{"id":962,"date":"2022-04-20T20:48:36","date_gmt":"2022-04-20T20:48:36","guid":{"rendered":"https:\/\/pressbooks.hcfl.edu\/bio1\/chapter\/dna-repair\/"},"modified":"2025-08-29T19:14:23","modified_gmt":"2025-08-29T19:14:23","slug":"dna-repair","status":"publish","type":"chapter","link":"https:\/\/pressbooks.hcfl.edu\/bio1\/chapter\/dna-repair\/","title":{"raw":"DNA Repair","rendered":"DNA Repair"},"content":{"raw":"<div class=\"textbox textbox--learning-objectives\"><header class=\"textbox__header\">\n<h2 class=\"textbox__title\">Learning Objectives<\/h2>\n<\/header>\n<div class=\"textbox__content\">\n\nBy the end of this section, you will be able to do the following:\n<ul>\n \t<li>Discuss the different types of mutations in DNA<\/li>\n \t<li>Explain DNA repair mechanisms<\/li>\n<\/ul>\n<\/div>\n<\/div>\nDNA replication is a highly accurate process, but mistakes can occasionally occur, such as a DNA polymerase inserting a wrong base. Uncorrected mistakes may sometimes lead to serious consequences, such as cancer. Repair mechanisms correct the mistakes. In rare cases, mistakes are not corrected, leading to mutations; in other cases, repair enzymes are themselves mutated or defective.\n<p id=\"fs-id2348700\">Most of the mistakes during DNA replication are promptly corrected by the proofreading ability of DNA polymerase itself. (Figure 14.17). In\u00a0<strong><span id=\"term546\" data-type=\"term\">proofreading<\/span><\/strong>, the DNA pol reads the newly added base before adding the next one, so a correction can be made. The polymerase checks whether the newly added base has paired correctly with the base in the template strand. If it is the right base, the next nucleotide is added. If an incorrect base has been added, the enzyme makes a cut at the phosphodiester bond and releases the wrong nucleotide. This is performed by the 3' exonuclease action of DNA pol. Once the incorrect nucleotide has been removed, it can be replaced by the correct one.<\/p>\n\n<div id=\"fig-ch14_06_01\" class=\"os-figure\">\n<figure data-id=\"fig-ch14_06_01\"><span id=\"fs-id2321057\" data-type=\"media\" data-alt=\"Illustration shows D N A polymerase replicating a strand of D N A. The enzyme has accidentally inserted G opposite A, resulting in a bulge. The enzyme backs up to fix the error.\"><\/span><\/figure>\n<div class=\"os-caption-container\" style=\"text-align: center\"><span class=\"os-caption\">\u00a0<\/span><\/div>\n<div>\n\n[caption id=\"attachment_957\" align=\"aligncenter\" width=\"1024\"]<img class=\"wp-image-957 size-large\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/77-1024x379.png\" alt=\"Illustration shows D N A polymerase replicating a strand of D N A. The enzyme has accidentally inserted G opposite A, resulting in a bulge. The enzyme backs up to fix the error.\" width=\"1024\" height=\"379\"> Figure\u00a014.17\u00a0Proofreading by DNA polymerase corrects errors during replication.[\/caption]\n\n<\/div>\n<\/div>\n<p id=\"fs-id2186525\">Some errors are not corrected during replication, but are instead corrected after replication is completed; this type of repair is known as\u00a0<strong><span id=\"term547\" data-type=\"term\">mismatch repair<\/span>\u00a0<\/strong>(Figure 14.18). Specific repair enzymes recognize the mispaired nucleotide and excise part of the strand that contains it; the excised region is then resynthesized. If the mismatch remains uncorrected, it may lead to more permanent damage when the mismatched DNA is replicated. How do mismatch repair enzymes recognize which of the two bases is the incorrect one? In\u00a0<em data-effect=\"italics\">E. coli<\/em>, after replication, the nitrogenous base adenine acquires a methyl group; the parental DNA strand will have methyl groups, whereas the newly synthesized strand lacks them. Thus, DNA polymerase is able to remove the wrongly incorporated bases from the newly synthesized, non-methylated strand. In eukaryotes, the mechanism is not very well understood, but it is believed to involve recognition of unsealed nicks in the new strand, as well as a short-term continuing association of some of the replication proteins with the new daughter strand after replication has completed.<\/p>\n\n<div id=\"fig-ch14_06_02\" class=\"os-figure\">\n<figure data-id=\"fig-ch14_06_02\"><span id=\"fs-id2025027\" data-type=\"media\" data-alt=\"The top illustration shows a replicated D N A strand with G T base mismatch. The bottom illustration shows the repaired D N A, which has the correct G C base pairing.