Gene - Revision history

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-<p>A <strong>gene</strong> is a locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions and/or other functional sequence regions.<sup class="reference" id="_ref-Pearson_2006_0">[1]</sup><sup class="reference" id="_ref-Rethink_0">[2]</sup> The physical development and phenotype of organisms can be thought of as a product of genes interacting with each other and with the environment<sup class="reference" id="_ref-0">[3]</sup> A concise definition of gene taking into account complex patterns of regulation and transcription, genic conservation and non-coding RNA genes, has been proposed by Gerstein et al.<sup class="reference" id="_ref-Gerstein_2007_0">[4]</sup> &quot;A gene is a union of genomic sequences encoding a coherent set of potentially overlapping functional products&quot;.</p>
+<p>A <strong>gene</strong> is a locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions and/or other functional sequence regions.&nbsp;<br />
 <p>Colloquially, the term <strong>gene</strong> is often used to refer to an inheritable trait which is usually accompanied by a phenotype as in (&quot;tall genes&quot; or &quot;bad genes&quot;) -- the proper scientific term for this is allele.</p>
 <p>In cells, genes consist of a long strand of DNA that contains a promoter, which controls the activity of a gene, and coding and non-coding sequence. Coding sequence determines what the gene produces, while non-coding sequence can regulate the conditions of gene expression. When a gene is active, the coding and non-coding sequence is copied in a process called transcription, producing an RNA copy of the gene's information. This RNA can then direct the synthesis of proteins via the genetic code. However, RNAs can also be used directly, for example as part of the ribosome. These molecules resulting from gene expression, whether RNA or protein, are known as gene products.</p>
 <p>Genes often contain regions that do not encode products, but regulate gene expression. The genes of eukaryotic organisms can contain regions called introns that are removed from the messenger RNA in a process called splicing. The regions encoding gene products are called exons. 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 prokaryotes (bacteria and archaea), 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 operons with promoter and operator sequences that regulate transcription of a single long RNA. This RNA contains multiple coding sequences. Each coding sequence is preceded by a Shine-Dalgarno sequence that ribosomes recognize.</p>
-<p>The total set of genes in an organism is known as its genome. An organism's genome size is generally lower in prokaryotes, both in number of base pairs and number of genes, than even single-celled eukaryotes. 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 amoeba <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">[5]</sup> The estimated number of genes in the human genome has been repeatedly revised downward since the completion of the Human Genome Project; 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>
+<p>The total set of genes in an organism is known as its genome. An organism's genome size is generally lower in prokaryotes, both in number of base pairs and number of genes, than even single-celled eukaryotes. 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 amoeba <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">[5]</sup> The estimated number of genes in the human genome has been repeatedly revised downward since the completion of the Human Genome Project; 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">[6]</sup> A recent <em>Science</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">[7]</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">[8]</sup></p>
 <p><span class="mw-headline"><font size="5"><br />
 History</font></span></p>
