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Available in 9 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-9" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">9 languages</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-ar mw-list-item"><a href="https://ar.wikipedia.org/wiki/%D9%85%D8%AB%D9%86%D9%88%D9%8A_%D8%A7%D9%84%D8%A8%D9%8A%D8%B1%D9%8A%D9%85%D9%8A%D8%AF%D9%8A%D9%86" title="مثنوي البيريميدين – Arabic" lang="ar" hreflang="ar" data-title="مثنوي البيريميدين" data-language-autonym="العربية" data-language-local-name="Arabic" class="interlanguage-link-target"><span>العربية</span></a></li><li class="interlanguage-link interwiki-el mw-list-item"><a href="https://el.wikipedia.org/wiki/%CE%94%CE%B9%CE%BC%CE%B5%CF%81%CE%AE_%CE%B8%CF%85%CE%BC%CE%AF%CE%BD%CE%B7%CF%82" title="Διμερή θυμίνης – Greek" lang="el" hreflang="el" data-title="Διμερή θυμίνης" data-language-autonym="Ελληνικά" data-language-local-name="Greek" class="interlanguage-link-target"><span>Ελληνικά</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Dim%C3%A8re_de_pyrimidine" title="Dimère de pyrimidine – French" lang="fr" hreflang="fr" data-title="Dimère de pyrimidine" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-gl mw-list-item"><a href="https://gl.wikipedia.org/wiki/D%C3%ADmero_de_pirimidina" title="Dímero de pirimidina – Galician" lang="gl" hreflang="gl" data-title="Dímero de pirimidina" data-language-autonym="Galego" data-language-local-name="Galician" class="interlanguage-link-target"><span>Galego</span></a></li><li class="interlanguage-link interwiki-pt mw-list-item"><a href="https://pt.wikipedia.org/wiki/D%C3%ADmero_de_pirimidina" title="Dímero de pirimidina – Portuguese" lang="pt" hreflang="pt" data-title="Dímero de pirimidina" data-language-autonym="Português" data-language-local-name="Portuguese" class="interlanguage-link-target"><span>Português</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/%D0%9F%D0%B8%D1%80%D0%B8%D0%BC%D0%B8%D0%B4%D0%B8%D0%BD%D0%BE%D0%B2%D1%8B%D0%B9_%D0%B4%D0%B8%D0%BC%D0%B5%D1%80" title="Пиримидиновый димер – Russian" lang="ru" hreflang="ru" data-title="Пиримидиновый димер" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-sr mw-list-item"><a href="https://sr.wikipedia.org/wiki/Pirimidinski_dimer" title="Pirimidinski dimer – Serbian" lang="sr" hreflang="sr" data-title="Pirimidinski dimer" data-language-autonym="Српски / srpski" data-language-local-name="Serbian" class="interlanguage-link-target"><span>Српски / srpski</span></a></li><li class="interlanguage-link interwiki-sh mw-list-item"><a href="https://sh.wikipedia.org/wiki/Pirimidinski_dimer" title="Pirimidinski dimer – Serbo-Croatian" lang="sh" hreflang="sh" data-title="Pirimidinski dimer" data-language-autonym="Srpskohrvatski / српскохрватски" data-language-local-name="Serbo-Croatian" class="interlanguage-link-target"><span>Srpskohrvatski / српскохрватски</span></a></li><li class="interlanguage-link interwiki-zh mw-list-item"><a href="https://zh.wikipedia.org/wiki/%E5%98%A7%E5%95%B6%E4%BA%8C%E8%81%9A%E4%BD%93" title="嘧啶二聚体 – Chinese" lang="zh" hreflang="zh" data-title="嘧啶二聚体" data-language-autonym="中文" data-language-local-name="Chinese" 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class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Type of damage to DNA</div> <figure class="mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:DNA_UV_mutation.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/f/fd/DNA_UV_mutation.svg/300px-DNA_UV_mutation.svg.png" decoding="async" width="300" height="231" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/f/fd/DNA_UV_mutation.svg/450px-DNA_UV_mutation.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/f/fd/DNA_UV_mutation.svg/600px-DNA_UV_mutation.svg.png 2x" data-file-width="387" data-file-height="298" /></a><figcaption>Formation of thymine dimer lesion in DNA. The <a href="/wiki/Photon" title="Photon">photon</a> causes two consecutive bases on one strand to bind together, destroying the normal base-pairing double-strand structure in that area.</figcaption></figure> <p><b>Pyrimidine dimers</b> represent <a href="/wiki/Molecular_lesion" title="Molecular lesion">molecular lesions</a> originating from <a href="/wiki/Thymine" title="Thymine">thymine</a> or <a href="/wiki/Cytosine" title="Cytosine">cytosine</a> bases within <a href="/wiki/DNA" title="DNA">DNA</a>, resulting from <a href="/wiki/Photochemical_reactions" class="mw-redirect" title="Photochemical reactions">photochemical reactions</a>.