\"><\/span><\/figure>\n<div class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/div>\n<div>\n\n[caption id=\"attachment_958\" align=\"aligncenter\" width=\"978\"]<img class=\"wp-image-958 size-large\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/78-978x1024.png\" alt=\"The top illustration shows a replicated D N A strand with G T base mismatch. The bottom illustration shows the repaired D N A, which has the correct G C base pairing.\" width=\"978\" height=\"1024\"> Figure\u00a014.18\u00a0In mismatch repair, the incorrectly added base is detected after replication. The mismatch repair proteins detect this base and remove it from the newly synthesized strand by nuclease action. The gap is now filled with the correctly paired base.[\/caption]\n\n<\/div>\n<\/div>\n<p id=\"fs-id3000290\">Another type of repair mechanism,\u00a0<strong><span id=\"term548\" data-type=\"term\">nucleotide excision repair<\/span><\/strong>, is similar to mismatch repair, except that it is used to remove damaged bases rather than mismatched ones. The repair enzymes replace abnormal bases by making a cut on both the 3' and 5' ends of the damaged base (Figure 14.19). The segment of DNA is removed and replaced with the correctly paired nucleotides by the action of DNA pol. Once the bases are filled in, the remaining gap is sealed with a phosphodiester linkage catalyzed by DNA ligase. This repair mechanism is often employed when UV exposure causes the formation of pyrimidine dimers.<\/p>\n\n<div>\n<figure data-id=\"fig-ch14_06_03\"><span id=\"fs-id1695253\" data-type=\"media\" data-alt=\"Illustration shows a D N A strand in which a thymine dimer has formed. Excision repair enzyme cut out the section of D N A that contains the dimer so it can be replaced with normal base pairs. Nuclease cuts the D N A on each side of the dimer, and removes the single strand of D N A containing the dimer. DNA Polymerase synthesizes new D N A to repair the hole. D N A ligase forms the final phosphidiester bond between the new D N A and old D N A.\"><\/span><\/figure>\n<p class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/p>\n\n\n[caption id=\"attachment_959\" align=\"aligncenter\" width=\"800\"]<img class=\"wp-image-959 size-full\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_681_Image_0001.jpg\" alt=\"Illustration shows a D N A strand in which a thymine dimer has formed. Excision repair enzyme cut out the section of D N A that contains the dimer so it can be replaced with normal base pairs. Nuclease cuts the D N A on each side of the dimer, and removes the single strand of D N A containing the dimer. DNA Polymerase synthesizes new D N A to repair the hole. D N A ligase forms the final phosphidiester bond between the new D N A and old D N A.\" width=\"800\" height=\"821\"> Figure\u00a014.19\u00a0Nucleotide excision repairs thymine dimers. When exposed to UV light, thymines lying adjacent to each other can form thymine dimers. In normal cells, they are excised and replaced. Credit: Rao, A., Fletcher, S. and Tag, A. Department of Biology, Texas A&amp;M University.[\/caption]\n\n<\/div>\n<span style=\"text-align: justify;font-size: 1em\">A well-studied example of mistakes not being corrected is seen in people suffering from xeroderma pigmentosa (<\/span><span style=\"font-size: 1em\">Figure 14.20<\/span><span style=\"text-align: justify;font-size: 1em\">). Affected individuals have skin that is highly sensitive to UV rays from the sun. When individuals are exposed to UV light, pyrimidine dimers, especially those of thymine, are formed; people with xeroderma pigmentosa are not able to repair the damage. These are not repaired because of a defect in the nucleotide excision repair enzymes, whereas in normal individuals, the thymine dimers are excised and the defect is corrected. The thymine dimers distort the structure of the DNA double helix, and this may cause problems during DNA replication. People with xeroderma pigmentosa may have a higher risk of contracting skin cancer than those who don't have the condition.<\/span>\n<div id=\"fig-ch14_06_04\" class=\"os-figure\">\n<figure data-id=\"fig-ch14_06_04\"><span id=\"fs-id2583496\" data-type=\"media\" data-alt=\"Photo shows a person with mottled skin lesions that result from xermoderma pigmentosa.\"><\/span><\/figure>\n<div class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/div>\n<div>\n\n[caption id=\"attachment_960\" align=\"aligncenter\" width=\"300\"]<img class=\"wp-image-960 size-medium\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_682_Image_0001-300x253.jpg\" alt=\"Photo shows a person with mottled skin lesions that result from xermoderma pigmentosa.