-<p>The existence of genes was first suggested by <a title="Gregor Mendel" href="http://en.wikipedia.org/wiki/Gregor_Mendel">Gregor Mendel</a> (1822-1884), who, in the <a title="1860s" href="http://en.wikipedia.org/wiki/1860s">1860s</a>, studied inheritance in <a title="Pea" href="http://en.wikipedia.org/wiki/Pea">pea</a> plants and <a title="Hypothesis" href="http://en.wikipedia.org/wiki/Hypothesis">hypothesized</a> a factor that conveys traits from parent to offspring. He spent over 10 years of his life on one experiment. Although he did not use the term <em>gene</em>, he explained his results in terms of inherited characteristics. Mendel was also the first to hypothesize <a class="mw-redirect" title="Independent assortment" href="http://en.wikipedia.org/wiki/Independent_assortment">independent assortment</a>, the distinction between <a class="mw-redirect" title="Dominant gene" href="http://en.wikipedia.org/wiki/Dominant_gene">dominant</a> and <a class="mw-redirect" title="Recessive" href="http://en.wikipedia.org/wiki/Recessive">recessive</a> traits, the distinction between a <a class="mw-redirect" title="Heterozygote" href="http://en.wikipedia.org/wiki/Heterozygote">heterozygote</a> and <a class="mw-redirect" title="Homozygote" href="http://en.wikipedia.org/wiki/Homozygote">homozygote</a>, and the difference between what would later be described as <a title="Genotype" href="http://en.wikipedia.org/wiki/Genotype">genotype</a> and <a title="Phenotype" href="http://en.wikipedia.org/wiki/Phenotype">phenotype</a>. Mendel's concept was given a name by <a title="Hugo de Vries" href="http://en.wikipedia.org/wiki/Hugo_de_Vries">Hugo de Vries</a> in 1889, who, at that time probably unaware of Mendel's work, in his book <em>Intracellular Pangenesis</em> coined the term &quot;pangen&quot; for &quot;the smallest particle [representing] one hereditary characteristic&quot;<sup class="reference" id="_ref-pangen_0"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-pangen">[9]</a></sup>. <a title="Wilhelm Johannsen" href="http://en.wikipedia.org/wiki/Wilhelm_Johannsen">Wilhelm Johannsen</a> abbreviated this term to &quot;gene&quot; (&quot;gen&quot; in Danish and German) two decades later.</p>
+<p>The existence of genes was first suggested by Gregor Mendel (1822-1884), who, in the 1860s, studied inheritance in pea plants and hypothesized a factor that conveys traits from parent to offspring. He spent over 10 years of his life on one experiment. Although he did not use the term <em>gene</em>, he explained his results in terms of inherited characteristics. Mendel was also the first to hypothesize independent assortment, the distinction between dominant and recessive traits, the distinction between a heterozygote and homozygote, and the difference between what would later be described as genotype and phenotype. Mendel's concept was given a name by Hugo de Vries in 1889, who, at that time probably unaware of Mendel's work, in his book <em>Intracellular Pangenesis</em> coined the term &quot;pangen&quot; for &quot;the smallest particle [representing] one hereditary characteristic&quot;<sup class="reference" id="_ref-pangen_0">[9]</sup>. Wilhelm Johannsen abbreviated this term to &quot;gene&quot; (&quot;gen&quot; in Danish and German) two decades later.</p>
-<p>In the early 1900s, Mendel's work received renewed attention from scientists. In 1910, <a title="Thomas Hunt Morgan" href="http://en.wikipedia.org/wiki/Thomas_Hunt_Morgan">Thomas Hunt Morgan</a> showed that genes reside on specific <a title="Chromosome" href="http://en.wikipedia.org/wiki/Chromosome">chromosomes</a>. He later showed that genes occupy specific locations on the chromosome. With this knowledge, Morgan and his students began the first chromosomal map of the fruit fly <em><a title="Drosophila melanogaster" href="http://en.wikipedia.org/wiki/Drosophila_melanogaster">Drosophila</a></em>. In 1928, <a title="Frederick Griffith" href="http://en.wikipedia.org/wiki/Frederick_Griffith">Frederick Griffith</a> showed that genes could be transferred. In what is now known as <a title="Griffith's experiment" href="http://en.wikipedia.org/wiki/Griffith%27s_experiment">Griffith's experiment</a>, injections into a mouse of a deadly strain of <a title="Bacteria" href="http://en.wikipedia.org/wiki/Bacteria">bacteria</a> that had been heat-killed transferred genetic information to a safe strain of the same bacteria, killing the mouse.</p>