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> These lesions, commonly linked to <b>direct DNA damage</b>,<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> are induced by <a href="/wiki/Ultraviolet_light" class="mw-redirect" title="Ultraviolet light">ultraviolet light</a> (UV), particularly <a href="/wiki/UVC_(radiation)" class="mw-redirect" title="UVC (radiation)">UVC</a>, result in the formation of <a href="/wiki/Covalent_bonds" class="mw-redirect" title="Covalent bonds">covalent bonds</a> between adjacent nitrogenous bases along the <a href="/wiki/Nucleotide" title="Nucleotide">nucleotide</a> chain near their carbon–carbon double bonds,<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> the photo-coupled dimers are <a href="/wiki/Fluorescent" class="mw-redirect" title="Fluorescent">fluorescent</a>.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> Such <a href="/wiki/Dimerization" title="Dimerization">dimerization</a>, which can also occur in <a href="/wiki/Double-stranded_RNA" title="Double-stranded RNA">double-stranded RNA</a> (dsRNA) involving <a href="/wiki/Uracil" title="Uracil">uracil</a> or <a href="/wiki/Cytosine" title="Cytosine">cytosine</a>, leads to the creation of <b>cyclobutane pyrimidine dimers</b> (CPDs) and <b>6–4 photoproducts</b>. These pre-<a href="/wiki/Mutagenic" class="mw-redirect" title="Mutagenic">mutagenic</a> lesions modify the <a href="/wiki/DNA_helix" class="mw-redirect" title="DNA helix">DNA helix</a> structure, resulting in abnormal non-canonical <a href="/wiki/Base_pairing" class="mw-redirect" title="Base pairing">base pairing</a> and, consequently, adjacent thymines or cytosines in DNA will form a <a href="/wiki/Cyclobutane" title="Cyclobutane">cyclobutane</a> ring when joined together and cause a distortion in the DNA. This distortion prevents <a href="/wiki/DNA_replication" title="DNA replication">DNA replication</a> and <a href="/wiki/Dna_transcription" class="mw-redirect" title="Dna transcription">transcription</a> mechanisms beyond the dimerization site.<sup id="cite_ref-Cooper_2000_6-0" class="reference"><a href="#cite_note-Cooper_2000-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> </p><p>While up to 100 such reactions per second may transpire in a <a href="/wiki/Skin_cell" class="mw-redirect" title="Skin cell">skin cell</a> exposed to sunlight resulting in <a href="/wiki/DNA_damage" class="mw-redirect" title="DNA damage">DNA damage</a>, they are typically rectified promptly through <a href="/wiki/DNA_repair" title="DNA repair">DNA repair</a>, such as through <a href="/wiki/Photolyase" title="Photolyase">photolyase</a> reactivation or <a href="/wiki/Nucleotide_excision_repair" title="Nucleotide excision repair">nucleotide excision repair</a>, with the latter being prevalent in humans. Conversely, certain bacteria utilize photolyase, powered by sunlight, to repair pyrimidine dimer-induced DNA damage. Unrepaired lesions may lead to erroneous nucleotide incorporation by <a href="/wiki/Polymerase" title="Polymerase">polymerase</a> machinery. Overwhelming DNA damage can precipitate <a href="/wiki/Mutations" class="mw-redirect" title="Mutations">mutations</a> within an organism's <a href="/wiki/Genome" title="Genome">genome</a>, potentially culminating in <a href="/wiki/Cancer" title="Cancer">cancer</a> cell formation.<sup id="cite_ref-Kemp_2012_7-0" class="reference"><a href="#cite_note-Kemp_2012-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> Unrectified lesions may also interfere with polymerase function, induce transcription or <a href="/wiki/Replication_error_phenotype" title="Replication error phenotype">replication errors</a>, or halt replication. Notably, pyrimidine dimers contribute to <a href="/wiki/Sunburn" title="Sunburn">sunburn</a> and <a href="/wiki/Melanin" title="Melanin">melanin</a> production, and are a primary factor in <a href="/wiki/Melanoma" title="Melanoma">melanoma</a> development in humans. </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Types_of_pyrimidine_dimers">Types of pyrimidine dimers</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pyrimidine_dimer&action=edit&section=1" title="Edit section: Types of pyrimidine dimers"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:CPD_%26_6-4PP_thymidine_photodimer.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/b/b8/CPD_%26_6-4PP_thymidine_photodimer.svg/220px-CPD_%26_6-4PP_thymidine_photodimer.svg.png" decoding="async" width="220" height="69" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/b/b8/CPD_%26_6-4PP_thymidine_photodimer.svg/330px-CPD_%26_6-4PP_thymidine_photodimer.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/b/b8/CPD_%26_6-4PP_thymidine_photodimer.svg/440px-CPD_%26_6-4PP_thymidine_photodimer.svg.png 2x" data-file-width="512" data-file-height="161" /></a><figcaption>Cyclobutane dimer (CPD) (left), 6,4-dimer (6-4PP) (right)</figcaption></figure> <p>Pyrimidine dimers encompass several types, each with distinct structures and implications for DNA integrity.