\" width=\"300\" height=\"253\"> Figure\u00a014.20\u00a0Xeroderma pigmentosa is a condition in which thymine dimerization from exposure to UV light is not repaired. Exposure to sunlight results in skin lesions. (credit: James Halpern et al.)[\/caption]\n\n<\/div>\n<\/div>\n<p id=\"fs-id1310247\">Errors during DNA replication are not the only reason why mutations arise in DNA.\u00a0<strong><span id=\"term549\" data-type=\"term\">Mutations<\/span><\/strong>, variations in the nucleotide sequence of a genome, can also occur because of damage to DNA. Such mutations may be of two types: induced or spontaneous.\u00a0<strong><span id=\"term550\" data-type=\"term\">Induced mutations<\/span><\/strong>\u00a0are those that result from an exposure to chemicals, UV rays, x-rays, or some other environmental agent. For example, Charlotte Auerbach and J.M Robson discovered the mutation-inducing effects of mustard gas.\u00a0<strong><span id=\"term551\" data-type=\"term\">Spontaneous mutations<\/span><\/strong>\u00a0occur without any exposure to any environmental agent; they are a result of natural reactions taking place within the body.<\/p>\n<p id=\"fs-id1368727\">Mutations may have a wide range of effects. Point mutations are those mutations that affect a single base pair. The most common nucleotide mutations are substitutions, in which one base is replaced by another. These substitutions can be of two types, either transitions or transversions.\u00a0<strong><span id=\"term552\" data-type=\"term\">Transition substitution<\/span><\/strong>\u00a0refers to a purine or pyrimidine being replaced by a base of the same kind; for example, a purine such as adenine may be replaced by the purine guanine.\u00a0<strong><span id=\"term553\" data-type=\"term\">Transversion substitution<\/span><\/strong>\u00a0refers to a purine being replaced by a pyrimidine, or vice versa; for example, cytosine, a pyrimidine, is replaced by adenine, a purine. Some point mutations are not detectable in the final product; these are known as silent mutations. Silent mutations are usually due to a substitution in the third base of a codon, which often represents the same amino acid as the original codon. Other point mutations can result in the replacement of one amino acid by another, which may alter the function of the protein. Point mutations that generate a stop codon can terminate a protein early.<\/p>\n<p id=\"fs-id1378727\">Some mutations can result in an increased number of copies of the same codon. These are called trinucleotide repeat expansions and result in repeated regions of the same amino acid. Mutations can also be the result of the addition of a base, known as an insertion, or the removal of a base, also known as deletion. If an insertion or deletion results in the alteration of the translational reading frame (a frameshift mutation), the resultant protein is usually nonfunctional. Sometimes a piece of DNA from one chromosome may get translocated to another chromosome or to another region of the same chromosome; this is also known as translocation. These mutation types are shown in\u00a0Figure 14.21.<\/p>\n\n<div class=\"textbox\">\n<h3 id=\"6\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Visual Connection<\/span><\/h3>\n<div id=\"fig-ch14_06_05\" class=\"os-figure\">\n<figure data-id=\"fig-ch14_06_05\"><span id=\"fs-id2221137\" data-type=\"media\" data-alt=\"Illustration shows different types of point mutations that result from a single amino acid substitution. In a silent mutation, no change in the amino acid sequence occurs. In a missense mutation, one amino acid is substituted for another. In a nonsense mutation, a stop codon is substituted for an amino acid. In a frameshift mutation, one or more bases is added or deleted, resulting in a change in the reading frame.\"><\/span><\/figure>\n<p class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/p>\n\n\n[caption id=\"attachment_961\" align=\"aligncenter\" width=\"469\"]<img class=\"wp-image-961 size-full\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/79.png\" alt=\"Illustration shows different types of point mutations that result from a single amino acid substitution. In a silent mutation, no change in the amino acid sequence occurs. In a missense mutation, one amino acid is substituted for another. In a nonsense mutation, a stop codon is substituted for an amino acid. In a frameshift mutation, one or more bases is added or deleted, resulting in a change in the reading frame.\" width=\"469\" height=\"599\"> Figure\u00a014.21\u00a0Mutations can lead to changes in the protein sequence encoded by the DNA.