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<p>Genes often contain regions that do not encode products, but <a class="mw-redirect" title="Gene regulation" href="http://en.wikipedia.org/wiki/Gene_regulation">regulate gene expression</a>. The genes of <a title="Eukaryote" href="http://en.wikipedia.org/wiki/Eukaryote">eukaryotic</a> organisms can contain regions called <a title="Intron" href="http://en.wikipedia.org/wiki/Intron">introns</a> that are removed from the messenger RNA in a process called <a title="Splicing (genetics)" href="http://en.wikipedia.org/wiki/Splicing_%28genetics%29">splicing</a>. The regions encoding gene products are called <a title="Exon" href="http://en.wikipedia.org/wiki/Exon">exons</a>. In eukaryotes, a single gene can encode multiple proteins, which are produced through the creation of different arrangements of exons through <em>alternative splicing</em>. In <a title="Prokaryote" href="http://en.wikipedia.org/wiki/Prokaryote">prokaryotes</a> (<a title="Bacteria" href="http://en.wikipedia.org/wiki/Bacteria">bacteria</a> and <a title="Archaea" href="http://en.wikipedia.org/wiki/Archaea">archaea</a>), introns are less common and genes often contain a single uninterrupted stretch of DNA, called a <em>cistron</em>, that codes for a product. Prokaryotic genes are often arranged in groups called <a class="mw-redirect" title="Operons" href="http://en.wikipedia.org/wiki/Operons">operons</a> with <a title="Promoter" href="http://en.wikipedia.org/wiki/Promoter">promoter</a> and <a title="Operator" href="http://en.wikipedia.org/wiki/Operator">operator</a> sequences that regulate <a title="Transcription (genetics)" href="http://en.wikipedia.org/wiki/Transcription_%28genetics%29">transcription</a> of a single long <a title="RNA" href="http://en.wikipedia.org/wiki/RNA">RNA</a>. This RNA contains multiple coding sequences. Each coding sequence is preceded by a <a title="Shine-Dalgarno sequence" href="http://en.wikipedia.org/wiki/Shine-Dalgarno_sequence">Shine-Dalgarno sequence</a> that ribosomes recognize.</p>

<p>The total set of genes in an organism is known as its <a title="Genome" href="http://en.wikipedia.org/wiki/Genome">genome</a>. An organism's <a title="Genome size" href="http://en.wikipedia.org/wiki/Genome_size">genome size</a> is generally lower in <a title="Prokaryote" href="http://en.wikipedia.org/wiki/Prokaryote">prokaryotes</a>, both in number of <a title="Base pair" href="http://en.wikipedia.org/wiki/Base_pair">base pairs</a> and number of genes, than even single-celled <a title="Eukaryote" href="http://en.wikipedia.org/wiki/Eukaryote">eukaryotes</a>. However, there is no clear relationship between genome sizes and complexity in eukaryotic organisms. One of the largest known genomes belongs to the single-celled <a title="Amoeba" href="http://en.wikipedia.org/wiki/Amoeba">amoeba</a> <em>Amoeba dubia</em>, with over 670 billion base pairs, some 200 times larger than the human genome.<sup class="reference" id="_ref-Cavalier-Smith_0"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Cavalier-Smith">[5]</a></sup> The estimated number of genes in the <a title="Human genome" href="http://en.wikipedia.org/wiki/Human_genome">human genome</a> has been repeatedly revised downward since the completion of the <a title="Human Genome Project" href="http://en.wikipedia.org/wiki/Human_Genome_Project">Human Genome Project</a>; current estimates place the human genome at just under 3 billion base pairs and about 20,000&ndash;25,000 genes.<sup class="reference" id="_ref-IHSGC2004_0"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-IHSGC2004">[6]</a></sup> A recent <em><a title="Science (journal)" href="http://en.wikipedia.org/wiki/Science_%28journal%29">Science</a></em> article gives a final number of 20,488, with perhaps 100 more yet to be discovered .<sup class="reference" id="_ref-gene_count2007_0"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-gene_count2007">[7]</a></sup> The gene density of a genome is a measure of the number of genes per million base pairs (called a megabase, Mb); prokaryotic genomes have much higher gene densities than eukaryotes. The gene density of the human genome is roughly 12&ndash;15 genes/Mb.<sup class="reference" id="_ref-Watson_2004_0"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Watson_2004">[8]</a></sup></p>

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<div class="noprint relarticle mainarticle"><em>Main article: <a title="History of genetics" href="http://en.wikipedia.org/wiki/History_of_genetics">History of genetics</a></em></div>