+<p>In the early 1900s, Mendel's work received renewed attention from scientists. In 1910, Thomas Hunt Morgan showed that genes reside on specific chromosomes. He later showed that genes occupy specific locations on the chromosome. With this knowledge, Morgan and his students began the first chromosomal map of the fruit fly <em>Drosophila</em>. In 1928, Frederick Griffith showed that genes could be transferred. In what is now known as Griffith's experiment, injections into a mouse of a deadly strain of bacteria that had been heat-killed transferred genetic information to a safe strain of the same bacteria, killing the mouse.</p>
-<p>In 1941, <a title="George Wells Beadle" href="http://en.wikipedia.org/wiki/George_Wells_Beadle">George Wells Beadle</a> and <a title="Edward Lawrie Tatum" href="http://en.wikipedia.org/wiki/Edward_Lawrie_Tatum">Edward Lawrie Tatum</a> showed that mutations in genes caused errors in certain steps in <a title="Metabolic pathway" href="http://en.wikipedia.org/wiki/Metabolic_pathway">metabolic pathways</a>. This showed that specific genes code for specific proteins, leading to the &quot;one gene, one enzyme&quot; hypothesis.<sup class="reference" id="_ref-Gerstein_0"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Gerstein">[10]</a></sup> <a title="Oswald Avery" href="http://en.wikipedia.org/wiki/Oswald_Avery">Oswald Avery</a>, <a class="mw-redirect" title="Collin Macleod" href="http://en.wikipedia.org/wiki/Collin_Macleod">Collin Macleod</a>, and <a title="Maclyn McCarty" href="http://en.wikipedia.org/wiki/Maclyn_McCarty">Maclyn McCarty</a> showed in 1944 that DNA holds the gene's information. In 1953, <a title="James D. Watson" href="http://en.wikipedia.org/wiki/James_D._Watson">James D. Watson</a> and <a title="Francis Crick" href="http://en.wikipedia.org/wiki/Francis_Crick">Francis Crick</a> demonstrated the molecular structure of <a title="DNA" href="http://en.wikipedia.org/wiki/DNA">DNA</a>. Together, these discoveries established the <a title="Central dogma of molecular biology" href="http://en.wikipedia.org/wiki/Central_dogma_of_molecular_biology">central dogma of molecular biology</a>, which states that proteins are translated from <a title="RNA" href="http://en.wikipedia.org/wiki/RNA">RNA</a> which is transcribed from DNA. This dogma has since been shown to have exceptions, such as <a title="Reverse transcription" href="http://en.wikipedia.org/wiki/Reverse_transcription">reverse transcription</a> in <a title="Retrovirus" href="http://en.wikipedia.org/wiki/Retrovirus">retroviruses</a>.</p>
+<p>In 1941, George Wells Beadle and Edward Lawrie Tatum showed that mutations in genes caused errors in certain steps in metabolic pathways. This showed that specific genes code for specific proteins, leading to the &quot;one gene, one enzyme&quot; hypothesis.<sup class="reference" id="_ref-Gerstein_0">[10]</sup> Oswald Avery, Collin Macleod, and Maclyn McCarty showed in 1944 that DNA holds the gene's information. In 1953, James D. Watson and Francis Crick demonstrated the molecular structure of DNA. Together, these discoveries established the central dogma of molecular biology, which states that proteins are translated from RNA which is transcribed from DNA. This dogma has since been shown to have exceptions, such as reverse transcription in retroviruses.</p>
-<p>In <a title="1972" href="http://en.wikipedia.org/wiki/1972">1972</a>, <a title="Walter Fiers" href="http://en.wikipedia.org/wiki/Walter_Fiers">Walter Fiers</a> and his team at the Laboratory of Molecular Biology of the <a class="mw-redirect" title="University of Ghent" href="http://en.wikipedia.org/wiki/University_of_Ghent">University of Ghent</a> (<a title="Ghent" href="http://en.wikipedia.org/wiki/Ghent">Ghent</a>, <a title="Belgium" href="http://en.wikipedia.org/wiki/Belgium">Belgium</a>) were the first to determine the sequence of a gene: the gene for <a title="Bacteriophage MS2" href="http://en.wikipedia.org/wiki/Bacteriophage_MS2">Bacteriophage MS2</a> coat protein.<sup class="reference" id="_ref-Min_1972_0"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Min_1972">[11]</a></sup> <a title="Richard J. Roberts" href="http://en.wikipedia.org/wiki/Richard_J._Roberts">Richard J. Roberts</a> and <a class="mw-redirect" title="Phillip Sharp" href="http://en.wikipedia.org/wiki/Phillip_Sharp">Phillip Sharp</a> discovered in 1977 that genes can be split into segments. This leads to the idea that one gene can make several proteins. Recently (as of <a title="2003" href="http://en.wikipedia.org/wiki/2003">2003</a>-<a title="2006" href="http://en.wikipedia.org/wiki/2006">2006</a>), <a title="Biology" href="http://en.wikipedia.org/wiki/Biology">biological</a> results let the notion of gene appear more slippery. In particular, genes do not seem to sit side by side on <a title="DNA" href="http://en.wikipedia.org/wiki/DNA">DNA</a> like discrete beads. Instead, <a title="Region" href="http://en.wikipedia.org/wiki/Region">regions</a> of the DNA producing distinct proteins may overlap, so that the idea emerges that &quot;genes are one long <a title="Continuum (theory)" href="http://en.wikipedia.org/wiki/Continuum_%28theory%29">continuum</a>&quot;.