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (November 2024)">citation needed</span></a></i>]</sup> </p><p>Cyclobutane pyrimidine dimer (CPD) is a dimer which features a four-membered ring formed by the fusion of two double-bonded carbons from adjacent pyrimidines. CPDs disrupt the formation of the <a href="/wiki/Base_pair" title="Base pair">base pair</a> during <a href="/wiki/DNA_replication" title="DNA replication">DNA replication</a>, potentially leading to <a href="/wiki/Mutations" class="mw-redirect" title="Mutations">mutations</a>.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-10" class="reference"><a href="#cite_note-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> </p><p>The 6–4 photoproduct (6–4 pyrimidine–<a href="/wiki/Pyrimidone" title="Pyrimidone">pyrimidone</a>, or 6–4 pyrimidine–pyrimidinone) is an alternate dimer configuration consisting of a single covalent bond linking the carbon at the 6 (C6) position of one pyrimidine ring and carbon at the 4 (C4) position of the adjoining base's ring.<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> This type of conversion occurs at one third the frequency of CPDs and has a higher <a href="/wiki/Mutagen" title="Mutagen">mutagenic</a> risk.<sup id="cite_ref-isbn0-8053-5015-2_12-0" class="reference"><a href="#cite_note-isbn0-8053-5015-2-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> </p><p>A third type of molecular lesion is a <a href="/wiki/Dewar_benzene" title="Dewar benzene">Dewar</a> pyrimidinone, resulting from the reversible <a href="/wiki/Isomerization" title="Isomerization">isomerization</a> of a 6–4 photoproduct under further light exposure.<sup id="cite_ref-13" class="reference"><a href="#cite_note-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Mutagenesis">Mutagenesis</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pyrimidine_dimer&action=edit&section=2" title="Edit section: Mutagenesis"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Mutagenesis" title="Mutagenesis">Mutagenesis</a></div> <p>Mutagenesis, the process of mutation formation, is significantly influenced by translesion <a href="/wiki/Polymerases" class="mw-redirect" title="Polymerases">polymerases</a> which often introduce mutations at sites of pyrimidine dimers. This occurrence is noted both in <a href="/wiki/Prokaryotes" class="mw-redirect" title="Prokaryotes">prokaryotes</a>, through the <a href="/wiki/SOS_response" title="SOS response">SOS response</a> to mutagenesis, and in <a href="/wiki/Eukaryotes" class="mw-redirect" title="Eukaryotes">eukaryotes</a>. Despite thymine-thymine CPDs being the most common lesions induced by UV, translesion polymerases show a tendency to incorporate <a href="/wiki/Adenine" title="Adenine">adenines</a>, resulting in the accurate replication of thymine dimers more often than not. Conversely, cytosines that are part of CPDs are susceptible to <a href="/wiki/Deamination" title="Deamination">deamination</a>, leading to a cytosine to thymine transition, thereby contributing to the mutation process.<sup id="cite_ref-14" class="reference"><a href="#cite_note-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="DNA_repair">DNA repair</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pyrimidine_dimer&action=edit&section=3" title="Edit section: DNA repair"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Further information: <a href="/wiki/DNA_repair" title="DNA repair">DNA repair</a> and <a href="/wiki/Ultraviolet_light_and_cancer" class="mw-redirect" title="Ultraviolet light and cancer">ultraviolet light and cancer</a></div> <figure class="mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:Melanoma.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/6/6c/Melanoma.jpg/300px-Melanoma.jpg" decoding="async" width="300" height="209" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/6/6c/Melanoma.jpg/450px-Melanoma.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/6/6c/Melanoma.jpg/600px-Melanoma.jpg 2x" data-file-width="2450" data-file-height="1705" /></a><figcaption><a href="/wiki/Melanoma" title="Melanoma">Melanoma</a>, a type of skin cancer</figcaption></figure> <p>Pyrimidine dimers introduce local conformational changes in the <a href="/wiki/Nucleic_acid_structure" title="Nucleic acid structure">DNA structure</a>, which allow recognition of the lesion by repair enzymes.