[\/caption]\n<p class=\"os-caption-container\"><span style=\"font-size: 1rem\">A frameshift mutation that results in the insertion of three nucleotides is often less deleterious than a mutation that results in the insertion of one nucleotide. Why?<\/span><\/p>\n\n<\/div>\n<\/div>\n<div class=\"textbox\">\n<h3 id=\"6\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Link to Learning<\/span><\/h3>\n<div id=\"fig-ch14_06_05\" class=\"os-figure\">\n<div class=\"os-caption-container\"><\/div>\n<\/div>\nLearn more by watching this video <a href=\"https:\/\/www.youtube.com\/watch?v=sX6LncNjTFU\">Mechanisms of DNA Damage and Repair<\/a>\n\n<\/div>\n<div id=\"fs-id2046784\" class=\"visual-connection ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\"><section>\n<div class=\"os-note-body\">\n<div id=\"fig-ch14_06_05\" class=\"os-figure\">\n<p class=\"os-caption-container\" style=\"text-align: justify\">Mutations in repair genes have been known to cause cancer. Many mutated repair genes have been implicated in certain forms of pancreatic cancer, colon cancer, and colorectal cancer. Mutations can affect either somatic cells or germ cells. If many mutations accumulate in a somatic cell, they may lead to problems such as the uncontrolled cell division observed in cancer. If a mutation takes place in germ cells, the mutation will be passed on to the next generation, as in the case of hemophilia and xeroderma pigmentosa.<\/p>\n&nbsp;\n\n[h5p id=\"170\"]\n\n<\/div>\n<\/div>\n<\/section><\/div>\n&nbsp;\n\n[h5p id=\"171\"]\n\n&nbsp;\n\n[h5p id=\"172\"]\n\n&nbsp;\n\n[h5p id=\"173\"]\n\n&nbsp;\n\n[h5p id=\"174\"]\n\n&nbsp;\n\n[h5p id=\"175\"]\n\n&nbsp;","rendered":"<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<h2 class=\"textbox__title\">Learning Objectives<\/h2>\n<\/header>\n<div class=\"textbox__content\">\n<p>By the end of this section, you will be able to do the following:<\/p>\n<ul>\n<li>Discuss the different types of mutations in DNA<\/li>\n<li>Explain DNA repair mechanisms<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<p>DNA replication is a highly accurate process, but mistakes can occasionally occur, such as a DNA polymerase inserting a wrong base. Uncorrected mistakes may sometimes lead to serious consequences, such as cancer. Repair mechanisms correct the mistakes. In rare cases, mistakes are not corrected, leading to mutations; in other cases, repair enzymes are themselves mutated or defective.<\/p>\n<p id=\"fs-id2348700\">Most of the mistakes during DNA replication are promptly corrected by the proofreading ability of DNA polymerase itself. (Figure 14.17). In\u00a0<strong><span id=\"term546\" data-type=\"term\">proofreading<\/span><\/strong>, the DNA pol reads the newly added base before adding the next one, so a correction can be made. The polymerase checks whether the newly added base has paired correctly with the base in the template strand. If it is the right base, the next nucleotide is added. If an incorrect base has been added, the enzyme makes a cut at the phosphodiester bond and releases the wrong nucleotide. This is performed by the 3&#8242; exonuclease action of DNA pol. Once the incorrect nucleotide has been removed, it can be replaced by the correct one.<\/p>\n<div id=\"fig-ch14_06_01\" class=\"os-figure\">\n<figure data-id=\"fig-ch14_06_01\"><span id=\"fs-id2321057\" data-type=\"media\" data-alt=\"Illustration shows D N A polymerase replicating a strand of D N A. The enzyme has accidentally inserted G opposite A, resulting in a bulge. The enzyme backs up to fix the error.\"><\/span><\/figure>\n<div class=\"os-caption-container\" style=\"text-align: center\"><span class=\"os-caption\">\u00a0<\/span><\/div>\n<div>\n<figure id=\"attachment_957\" aria-describedby=\"caption-attachment-957\" style=\"width: 1024px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-957 size-large\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/77-1024x379.png\" alt=\"Illustration shows D N A polymerase replicating a strand of D N A. The enzyme has accidentally inserted G opposite A, resulting in a bulge. The enzyme backs up to fix the error.\" width=\"1024\" height=\"379\" srcset=\"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/77-1024x379.png 1024w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/77-300x111.png 300w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/77-768x284.png 768w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/77-65x24.png 65w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/77-225x83.png 225w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/77-350x130.png 350w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/77.png 1078w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><figcaption id=\"caption-attachment-957\" class=\"wp-caption-text\">Figure\u00a014.17\u00a0Proofreading by DNA polymerase corrects errors during replication.