume=57 |issue=6–8 |pages=629–37 |year=2013 |doi=10.1387/ijdb.130194hp |doi-access=free |pmid=24166445}}</ref><ref name="ran">{{cite journal |vauthors=Ran FA, Hsu PD, Wright J, Agarwala V, Scott DA, Zhang F |title=Genome engineering using the CRISPR-Cas9 system |journal=Nature Protocols |volume=8 |issue=11 |pages=2281–2308 |date=November 2013 |doi=10.1038/nprot.2013.143 |pmc=3969860 |pmid=24157548}}</ref> The related term [[synthetic biology]] is sometimes used to refer to extensive genetic engineering of an organism.<ref>{{cite journal |vauthors=Kittleson JT, Wu GC, Anderson JC |title=Successes and failures in modular genetic engineering |journal=Current Opinion in Chemical Biology |volume=16 |issue=3–4 |pages=329–36 |date=August 2012 |doi=10.1016/j.cbpa.2012.06.009 |pmid=22818777}}</ref>

Genetic engineering is now a routine research tool with [[model organism]]s. For example, genes are easily added to [[bacteria]]<ref>{{cite journal |vauthors=Berg P, Mertz JE |title=Personal reflections on the origins and emergence of recombinant DNA technology |journal=Genetics |volume=184 |issue=1 |pages=9–17 |date=January 2010 |doi=10.1534/genetics.109.112144 |pmc=2815933 |pmid=20061565}}</ref> and lineages of [[knockout mice]] with a specific gene's function disrupted are used to investigate that gene's function.<ref>{{cite journal |vauthors=Austin CP, Battey JF, Bradley A, Bucan M, Capecchi M, Collins FS, Dove WF, Duyk G, Dymecki S, Eppig JT, Grieder FB, Heintz N, Hicks G, Insel TR, Joyner A, Koller BH, Lloyd KC, Magnuson T, Moore MW, Nagy A, Pollock JD, Roses AD, Sands AT, Seed B, Skarnes WC, Snoddy J, Soriano P, Stewart DJ, Stewart F, Stillman B, Varmus H, Varticovski L, Verma IM, Vogt TF, von Melchner H, Witkowski J, Woychik RP, Wurst W, Yancopoulos GD, Young SG, Zambrowicz B |display-authors=6 |title=The knockout mouse project |journal=Nature Genetics |volume=36 |issue=9 |pages=921–4 |date=September 2004 |author-link9=Susan Dymecki |doi=10.1038/ng0904-921 |pmc=2716027 |pmid=15340423}}</ref><ref>{{cite journal |vauthors=Guan C, Ye C, Yang X, Gao J |title=A review of current large-scale mouse knockout efforts |journal=Genesis |volume=48 |issue=2 |pages=73–85 |date=February 2010 |doi=10.1002/dvg.20594 |pmid=20095055 |s2cid=34470273}}</ref> Many organisms have been genetically modified for applications in [[agriculture]], industrial biotechnology, and [[medicine]].

For multicellular organisms, typically the [[embryo]] is engineered which grows into the adult [[genetically modified organism]].<ref name=Deng_2007>{{cite journal |vauthors=Deng C |title=In celebration of Dr. Mario R. Capecchi's Nobel Prize |journal=International Journal of Biological Sciences |volume=3 |issue=7 |pages=417–9 |date=October 2007 |doi=10.7150/ijbs.3.417 |pmc=2043165 |pmid=17998949}}</ref> However, the genomes of cells in an adult organism can be edited using [[gene therapy]] techniques to treat genetic diseases.