<sup id="cite_ref-15" class="reference"><a href="#cite_note-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup> In most organisms (excluding <a href="/wiki/Placentalia" title="Placentalia">placental mammals</a> such as humans) they can be repaired by photoreactivation.<sup id="cite_ref-16" class="reference"><a href="#cite_note-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> Photoreactivation is a repair process in which <a href="/wiki/Photolyase" title="Photolyase">photolyase</a> enzymes reverse CPDs using <a href="/wiki/Photochemical" class="mw-redirect" title="Photochemical">photochemical</a> reactions. In addition, some photolyases can also repair 6-4 photoproducts of UV induced DNA damage. Photolyase enzymes utilize <a href="/wiki/Flavin_adenine_dinucleotide" title="Flavin adenine dinucleotide">flavin adenine dinucleotide (FAD)</a> as a cofactor in the repair process.<sup id="cite_ref-Friedberg,_Errol_C._2003_17-0" class="reference"><a href="#cite_note-Friedberg,_Errol_C._2003-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup> </p><p>The UV dose that reduces a population of wild-type yeast cells to 37% survival is equivalent (assuming a <a href="/wiki/Poisson_distribution" title="Poisson distribution">Poisson distribution</a> of hits) to the UV dose that causes an average of one lethal hit to each of the cells of the population.<sup id="cite_ref-Cox1974_18-0" class="reference"><a href="#cite_note-Cox1974-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> The number of pyrimidine dimers induced per <a href="/wiki/Ploidy" title="Ploidy">haploid</a> <a href="/wiki/Genome" title="Genome">genome</a> at this dose was measured as 27,000.<sup id="cite_ref-Cox1974_18-1" class="reference"><a href="#cite_note-Cox1974-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> A mutant yeast strain defective in the three pathways by which pyrimidine dimers were known to be <a href="/wiki/DNA_repair" title="DNA repair">repaired</a> in yeast was also tested for UV sensitivity. It was found in this case that only one or, at most, two unrepaired pyrimidine dimers per haploid genome are lethal to the cell.<sup id="cite_ref-Cox1974_18-2" class="reference"><a href="#cite_note-Cox1974-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> These findings thus indicate that the repair of thymine dimers in wild-type yeast is highly efficient.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (November 2024)">citation needed</span></a></i>]</sup> </p><p><a href="/wiki/Nucleotide_excision_repair" title="Nucleotide excision repair">Nucleotide excision repair</a>, sometimes termed "dark reactivation", is a more general mechanism for repair of lesions and is the most common form of DNA repair for pyrimidine dimers in humans. This process works by using cellular machinery to locate the dimerized nucleotides and excise the lesion. Once the CPD is removed, there is a gap in the DNA strand that must be filled. DNA machinery uses the undamaged complementary strand to synthesize nucleotides off of and consequently fill in the gap on the previously damaged strand.<sup id="cite_ref-Cooper_2000_6-1" class="reference"><a href="#cite_note-Cooper_2000-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> </p><p><a href="/wiki/Xeroderma_pigmentosum" title="Xeroderma pigmentosum">Xeroderma pigmentosum</a> (XP) is a rare genetic disease in humans in which genes that encode for NER proteins are mutated and result in decreased ability to combat pyrimidine dimers that form as a result of UV damage. Individuals with XP are also at a much higher risk of cancer than others, with a greater than 5,000 fold increased risk of developing skin cancers.<sup id="cite_ref-Kemp_2012_7-1" class="reference"><a href="#cite_note-Kemp_2012-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> Some common features and symptoms of XP include skin discoloration, and the formation of multiple tumors proceeding UV exposure.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (November 2024)">citation needed</span></a></i>]</sup> </p><p>A few organisms have other ways to perform repairs: </p> <ul><li><a href="/wiki/Spore_photoproduct_lyase" title="Spore photoproduct lyase">Spore photoproduct lyase</a> is found in spore-forming bacteria. It returns thymine dimers to their original state.<sup id="cite_ref-19" class="reference"><a href="#cite_note-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup></li> <li><a href="/wiki/Deoxyribodipyrimidine_endonucleosidase" title="Deoxyribodipyrimidine endonucleosidase">Deoxyribodipyrimidine endonucleosidase</a> is found in <a href="/wiki/Bacteriophage_T4" class="mw-redirect" title="Bacteriophage T4">bacteriophage T4</a>. It is a <a href="/wiki/Base_excision_repair" title="Base excision repair">base excision repair</a> enzyme specific for pyrimidine dimers. It is then able to cut open the <a href="/wiki/AP_site" title="AP site">AP site</a>.