<\/figcaption><\/figure>\n<\/div>\n<\/div>\n<p id=\"fs-id2186525\">Some errors are not corrected during replication, but are instead corrected after replication is completed; this type of repair is known as\u00a0<strong><span id=\"term547\" data-type=\"term\">mismatch repair<\/span>\u00a0<\/strong>(Figure 14.18). Specific repair enzymes recognize the mispaired nucleotide and excise part of the strand that contains it; the excised region is then resynthesized. If the mismatch remains uncorrected, it may lead to more permanent damage when the mismatched DNA is replicated. How do mismatch repair enzymes recognize which of the two bases is the incorrect one? In\u00a0<em data-effect=\"italics\">E. coli<\/em>, after replication, the nitrogenous base adenine acquires a methyl group; the parental DNA strand will have methyl groups, whereas the newly synthesized strand lacks them. Thus, DNA polymerase is able to remove the wrongly incorporated bases from the newly synthesized, non-methylated strand. In eukaryotes, the mechanism is not very well understood, but it is believed to involve recognition of unsealed nicks in the new strand, as well as a short-term continuing association of some of the replication proteins with the new daughter strand after replication has completed.<\/p>\n<div id=\"fig-ch14_06_02\" class=\"os-figure\">\n<figure data-id=\"fig-ch14_06_02\"><span id=\"fs-id2025027\" data-type=\"media\" data-alt=\"The top illustration shows a replicated D N A strand with G T base mismatch. The bottom illustration shows the repaired D N A, which has the correct G C base pairing.\"><\/span><\/figure>\n<div class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/div>\n<div>\n<figure id=\"attachment_958\" aria-describedby=\"caption-attachment-958\" style=\"width: 978px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-958 size-large\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/78-978x1024.png\" alt=\"The top illustration shows a replicated D N A strand with G T base mismatch. The bottom illustration shows the repaired D N A, which has the correct G C base pairing.\" width=\"978\" height=\"1024\" srcset=\"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/78-978x1024.png 978w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/78-286x300.png 286w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/78-768x804.png 768w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/78-65x68.png 65w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/78-225x236.png 225w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/78-350x367.png 350w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/78.png 1077w\" sizes=\"auto, (max-width: 978px) 100vw, 978px\" \/><figcaption id=\"caption-attachment-958\" class=\"wp-caption-text\">Figure\u00a014.18\u00a0In mismatch repair, the incorrectly added base is detected after replication. The mismatch repair proteins detect this base and remove it from the newly synthesized strand by nuclease action. The gap is now filled with the correctly paired base.<\/figcaption><\/figure>\n<\/div>\n<\/div>\n<p id=\"fs-id3000290\">Another type of repair mechanism,\u00a0<strong><span id=\"term548\" data-type=\"term\">nucleotide excision repair<\/span><\/strong>, is similar to mismatch repair, except that it is used to remove damaged bases rather than mismatched ones. The repair enzymes replace abnormal bases by making a cut on both the 3&#8242; and 5&#8242; ends of the damaged base (Figure 14.19). The segment of DNA is removed and replaced with the correctly paired nucleotides by the action of DNA pol. Once the bases are filled in, the remaining gap is sealed with a phosphodiester linkage catalyzed by DNA ligase. This repair mechanism is often employed when UV exposure causes the formation of pyrimidine dimers.<\/p>\n<div>\n<figure data-id=\"fig-ch14_06_03\"><span id=\"fs-id1695253\" data-type=\"media\" data-alt=\"Illustration shows a D N A strand in which a thymine dimer has formed. Excision repair enzyme cut out the section of D N A that contains the dimer so it can be replaced with normal base pairs. Nuclease cuts the D N A on each side of the dimer, and removes the single strand of D N A containing the dimer. DNA Polymerase synthesizes new D N A to repair the hole. D N A ligase forms the final phosphidiester bond between the new D N A and old D N A.