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<p><a title="Richard Dawkins" href="http://en.wikipedia.org/wiki/Richard_Dawkins">Richard Dawkins</a>' <em><a title="The Selfish Gene" href="http://en.wikipedia.org/wiki/The_Selfish_Gene">The Selfish Gene</a></em> and <em><a title="The Extended Phenotype" href="http://en.wikipedia.org/wiki/The_Extended_Phenotype">The Extended Phenotype</a></em> defended the idea that the gene is the only <a title="DNA replication" href="http://en.wikipedia.org/wiki/DNA_replication">replicator</a> in living systems. This means that only genes transmit their structure largely intact and are potentially immortal in the form of copies. So, genes should be the <a title="Unit of selection" href="http://en.wikipedia.org/wiki/Unit_of_selection">unit of selection</a>. In <em><a title="The Selfish Gene" href="http://en.wikipedia.org/wiki/The_Selfish_Gene">The Selfish Gene</a></em> Dawkins attempts to redefine the word 'gene' to mean "an inheritable unit" instead of the generally accepted definition of "a section of DNA coding for a particular protein". In <em><a class="mw-redirect" title="River Out of Eden" href="http://en.wikipedia.org/wiki/River_Out_of_Eden">River Out of Eden</a></em>, Dawkins further refined the idea of gene-centric selection by describing life as a river of compatible genes flowing through <a class="mw-redirect" title="Geological time" href="http://en.wikipedia.org/wiki/Geological_time">geological time</a>. Scoop up a bucket of genes from the river of genes, and we have an <a title="Organism" href="http://en.wikipedia.org/wiki/Organism">organism</a> serving as temporary bodies or <a class="mw-redirect" title="Survival machine" href="http://en.wikipedia.org/wiki/Survival_machine">survival machines</a>. A river of genes may fork into two branches representing two non-<a title="Hybrid" href="http://en.wikipedia.org/wiki/Hybrid">interbreeding</a> <a title="Species" href="http://en.wikipedia.org/wiki/Species"><font color="#810081">species</font></a> as a result of geographical separation.</p>
<p><a id="Gene_targeting_and_implications" name="Gene_targeting_and_implications"></a></p>
<h2><span class="editsection">[<a title="Edit section: Gene targeting and implications" href="http://en.wikipedia.org/w/index.php?title=Gene&action=edit&section=19">edit</a>]</span> <span class="mw-headline">Gene targeting and implications</span></h2>
<p>Gene targeting is commonly referred to techniques for altering or disrupting mouse genes and provides the mouse models for studying the roles of individual genes in <a class="mw-redirect" title="Embryonic development" href="http://en.wikipedia.org/wiki/Embryonic_development">embryonic development</a>, human disorders, aging and diseases. The mouse models, where one or more of its genes are deactivated or made inoperable, are called <a class="mw-redirect" title="Knockout mice" href="http://en.wikipedia.org/wiki/Knockout_mice">knockout mice</a>. Since the first reports in which <a title="Homologous recombination" href="http://en.wikipedia.org/wiki/Homologous_recombination">homologous recombination</a> in <a title="Embryonic stem cell" href="http://en.wikipedia.org/wiki/Embryonic_stem_cell">embryonic stem cells</a> was used to generate gene-targeted mice,<sup class="reference" id="_ref-2"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-2">[19]</a></sup> gene targeting has proven to be a powerful means of precisely manipulating the mammalian genome, producing at least ten thousand mutant mouse strains and it is now possible to introduce mutations that can be activated at specific time points, or in specific cells or organs, both during development and in the adult animal.<sup class="reference" id="_ref-3"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-3">[20]</a></sup><sup class="reference" id="_ref-Deng_2007_0"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Deng_2007">[21]</a></sup></p>