==See also==
{{Columns-list|colwidth=22em|
* [[Biological patent]]
* [[Epigenetics]]
* [[Gene-centered view of evolution|Gene-centric view of evolution]] {{nb5}}
* [[Gene dosage]]
* [[Gene redundancy]]
* [[Gene silencing]]
* [[Genetic algorithm]]
* [[Haplotype]]
* [[List of gene prediction software]]
* [[Lists of human genes]]
* [[Predictive medicine]]
* [[Quantitative trait locus]]
* [[Selfish genetic element]]
* [[Whole genome sequencing]]
}}

==References==

===Citations===
{{Reflist}}

===Sources===
; Main textbook
* {{cite book |vauthors=Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P |author1-link=Bruce Alberts |author3-link=Julian Lewis (biologist) |author4-link=Martin Raff |author6-link=Peter Walter |title=Molecular Biology of the Cell |edition=Fourth |publisher=Garland Science |location=New York |year=2002 |isbn=978-0-8153-3218-3 |url=https://www.ncbi.nlm.nih.gov/books/NBK21054/}} – A molecular biology textbook available free online through NCBI Bookshelf.{{Open access}}

{{Hidden begin
|title=Referenced chapters of [https://www.ncbi.nlm.nih.gov/books/NBK21054/ ''Molecular Biology of the Cell'']
|left=true
|bg=#DBE9F4
|style=width: 95%
|border=width: 1px; border-color:#999999;
|titlestyle=background: #DBE9F4; padding-left:4px;
}}
:[https://www.ncbi.nlm.nih.gov/books/NBK21052/ Glossary]

:[https://www.ncbi.nlm.nih.gov/books/NBK26864/ Ch 1: Cells and genomes]
::[https://www.ncbi.nlm.nih.gov/books/NBK26864/ 1.1: The Universal Features of Cells on Earth]

:[https://www.ncbi.nlm.nih.gov/books/NBK21068/ Ch 2: Cell Chemistry and Biosynthesis]
::[https://www.ncbi.nlm.nih.gov/books/NBK26883/ 2.1: The Chemical Components of a Cell]

:[https://www.ncbi.nlm.nih.gov/books/NBK26830/ Ch 3: Proteins]

:[https://www.ncbi.nlm.nih.gov/books/NBK21074/ Ch 4: DNA and Chromosomes]
::[https://www.ncbi.nlm.nih.gov/books/NBK26821/ 4.1: The Structure and Function of DNA]
::[https://www.ncbi.nlm.nih.gov/books/NBK26834/ 4.2: Chromosomal DNA and Its Packaging in the Chromatin Fiber]

:[https://www.ncbi.nlm.nih.gov/books/NBK21064/ Ch 5: DNA Replication, Repair, and Recombination]
::[https://www.ncbi.nlm.nih.gov/books/NBK26850/ 5.2: DNA Replication Mechanisms]
::[https://www.ncbi.nlm.nih.gov/books/NBK26879/ 5.4: DNA Repair]
::[https://www.ncbi.nlm.nih.gov/books/NBK26898/ 5.5: General Recombination]

:[https://www.ncbi.nlm.nih.gov/books/NBK21050/ Ch 6: How Cells Read the Genome: From DNA to Protein]
::[https://www.ncbi.nlm.nih.gov/books/NBK26887/ 6.1: DNA to RNA]
::[https://www.ncbi.nlm.nih.gov/books/NBK26829/ 6.2: RNA to Protein]

:[https://www.ncbi.nlm.nih.gov/books/NBK21057/ Ch 7: Control of Gene Expression]
::[https://www.ncbi.nlm.nih.gov/books/NBK26885/ 7.1: An Overview of Gene Control]
::[https://www.ncbi.nlm.nih.gov/books/NBK26806/ 7.2: DNA-Binding Motifs in Gene Regulatory Proteins]
::[https://www.ncbi.nlm.nih.gov/books/NBK26872/ 7.3: How Genetic Switches Work]
::[https://www.ncbi.nlm.nih.gov/books/NBK26890/ 7.5: Posttranscriptional Controls]
::[https://www.ncbi.nlm.nih.gov/books/NBK26836/ 7.6: How Genomes Evolve]

:[https://www.ncbi.nlm.nih.gov/books/NBK21063/ Ch 14: Energy Conversion: Mitochondria and Chloroplasts]
::[https://www.ncbi.nlm.nih.gov/books/NBK26924/ 14.4: The Genetic Systems of Mitochondria and Plastids]