</li></ul><p> Another type of repair mechanism that is conserved in humans and other non-mammals is <a href="/wiki/Translesion" class="mw-redirect" title="Translesion">translesion synthesis.</a> Typically, the lesion associated with the pyrimidine dimer blocks cellular machinery from synthesizing past the damaged site. However, in translesion synthesis, the CPD is bypassed by translesion polymerases, and replication and or transcription machinery can continue past the lesion. One specific translesion DNA polymerase, DNA polymerase η, is deficient in individuals with XPD.<sup id="cite_ref-20" class="reference"><a href="#cite_note-20"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup></p><div style="clear:both;" class=""></div> <div class="mw-heading mw-heading2"><h2 id="Effect_of_topical_sunscreen_and_effect_of_absorbed_sunscreen">Effect of topical sunscreen and effect of absorbed sunscreen</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pyrimidine_dimer&action=edit&section=4" title="Edit section: Effect of topical sunscreen and effect of absorbed sunscreen"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Direct DNA damage is reduced by sunscreen, which also reduces the risk of developing a sunburn. When the sunscreen is at the surface of the skin, it filters the UV rays, which attenuates the intensity. Even when the sunscreen molecules have penetrated into the skin, they protect against direct DNA damage, because the UV light is absorbed by the sunscreen and not by the DNA.<sup id="cite_ref-KnowlandGulston1999_21-0" class="reference"><a href="#cite_note-KnowlandGulston1999-21"><span class="cite-bracket">[</span>21<span class="cite-bracket">]</span></a></sup> Sunscreen primarily works by absorbing the UV light from the sun through the use of organic compounds, such as oxybenzone or avobenzone. These compounds are able to absorb UV energy from the sun and transition into higher-energy states. Eventually, these molecules return to lower energy states, and in doing so, the initial energy from the UV light can be transformed into heat. This process of absorption works to reduce the risk of DNA damage and the formation of pyrimidine dimers. UVA light makes up 95% of the UV light that reaches earth, whereas UVB light makes up only about 5%. UVB light is the form of UV light that is responsible for tanning and burning. Sunscreens work to protect from both UVA and UVB rays. Overall, sunburns exemplify DNA damage caused by UV rays, and this damage can come in the form of free radical species, as well as dimerization of adjacent nucleotides.<sup id="cite_ref-22" class="reference"><a href="#cite_note-22"><span class="cite-bracket">[</span>22<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pyrimidine_dimer&action=edit&section=5" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/DNA_repair" title="DNA repair">DNA repair</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pyrimidine_dimer&action=edit&section=6" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap mw-references-columns"><ol class="references"> <li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite id="CITEREFGoodsell2001" class="citation journal cs1">Goodsell DS (2001). <a rel="nofollow" class="external text" href="https://doi.org/10.1634%2Ftheoncologist.6-3-298">"The molecular perspective: ultraviolet light and pyrimidine dimers"</a>. <i>The Oncologist</i>. <b>6</b> (3): 298–299. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://doi.org/10.1634%2Ftheoncologist.6-3-298">10.1634/theoncologist.6-3-298</a></span>. <a href="/wiki/PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/11423677">11423677</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:36511461">36511461</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=The+Oncologist&rft.atitle=The+molecular+perspective%3A+ultraviolet+light+and+pyrimidine+dimers&rft.volume=6&rft.issue=3&rft.pages=298-299&rft.date=2001&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A36511461%23id-name%3DS2CID&rft_id=info%3Apmid%2F11423677&rft_id=info%3Adoi%2F10.1634%2Ftheoncologist.6-3-298&rft.aulast=Goodsell&rft.aufirst=DS&rft_id=https%3A%2F%2Fdoi.org%2F10.1634%252Ftheoncologist.6-3-298&rfr_id=info%3Asid%2Fen.wikipedia.org%3APyrimidine+dimer" class="Z3988"></span></span> </li> <li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFFriedbergWalkerSiedeWood2006" class="citation book cs1">Friedberg EC, Walker GC, Siede W, Wood RD, Schultz RA, Ellenberger T, eds. (2006). <i>DNA repair and mutagenesis</i>. 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