\"><\/span><\/figure>\n<p class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/p>\n<figure id=\"attachment_959\" aria-describedby=\"caption-attachment-959\" style=\"width: 800px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-959 size-full\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_681_Image_0001.jpg\" alt=\"Illustration shows a D N A strand in which a thymine dimer has formed. Excision repair enzyme cut out the section of D N A that contains the dimer so it can be replaced with normal base pairs. Nuclease cuts the D N A on each side of the dimer, and removes the single strand of D N A containing the dimer. DNA Polymerase synthesizes new D N A to repair the hole. D N A ligase forms the final phosphidiester bond between the new D N A and old D N A.\" width=\"800\" height=\"821\" srcset=\"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_681_Image_0001.jpg 800w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_681_Image_0001-292x300.jpg 292w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_681_Image_0001-768x788.jpg 768w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_681_Image_0001-65x67.jpg 65w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_681_Image_0001-225x231.jpg 225w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_681_Image_0001-350x359.jpg 350w\" sizes=\"auto, (max-width: 800px) 100vw, 800px\" \/><figcaption id=\"caption-attachment-959\" class=\"wp-caption-text\">Figure\u00a014.19\u00a0Nucleotide excision repairs thymine dimers. When exposed to UV light, thymines lying adjacent to each other can form thymine dimers. In normal cells, they are excised and replaced. Credit: Rao, A., Fletcher, S. and Tag, A. Department of Biology, Texas A&amp;M University.<\/figcaption><\/figure>\n<\/div>\n<p><span style=\"text-align: justify;font-size: 1em\">A well-studied example of mistakes not being corrected is seen in people suffering from xeroderma pigmentosa (<\/span><span style=\"font-size: 1em\">Figure 14.20<\/span><span style=\"text-align: justify;font-size: 1em\">). Affected individuals have skin that is highly sensitive to UV rays from the sun. When individuals are exposed to UV light, pyrimidine dimers, especially those of thymine, are formed; people with xeroderma pigmentosa are not able to repair the damage. These are not repaired because of a defect in the nucleotide excision repair enzymes, whereas in normal individuals, the thymine dimers are excised and the defect is corrected. The thymine dimers distort the structure of the DNA double helix, and this may cause problems during DNA replication. People with xeroderma pigmentosa may have a higher risk of contracting skin cancer than those who don&#8217;t have the condition.<\/span><\/p>\n<div id=\"fig-ch14_06_04\" class=\"os-figure\">\n<figure data-id=\"fig-ch14_06_04\"><span id=\"fs-id2583496\" data-type=\"media\" data-alt=\"Photo shows a person with mottled skin lesions that result from xermoderma pigmentosa.\"><\/span><\/figure>\n<div class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/div>\n<div>\n<figure id=\"attachment_960\" aria-describedby=\"caption-attachment-960\" style=\"width: 300px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-960 size-medium\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_682_Image_0001-300x253.jpg\" alt=\"Photo shows a person with mottled skin lesions that result from xermoderma pigmentosa.\" width=\"300\" height=\"253\" srcset=\"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_682_Image_0001-300x253.jpg 300w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_682_Image_0001-1024x864.jpg 1024w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_682_Image_0001-768x648.jpg 768w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_682_Image_0001-65x55.jpg 65w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_682_Image_0001-225x190.jpg 225w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_682_Image_0001-350x295.jpg 350w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_682_Image_0001.jpg 1270w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><figcaption id=\"caption-attachment-960\" class=\"wp-caption-text\">Figure\u00a014.20\u00a0Xeroderma pigmentosa is a condition in which thymine dimerization from exposure to UV light is not repaired. Exposure to sunlight results in skin lesions. (credit: James Halpern et al.)<\/figcaption><\/figure>\n<\/div>\n<\/div>\n<p id=\"fs-id1310247\">Errors during DNA replication are not the only reason why mutations arise in DNA.