<sup class="reference" id="_ref-Pearson_2006_1"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Pearson_2006">[1]</a></sup></p>
+<p>In 1972, Walter Fiers and his team at the Laboratory of Molecular Biology of the University of Ghent (Ghent, Belgium) were the first to determine the sequence of a gene: the gene for Bacteriophage MS2 coat protein.<sup class="reference" id="_ref-Min_1972_0">[11]</sup> Richard J. Roberts and Phillip Sharp discovered in 1977 that genes can be split into segments. This leads to the idea that one gene can make several proteins. Recently (as of 2003-2006), biological results let the notion of gene appear more slippery. In particular, genes do not seem to sit side by side on DNA like discrete beads. Instead, regions of the DNA producing distinct proteins may overlap, so that the idea emerges that &quot;genes are one long continuum&quot;.<sup class="reference" id="_ref-Pearson_2006_1">[1]</sup></p>
-<p><a id="Mendelian_inheritance_and_classical_genetics" name="Mendelian_inheritance_and_classical_genetics"></a></p>
+<p>&nbsp;</p>
-<h2><span class="editsection">[<a title="Edit section: Mendelian inheritance and classical genetics" href="http://en.wikipedia.org/w/index.php?title=Gene&amp;action=edit&amp;section=2">edit</a>]</span> <span class="mw-headline">Mendelian inheritance and classical genetics</span></h2>
+<h2><span class="mw-headline">Mendelian inheritance and classical genetics</span></h2>
 <dl><dd>
 <div class="noprint relarticle mainarticle"><em>Main articles: <a title="Mendelian inheritance" href="http://en.wikipedia.org/wiki/Mendelian_inheritance">Mendelian inheritance</a> and <a title="Classical genetics" href="http://en.wikipedia.org/wiki/Classical_genetics">Classical genetics</a></em></div>

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 <p>The genetic code is the set of rules by which a gene is translated into a functional protein. Each gene consists of a specific sequence of nucleotides encoded in a DNA (or sometimes RNA) strand; a correspondence between nucleotides, the basic building blocks of genetic material, and amino acids, the basic building blocks of proteins, must be established for genes to be successfully translated into functional proteins. Sets of three nucleotides, known as <a class="mw-redirect" title="Codon" href="http://en.wikipedia.org/wiki/Codon">codons</a>, each correspond to a specific amino acid or to a signal; three codons are known as &quot;stop codons&quot; and, instead of specifying a new amino acid, alert the translation machinery that the end of the gene has been reached. There are 64 possible codons (four possible nucleotides at each of three positions, hence 4<sup>3</sup> possible codons) and only 20 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.</p>
 <p><a id="Transcription" name="Transcription"></a></p>
-<h3><span class="editsection">[<a title="Edit section: Transcription" href="http://en.wikipedia.org/w/index.php?title=Gene&amp;action=edit&amp;section=9">edit</a>]</span> <span class="mw-headline">Transcription</span></h3>
+<h3><span class="mw-headline">Transcription</span></h3>