:[https://www.ncbi.nlm.nih.gov/books/NBK21065/ Ch 18: The Mechanics of Cell Division]
::[https://www.ncbi.nlm.nih.gov/books/NBK26931/ 18.1: An Overview of M Phase]
::[https://www.ncbi.nlm.nih.gov/books/NBK26934/ 18.2: Mitosis]

:[https://www.ncbi.nlm.nih.gov/books/NBK21049/ Ch 20: Germ Cells and Fertilization]
::[https://www.ncbi.nlm.nih.gov/books/NBK26840/ 20.2: Meiosis]
{{Hidden end}}

==Further reading==
{{Refbegin}}
* {{cite book |vauthors=Watson JD, Baker TA, Bell SP, Gann A, Levine M, Losick R |author1-link=James Watson |author2-link=Tania A. Baker |author-link5=Michael Levine (biologist) |title=Molecular Biology of the Gene |edition=7th |date=2013 |publisher=Benjamin Cummings |isbn=978-0-321-90537-6}}
* {{cite book |vauthors=Dawkins R |author-link=Richard Dawkins |title=The Selfish Gene |publisher=Oxford University Press |year=1990 |isbn=978-0-19-286092-7 |title-link=The Selfish Gene}}
* {{cite book |vauthors=Ridley M |author-link=Matt Ridley |title=Genome: The Autobiography of a Species in 23 Chapters |publisher=Fourth Estate |year=1999 |isbn=978-0-00-763573-3 |title-link=Genome: The Autobiography of a Species in 23 Chapters}}
* {{cite book |vauthors=Brown T |title=Genomes |year=2002 |publisher=Wiley-Liss |location=New York |isbn=978-0-471-25046-3 |edition=2nd |pmid=20821850 |url=https://www.ncbi.nlm.nih.gov/books/NBK21128 |access-date=31 August 2017 |archive-date=6 November 2020 |archive-url=https://web.archive.org/web/20201106150314/https://www.ncbi.nlm.nih.gov/books/NBK21128/ |url-status=live }}
{{Refend}}

==External links==
{{Div col|colwidth=27em}}
* [https://ctdbase.org/ Comparative Toxicogenomics Database]
* [https://www.dnaftb.org/ DNA From The Beginning&nbsp;– a primer on genes and DNA]
* [https://www.ncbi.nlm.nih.gov/gene Gene&nbsp;– a searchable database of genes]
* [https://www.mdpi.com/journal/genes ''Genes'']&nbsp;– an Open Access journal
* [http://idconverter.bioinfo.cnio.es/ IDconverter&nbsp;– converts gene IDs between public databases] {{Webarchive|url=https://web.archive.org/web/20200929092205/http://idconverter.bioinfo.cnio.es/ |date=29 September 2020 }}
* [https://web.archive.org/web/20051017154959/http://www.ihop-net.org/UniPub/iHOP/ iHOP&nbsp;– Information Hyperlinked over Proteins]
* [https://web.archive.org/web/20110720065115/http://tagc.univ-mrs.fr/tbrowser/ TranscriptomeBrowser&nbsp;– Gene expression profile analysis]
* [https://archive.today/20121221003541/http://www.jcvi.org/pn-utility The Protein Naming Utility, a database to identify and correct deficient gene names]
* [https://www.mousephenotype.org/ IMPC (International Mouse Phenotyping Consortium)]&nbsp;– Encyclopedia of mammalian gene function
* [https://www.globalgenes.org/ Global Genes Project]&nbsp;– Leading non-profit organization supporting people living with genetic diseases
* [https://www.nature.com/collections/aghcdefffg Encode threads explorer], ''[[Nature (journal)|Nature]]''
** [https://www.nature.com/articles/nature28172 Characterization of intergenic regions and gene definition], ''[[Nature (journal)|Nature]]''
{{Div col end}}

{{Gene expression}}
{{Self-replicating organic structures}}

{{Authority control}}
[[Category:Genes| ]]
[[Category:Cloning]]
[[Category:Molecular biology]]
[[Category:Wikipedia articles with sections published in WikiJournal of Medicine]]