\u00a0<strong><span id=\"term549\" data-type=\"term\">Mutations<\/span><\/strong>, variations in the nucleotide sequence of a genome, can also occur because of damage to DNA. Such mutations may be of two types: induced or spontaneous.\u00a0<strong><span id=\"term550\" data-type=\"term\">Induced mutations<\/span><\/strong>\u00a0are those that result from an exposure to chemicals, UV rays, x-rays, or some other environmental agent. For example, Charlotte Auerbach and J.M Robson discovered the mutation-inducing effects of mustard gas.\u00a0<strong><span id=\"term551\" data-type=\"term\">Spontaneous mutations<\/span><\/strong>\u00a0occur without any exposure to any environmental agent; they are a result of natural reactions taking place within the body.<\/p>\n<p id=\"fs-id1368727\">Mutations may have a wide range of effects. Point mutations are those mutations that affect a single base pair. The most common nucleotide mutations are substitutions, in which one base is replaced by another. These substitutions can be of two types, either transitions or transversions.\u00a0<strong><span id=\"term552\" data-type=\"term\">Transition substitution<\/span><\/strong>\u00a0refers to a purine or pyrimidine being replaced by a base of the same kind; for example, a purine such as adenine may be replaced by the purine guanine.\u00a0<strong><span id=\"term553\" data-type=\"term\">Transversion substitution<\/span><\/strong>\u00a0refers to a purine being replaced by a pyrimidine, or vice versa; for example, cytosine, a pyrimidine, is replaced by adenine, a purine. Some point mutations are not detectable in the final product; these are known as silent mutations. Silent mutations are usually due to a substitution in the third base of a codon, which often represents the same amino acid as the original codon. Other point mutations can result in the replacement of one amino acid by another, which may alter the function of the protein. Point mutations that generate a stop codon can terminate a protein early.<\/p>\n<p id=\"fs-id1378727\">Some mutations can result in an increased number of copies of the same codon. These are called trinucleotide repeat expansions and result in repeated regions of the same amino acid. Mutations can also be the result of the addition of a base, known as an insertion, or the removal of a base, also known as deletion. If an insertion or deletion results in the alteration of the translational reading frame (a frameshift mutation), the resultant protein is usually nonfunctional. Sometimes a piece of DNA from one chromosome may get translocated to another chromosome or to another region of the same chromosome; this is also known as translocation. These mutation types are shown in\u00a0Figure 14.21.<\/p>\n<div class=\"textbox\">\n<h3 id=\"6\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Visual Connection<\/span><\/h3>\n<div id=\"fig-ch14_06_05\" class=\"os-figure\">\n<figure data-id=\"fig-ch14_06_05\"><span id=\"fs-id2221137\" data-type=\"media\" data-alt=\"Illustration shows different types of point mutations that result from a single amino acid substitution. In a silent mutation, no change in the amino acid sequence occurs. In a missense mutation, one amino acid is substituted for another. In a nonsense mutation, a stop codon is substituted for an amino acid. In a frameshift mutation, one or more bases is added or deleted, resulting in a change in the reading frame.\"><\/span><\/figure>\n<p class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/p>\n<figure id=\"attachment_961\" aria-describedby=\"caption-attachment-961\" style=\"width: 469px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-961 size-full\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/79.png\" alt=\"Illustration shows different types of point mutations that result from a single amino acid substitution. In a silent mutation, no change in the amino acid sequence occurs. In a missense mutation, one amino acid is substituted for another. In a nonsense mutation, a stop codon is substituted for an amino acid. In a frameshift mutation, one or more bases is added or deleted, resulting in a change in the reading frame.\" width=\"469\" height=\"599\" srcset=\"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/79.png 469w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/79-235x300.png 235w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/79-65x83.png 65w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/79-225x287.png 225w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/79-350x447.png 350w\" sizes=\"auto, (max-width: 469px) 100vw, 469px\" \/><figcaption id=\"caption-attachment-961\" class=\"wp-caption-text\">Figure\u00a014.21\u00a0Mutations can lead to changes in the protein sequence encoded by the DNA.