 <p>The process of genetic <a title="Transcription (genetics)" href="http://en.wikipedia.org/wiki/Transcription_%28genetics%29">transcription</a> produces a single-stranded <a title="RNA" href="http://en.wikipedia.org/wiki/RNA">RNA</a> molecule known as <a title="Messenger RNA" href="http://en.wikipedia.org/wiki/Messenger_RNA">messenger RNA</a>, whose nucleotide sequence is complementary to the DNA from which it was transcribed. The DNA strand whose sequence matches that of the RNA is known as the <a title="Coding strand" href="http://en.wikipedia.org/wiki/Coding_strand">coding strand</a> and the strand from which the RNA was synthesized is the <a class="mw-redirect" title="Template strand" href="http://en.wikipedia.org/wiki/Template_strand">template strand</a>. Transcription is performed by an <a title="Enzyme" href="http://en.wikipedia.org/wiki/Enzyme">enzyme</a> called an <a title="RNA polymerase" href="http://en.wikipedia.org/wiki/RNA_polymerase">RNA polymerase</a>, which reads the template strand in the <a class="mw-redirect" title="3' end" href="http://en.wikipedia.org/wiki/3%27_end">3'</a> to <a class="mw-redirect" title="5' end" href="http://en.wikipedia.org/wiki/5%27_end">5'</a> direction and synthesizes the RNA from <a class="mw-redirect" title="5' end" href="http://en.wikipedia.org/wiki/5%27_end">5'</a> to <a class="mw-redirect" title="3' end" href="http://en.wikipedia.org/wiki/3%27_end">3'</a>. To initiate transcription, the polymerase first recognizes and binds a <a title="Promoter" href="http://en.wikipedia.org/wiki/Promoter">promoter</a> region of the gene. Thus a major mechanism of <a class="mw-redirect" title="Gene regulation" href="http://en.wikipedia.org/wiki/Gene_regulation">gene regulation</a> is the blocking or sequestering of the promoter region, either by tight binding by <a title="Repressor" href="http://en.wikipedia.org/wiki/Repressor">repressor</a> molecules that physically block the polymerase, or by organizing the DNA so that the promoter region is not accessible.</p>
 <p>In <a title="Prokaryote" href="http://en.wikipedia.org/wiki/Prokaryote">prokaryotes</a>, transcription occurs in the <a title="Cytoplasm" href="http://en.wikipedia.org/wiki/Cytoplasm">cytoplasm</a>; for very long transcripts, translation may begin at the 5' end of the RNA while the 3' end is still being transcribed. In <a title="Eukaryote" href="http://en.wikipedia.org/wiki/Eukaryote">eukaryotes</a>, transcription necessarily occurs in the nucleus, where the cell's DNA is sequestered; the RNA molecule produced by the polymerase is known as the <a class="mw-redirect" title="Primary transcript" href="http://en.wikipedia.org/wiki/Primary_transcript">primary transcript</a> and must undergo <a title="Post-transcriptional modification" href="http://en.wikipedia.org/wiki/Post-transcriptional_modification">post-transcriptional modifications</a> before being exported to the cytoplasm for translation. The <a title="Splicing (genetics)" href="http://en.wikipedia.org/wiki/Splicing_%28genetics%29">splicing</a> of <a title="Intron" href="http://en.wikipedia.org/wiki/Intron">introns</a> present within the transcribed region is a modification unique to eukaryotes; <a title="Alternative splicing" href="http://en.wikipedia.org/wiki/Alternative_splicing">alternative splicing</a> 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.</p>
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 <p><a class="mw-redirect" title="Translation (genetics)" href="http://en.wikipedia.org/wiki/Translation_%28genetics%29">Translation</a> is the process by which a mature mRNA molecule is used as a template for synthesizing a new <a title="Protein" href="http://en.wikipedia.org/wiki/Protein">protein</a>. Translation is carried out by <a title="Ribosome" href="http://en.wikipedia.org/wiki/Ribosome">ribosomes</a>, large complexes of RNA and protein responsible for carrying out the chemical reactions to add new <a title="Amino acid" href="http://en.wikipedia.org/wiki/Amino_acid">amino acids</a> to a growing <a class="mw-redirect" title="Polypeptide chain" href="http://en.wikipedia.org/wiki/Polypeptide_chain">polypeptide chain</a> by the formation of <a title="Peptide bond" href="http://en.wikipedia.org/wiki/Peptide_bond">peptide bonds</a>. The genetic code is read three nucleotides at a time, in units called <a class="mw-redirect" title="Codon" href="http://en.wikipedia.org/wiki/Codon">codons</a>, via interactions with specialized RNA molecules called <a title="Transfer RNA" href="http://en.wikipedia.org/wiki/Transfer_RNA">transfer RNA</a> (tRNA). Each tRNA has three unpaired bases known as the <a class="mw-redirect" title="Anticodon" href="http://en.wikipedia.org/wiki/Anticodon">anticodon</a> that are complementary to the codon it reads; the tRNA is also <a class="mw-redirect" title="Covalent" href="http://en.wikipedia.org/wiki/Covalent">covalently</a> attached to the <a title="Amino acid" href="http://en.wikipedia.org/wiki/Amino_acid">amino