<p>Gene targeting strategies have been expanded to all kinds of modifications, including <a title="Point mutation" href="http://en.wikipedia.org/wiki/Point_mutation">point mutations</a>, isoform deletions, mutant allele correction, large pieces of chromosomal DNA <a title="Genetic insertion" href="http://en.wikipedia.org/wiki/Genetic_insertion">insertion</a> and <a title="Deletion (genetics)" href="http://en.wikipedia.org/wiki/Deletion_%28genetics%29">deletion</a>, tissue specific disruption combined with spatial and temporal regulation and so on. It is predicted that the ability to generate mouse models with predictable phenotypes will have a major impact on studies of all phases of development, <a title="Immunology" href="http://en.wikipedia.org/wiki/Immunology">immunology</a>, <a title="Neurobiology" href="http://en.wikipedia.org/wiki/Neurobiology">neurobiology</a>, <a title="Oncology" href="http://en.wikipedia.org/wiki/Oncology">oncology</a>, <a title="Physiology" href="http://en.wikipedia.org/wiki/Physiology">physiology</a>, <a title="Metabolism" href="http://en.wikipedia.org/wiki/Metabolism">metabolism</a>, and human diseases. Gene targeting is also in theory applicable to species from which <a title="Totipotency" href="http://en.wikipedia.org/wiki/Totipotency">totipotent</a> embryonic stem cells can be established, and therefore may offer a potential to the improvement of domestic animals and plants.<sup class="reference" id="_ref-Deng_2007_1"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Deng_2007">[21]</a></sup><sup class="reference" id="_ref-4"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-4">[22]</a></sup></p>
<p><a id="The_gene_concept_is_still_changing" name="The_gene_concept_is_still_changing"></a></p>
<h2><span class="editsection">[<a title="Edit section: The gene concept is still changing" href="http://en.wikipedia.org/w/index.php?title=Gene&action=edit&section=20">edit</a>]</span> <span class="mw-headline">The <em>gene</em> concept is still changing</span></h2>
<p>The concept of the gene has changed considerably (see <a title="Gene" href="http://en.wikipedia.org/wiki/Gene#history">history section</a>). Originally considered a "unit of inheritance" to a usually <a title="DNA" href="http://en.wikipedia.org/wiki/DNA">DNA</a>-based unit that can exert its effects on the organism through <a title="RNA" href="http://en.wikipedia.org/wiki/RNA">RNA</a> or <a title="Protein" href="http://en.wikipedia.org/wiki/Protein">protein</a> products. It was also previously believed that one gene makes one protein; this concept has been overthrown by the discovery of <a title="Alternative splicing" href="http://en.wikipedia.org/wiki/Alternative_splicing">alternative splicing</a> and <a title="Trans-splicing" href="http://en.wikipedia.org/wiki/Trans-splicing">trans-splicing</a>.<sup class="reference" id="_ref-Gerstein_2"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Gerstein">[10]</a></sup></p>
<p>And the definition of gene is still changing. The first cases of <a title="RNA" href="http://en.wikipedia.org/wiki/RNA">RNA</a>-based <a title="Biological inheritance" href="http://en.wikipedia.org/wiki/Biological_inheritance">inheritance</a> have been discovered in mammals.<sup class="reference" id="_ref-rass_1"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-rass">[14]</a></sup> In plants, cases of traits reappearing after several generations of absence have led researchers to hypothesise RNA-directed overwriting of genomic DNA.<sup class="reference" id="_ref-5"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-5">[23]</a></sup> Evidence is also accumulating that the <a title="Enhancer (genetics)" href="http://en.wikipedia.org/wiki/Enhancer_%28genetics%29">control regions</a> of a gene do not necessarily have to be close to the <a class="mw-redirect" title="Coding sequence" href="http://en.wikipedia.org/wiki/Coding_sequence">coding sequence</a> on the linear molecule or even on the same chromosome. Spilianakis and colleagues discovered that the <a class="mw-redirect" title="Promoter region" href="http://en.wikipedia.org/wiki/Promoter_region">promoter region</a> of the <a title="Interferon-gamma" href="http://en.wikipedia.org/wiki/Interferon-gamma">interferon-gamma</a> gene on chromosome 10 and the regulatory regions of the T(H)2 <a title="Cytokine" href="http://en.wikipedia.org/wiki/Cytokine">cytokine</a> locus on chromosome 11 come into close proximity in the <a title="Cell nucleus" href="http://en.wikipedia.org/wiki/Cell_nucleus">nucleus</a> possibly to be jointly regulated.<sup class="reference" id="_ref-6"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-6">[24]</a></sup></p>