<\/figcaption><\/figure>\n<p class=\"os-caption-container\"><span style=\"font-size: 1rem\">A frameshift mutation that results in the insertion of three nucleotides is often less deleterious than a mutation that results in the insertion of one nucleotide. Why?<\/span><\/p>\n<\/div>\n<\/div>\n<div class=\"textbox\">\n<h3 class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Link to Learning<\/span><\/h3>\n<div class=\"os-figure\">\n<div class=\"os-caption-container\"><\/div>\n<\/div>\n<p>Learn more by watching this video <a href=\"https:\/\/www.youtube.com\/watch?v=sX6LncNjTFU\">Mechanisms of DNA Damage and Repair<\/a><\/p>\n<\/div>\n<div id=\"fs-id2046784\" class=\"visual-connection ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<section>\n<div class=\"os-note-body\">\n<div class=\"os-figure\">\n<p class=\"os-caption-container\" style=\"text-align: justify\">Mutations in repair genes have been known to cause cancer. Many mutated repair genes have been implicated in certain forms of pancreatic cancer, colon cancer, and colorectal cancer. Mutations can affect either somatic cells or germ cells. If many mutations accumulate in a somatic cell, they may lead to problems such as the uncontrolled cell division observed in cancer. If a mutation takes place in germ cells, the mutation will be passed on to the next generation, as in the case of hemophilia and xeroderma pigmentosa.<\/p>\n<p>&nbsp;<\/p>\n<div id=\"h5p-170\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-170\" class=\"h5p-iframe\" data-content-id=\"170\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ch. 14 DNA Structure 1. Griffin\"><\/iframe><\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/section>\n<\/div>\n<p>&nbsp;<\/p>\n<div id=\"h5p-171\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-171\" class=\"h5p-iframe\" data-content-id=\"171\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ch. 14 DNA Structure 2. Component\"><\/iframe><\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<div id=\"h5p-172\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-172\" class=\"h5p-iframe\" data-content-id=\"172\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ch. 14 DNA Structure 3. Chargaff\"><\/iframe><\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<div id=\"h5p-173\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-173\" class=\"h5p-iframe\" data-content-id=\"173\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Chapter 14 DNA Structure 4. Bonds\"><\/iframe><\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<div id=\"h5p-174\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-174\" class=\"h5p-iframe\" data-content-id=\"174\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Chapter 14 DNA Structure 5. Replication\"><\/iframe><\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<div id=\"h5p-175\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-175\" class=\"h5p-iframe\" data-content-id=\"175\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Ch. 14 DNA Structure 6. Polymerases\"><\/iframe><\/div>\n<\/div>\n<p>&nbsp;<\/p>\n","protected":false},"author":130,"menu_order":7,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":["jung-choi","mary-ann-clark","matthew-douglas"],"pb_section_license":"cc-by"},"chapter-type":[],"contributor":[92,93,94],"license":[53],"class_list":["post-962","chapter","type-chapter","status-publish","hentry","contributor-jung-choi","contributor-mary-ann-clark","contributor-matthew-douglas","license-cc-by"],"part":925,"_links":{"self":[{"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/pressbooks\/v2\/chapters\/962","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/wp\/v2\/users\/130"}],"version-history":[{"count":2,"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/pressbooks\/v2\/chapters\/962\/revisions"}],"predecessor-version":[{"id":1064,"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/pressbooks\/v2\/chapters\/962\/revisions\/1064"}],"part":[{"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/pressbooks\/v2\/parts\/925"}],"metadata":[{"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/pressbooks\/v2\/chapters\/962\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/wp\/v2\/media?parent=962"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/pressbooks\/v2\/chapter-type?post=962"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/wp\/v2\/contributor?post=962"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-json\/wp\/v2\/license?post=962"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}