acid</a> specified by the complementary codon. When the tRNA binds to its complementary codon in an mRNA strand, the ribosome ligates its amino acid cargo to the new polypeptide chain, which is synthesized from <a title="N-terminus" href="http://en.wikipedia.org/wiki/N-terminus">amino terminus</a> to <a title="C-terminus" href="http://en.wikipedia.org/wiki/C-terminus">carboxyl terminus</a>. During and after its synthesis, the new protein must <a title="Protein folding" href="http://en.wikipedia.org/wiki/Protein_folding">fold</a> to its active <a title="Tertiary structure" href="http://en.wikipedia.org/wiki/Tertiary_structure">three-dimensional structure</a> before it can carry out its cellular function.</p>
 <p><a id="DNA_replication_and_inheritance" name="DNA_replication_and_inheritance"></a></p>
-<h2><span class="editsection">[<a title="Edit section: DNA replication and inheritance" href="http://en.wikipedia.org/w/index.php?title=Gene&amp;action=edit&amp;section=11">edit</a>]</span> <span class="mw-headline">DNA replication and inheritance</span></h2>
+<h2><span class="mw-headline">DNA replication and inheritance</span></h2>
 <p>The growth, development, and reproduction of organisms relies on <a title="Cell division" href="http://en.wikipedia.org/wiki/Cell_division">cell division</a>, or the process by which a single <a title="Cell (biology)" href="http://en.wikipedia.org/wiki/Cell_%28biology%29">cell</a> divides into two usually identical <a class="mw-redirect" title="Daughter cell" href="http://en.wikipedia.org/wiki/Daughter_cell">daughter cells</a>. This requires first making a duplicate copy of every gene in the <a title="Genome" href="http://en.wikipedia.org/wiki/Genome">genome</a> in a process called <a title="DNA replication" href="http://en.wikipedia.org/wiki/DNA_replication">DNA replication</a>. The copies are made by specialized <a title="Enzyme" href="http://en.wikipedia.org/wiki/Enzyme">enzymes</a> known as <a title="DNA polymerase" href="http://en.wikipedia.org/wiki/DNA_polymerase">DNA polymerases</a>, which &quot;read&quot; 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 <a title="Base pair" href="http://en.wikipedia.org/wiki/Base_pair">base pairing</a>, 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 <a title="Semiconservative replication" href="http://en.wikipedia.org/wiki/Semiconservative_replication">semiconservative</a>; that is, the copy of the genome inherited by each daughter cell contains one original and one newly synthesized strand of DNA.<sup class="reference" id="_ref-Watson_2004_1"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-Watson_2004">[8]</a></sup></p>
 <p>After DNA replication is complete, the cell must physically separate the two copies of the genome and divide into two distinct membrane-bound cells. 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> - this usually occurs via a relatively simple process called <a title="Binary fission" href="http://en.wikipedia.org/wiki/Binary_fission">binary fission</a>, in which each circular genome attaches to the <a title="Cell membrane" href="http://en.wikipedia.org/wiki/Cell_membrane">cell membrane</a> and is separated into the daughter cells as the membrane <a class="mw-redirect" title="Invaginate" href="http://en.wikipedia.org/wiki/Invaginate">invaginates</a> to split the <a title="Cytoplasm" href="http://en.wikipedia.org/wiki/Cytoplasm">cytoplasm</a> into two membrane-bound portions. Binary fission is extremely fast compared to the rates of cell division in <a title="Eukaryote" href="http://en.wikipedia.org/wiki/Eukaryote">eukaryotes</a>. Eukaryotic cell division is a more complex process known as the <a title="Cell cycle" href="http://en.wikipedia.org/wiki/Cell_cycle">cell cycle</a>; DNA replication occurs during a phase of this cycle known as <a title="S phase" href="http://en.wikipedia.org/wiki/S_phase">S phase</a>, while the process of segregating <a title="Chromosome" href="http://en.wikipedia.org/wiki/Chromosome">chromosomes</a> and splitting the <a title="Cytoplasm" href="http://en.wikipedia.org/wiki/Cytoplasm">cytoplasm</a> occurs during <a class="mw-redirect" title="M phase" href="http://en.wikipedia.org/wiki/M_phase">M phase</a>. In many single-celled eukaryotes such as <a title="Yeast" href="http://en.wikipedia.org/wiki/Yeast">yeast</a>, reproduction by <a title="Budding" href="http://en.wikipedia.org/wiki/Budding">budding</a> is common, which results in asymmetrical portions of cytoplasm in the two daughter cells.</p>