<p>The concept that genes are clearly delimited is also being eroded. There is evidence for fused proteins stemming from two adjacent genes that can produce two separate protein products. While it is not clear whether these fusion proteins are functional, the phenomena is more frequent than previously thought.<sup class="reference" id="_ref-7"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-7">[25]</a></sup> Even more ground-breaking than the discovery of fused genes is the observation that some proteins can be composed of <a class="mw-redirect" title="Exons" href="http://en.wikipedia.org/wiki/Exons">exons</a> from far away regions and even different chromosomes.<sup class="reference" id="_ref-8"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-8">[26]</a></sup><sup class="reference" id="_ref-Rethink_2"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Rethink">[2]</a></sup> This new data has led to an updated, and probably tentative, definition of a gene as "a union of genomic sequences encoding a coherent set of potentially overlapping functional products."<sup class="reference" id="_ref-Gerstein_3"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Gerstein">[10]</a></sup> This new definition categorizes genes by functional products, whether they be proteins or RNA, rather than specific DNA loci; all regulatory elements of DNA are therefore classified as <em>gene-associated</em> regions.<sup class="reference" id="_ref-Gerstein_4"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Gerstein">[10]</a></sup></p>
<p><a id="See_also" name="See_also"></a></p>
<h2><span class="editsection">[<a title="Edit section: See also" href="http://en.wikipedia.org/w/index.php?title=Gene&action=edit&section=21">edit</a>]</span> <span class="mw-headline">See also</span></h2>
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<p><a id="References" name="References"></a></p><h2><span class="editsection">[<a title="Edit section: References" href="http://en.wikipedia.org/w/index.php?title=Gene&action=edit&section=22">edit</a>]</span> <span class="mw-headline">References</span></h2p>
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<div style="MARGIN-LEFT: 60px"><a title="Wikibooks" href="http://en.wikipedia.org/wiki/Wikibooks">Wikibooks</a> has a book on the topic of
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<div style="FONT-SIZE: xx-small; MARGIN-LEFT: 60px; LINE-HEIGHT: 1.6em">This audio file was created from a revision dated <a title="2005" href="http://en.wikipedia.org/wiki/2005">2005</a>-<a title="April 21" href="http://en.wikipedia.org/wiki/April_21">04-21</a>, and may not reflect subsequent edits to the article. (<a title="Wikipedia:Media help" href="http://en.wikipedia.org/wiki/Wikipedia:Media_help">Audio help</a>)</div>
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<p><a id="Tutorial_and_news" name="Tutorial_and_news"></a> </p><h3><span class="editsection">[<a title="Edit section: Tutorial and news" href="http://en.wikipedia.org/w/index.php?title=Gene&action=edit&section=25">edit</a>]</span> <span class="mw-headline">Tutorial and news</span></h3>
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<li><a class="external text" title="http://www.scienceaid.co.uk/biology/genetics" rel="nofollow" href="http://www.scienceaid.co.uk/biology/genetics">Science aid: Genetics</a> for beginners </li>
<li><cite style="FONT-STYLE: normal">(2004) "Finishing the euchromatic sequence of the human genome". <em>Nature</em> 431 (7011): 931-45. <a class="external" title="http://www.ncbi.nlm.nih.gov/pubmed/15496913" href="http://www.ncbi.nlm.nih.gov/pubmed/15496913">PMID 15496913</a>.</cite><span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.atitle=Finishing+the+euchromatic+sequence+of+the+human+genome&rft.jtitle=Nature&rft.date=2004&rft.volume=431&rft.issue=7011&rft.pages=931-45&rft_id=info:pmid/15496913"> </span> </li>
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<p><a idspan class="References_and_databasesmw-headline" name="References_and_databases"></a></p><h3><span classfont size="editsection5">[<a title="Edit section: References and databases" href="http://en.wikipedia.org/w/index.php?title=Gene&action=edit&section=26">edit</a>]</span> <span class="mw-headline"font>References and databases</span></h3p>
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<li><a class="external text" title="http://www.gene.ucl.ac.uk/nomenclature" rel="nofollow" href="http://www.gene.ucl.ac.uk/nomenclature">HUGO Gene Nomenclature Committee, HGNC</a> </li>