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+<p><span class="mw-headline"><font size="5">References</font></span></p>
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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>
-<p><span class="mw-headline">History</span></p>
+<p><span class="mw-headline"><font size="5">History</font></span></p>
 <p>The existence of genes was first suggested by <a title="Gregor Mendel" href="http://en.wikipedia.org/wiki/Gregor_Mendel">Gregor Mendel</a> (1822-1884), who, in the <a title="1860s" href="http://en.wikipedia.org/wiki/1860s">1860s</a>, studied inheritance in <a title="Pea" href="http://en.wikipedia.org/wiki/Pea">pea</a> plants and <a title="Hypothesis" href="http://en.wikipedia.org/wiki/Hypothesis">hypothesized</a> a factor that conveys traits from parent to offspring. He spent over 10 years of his life on one experiment. Although he did not use the term <em>gene</em>, he explained his results in terms of inherited characteristics. Mendel was also the first to hypothesize <a class="mw-redirect" title="Independent assortment" href="http://en.wikipedia.org/wiki/Independent_assortment">independent assortment</a>, the distinction between <a class="mw-redirect" title="Dominant gene" href="http://en.wikipedia.org/wiki/Dominant_gene">dominant</a> and <a class="mw-redirect" title="Recessive" href="http://en.wikipedia.org/wiki/Recessive">recessive</a> traits, the distinction between a <a class="mw-redirect" title="Heterozygote" href="http://en.wikipedia.org/wiki/Heterozygote">heterozygote</a> and <a class="mw-redirect" title="Homozygote" href="http://en.wikipedia.org/wiki/Homozygote">homozygote</a>, and the difference between what would later be described as <a title="Genotype" href="http://en.wikipedia.org/wiki/Genotype">genotype</a> and <a title="Phenotype" href="http://en.wikipedia.org/wiki/Phenotype">phenotype</a>. Mendel's concept was given a name by <a title="Hugo de Vries" href="http://en.wikipedia.org/wiki/Hugo_de_Vries">Hugo de Vries</a> in 1889, who, at that time probably unaware of Mendel's work, in his book <em>Intracellular Pangenesis</em> coined the term &quot;pangen&quot; for &quot;the smallest particle [representing] one hereditary characteristic&quot;<sup class="reference" id="_ref-pangen_0"><a title="" href="http://en.wikipedia.org/wiki/Gene#_note-pangen">[9]</a></sup>. <a title="Wilhelm Johannsen" href="http://en.wikipedia.org/wiki/Wilhelm_Johannsen">Wilhelm Johannsen</a> abbreviated this term to &quot;gene&quot; (&quot;gen&quot; in Danish and German) two decades later.</p>
 <p>In the early 1900s, Mendel's work received renewed attention from scientists. In 1910, <a title="Thomas Hunt Morgan" href="http://en.wikipedia.org/wiki/Thomas_Hunt_Morgan">Thomas Hunt Morgan</a> showed that genes reside on specific <a title="Chromosome" href="http://en.wikipedia.org/wiki/Chromosome">chromosomes</a>. He later showed that genes occupy specific locations on the chromosome. With this knowledge, Morgan and his students began the first chromosomal map of the fruit fly <em><a title="Drosophila melanogaster" href="http://en.wikipedia.org/wiki/Drosophila_melanogaster">Drosophila</a></em>. In 1928, <a title="Frederick Griffith" href="http://en.wikipedia.org/wiki/Frederick_Griffith">Frederick Griffith</a> showed that genes could be transferred. In what is now known as <a title="Griffith's experiment" href="http://en.wikipedia.org/wiki/Griffith%27s_experiment">Griffith's experiment</a>, injections into a mouse of a deadly strain of <a title="Bacteria" href="http://en.wikipedia.org/wiki/Bacteria">bacteria</a> that had been heat-killed transferred genetic information to a safe strain of the same bacteria, killing the mouse.</p>

@@ Line 4: / Line 4: @@
 <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>
-<p><a id="History" name="History"></a></p>
+<p><span class="mw-headline">History</span></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>

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		+	<p style="LINE-HEIGHT: 20px" align="left"><a name="NDTITLE"><font size="2">염색체상에서 일정한 위치를 차지하고 있는 유전정보의 단위.</font></a></p>