{"id":2651,"date":"2016-06-06T20:54:18","date_gmt":"2016-06-06T20:54:18","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/biologyxwaymakerxmaster\/?post_type=chapter&#038;p=2651"},"modified":"2025-12-02T19:55:47","modified_gmt":"2025-12-02T19:55:47","slug":"reading-codons","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/chapter\/reading-codons\/","title":{"raw":"Genetic Code","rendered":"Genetic Code"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>Identify the components of the genetic code<\/li>\r\n<\/ul>\r\n<\/div>\r\nGiven the different numbers of \"letters\" in the mRNA and protein \"alphabets,\" scientists theorized that combinations of nucleotides corresponded to single amino acids. Scientists theorized that amino acids were encoded by <strong>nucleotide triplets<\/strong> and that the genetic code was\u00a0<strong>degenerate<\/strong>. In other words, a given amino acid could be encoded by more than one nucleotide triplet.\u00a0 These nucleotide triplets are called <strong>codons<\/strong>. Scientists painstakingly solved the <strong>genetic code<\/strong> by translating synthetic mRNAs in vitro and sequencing the proteins they specified (Figure 1).\r\n\r\n[caption id=\"attachment_1446\" align=\"aligncenter\" width=\"552\"]<img class=\"wp-image-1446 size-full\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02185922\/Figure_15_01_04.jpg\" alt=\"Figure shows all 64 codons. Sixty-two of these code for amino acids, and three are stop codons.\" width=\"552\" height=\"473\" \/> Figure 1. This figure shows the genetic code for translating each nucleotide triplet in mRNA into an amino acid or a termination signal in a nascent protein. (credit: modification of work by NIH)[\/caption]\r\n\r\nIn addition to instructing the addition of a specific amino acid to a polypeptide chain, three (<strong>UAA, UAG, UGA<\/strong>) of the 64 codons terminate protein synthesis and release the polypeptide from the translation machinery. These triplets are called <strong>stop codons, or nonsense codons<\/strong>. Another codon, <strong>AUG<\/strong>, also has a special function. In addition to specifying the amino acid methionine, it also serves as the <strong>start codon<\/strong> to initiate translation. The reading frame for translation is set by the AUG start codon near the 5' end of the mRNA.\r\n\r\nThe genetic code is <strong>universal<\/strong>. With a few exceptions, virtually all species use the same genetic code for protein synthesis. Conservation of codons means that a purified mRNA encoding the globin protein in horses could be transferred to a tulip cell, and the tulip would synthesize horse globin. That there is only one genetic code is powerful evidence that all of life on Earth shares a common origin, especially considering that there are about 1084 possible combinations of 20 amino acids and 64 triplet codons.\r\n<div class=\"textbox shaded\">Transcribe a gene and translate it to protein using complementary pairing and the genetic code at this\u00a0<a href=\"https:\/\/learn.genetics.utah.edu\/content\/basics\/transcribe\/\" target=\"_blank\" rel=\"noopener\">site<\/a>.<\/div>\r\nDegeneracy is believed to be a cellular mechanism to reduce the negative impact of random mutations. Codons that specify the same amino acid typically only differ by one nucleotide. In addition, amino acids with chemically similar side chains are encoded by similar codons. This nuance of the genetic code ensures that a single-nucleotide substitution mutation might either specify the same amino acid but have no effect or specify a similar amino acid, preventing the protein from being rendered completely nonfunctional.\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/02f33400-22f9-496b-87a1-83cea5e5bdb5\r\n\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>Identify the components of the genetic code<\/li>\n<\/ul>\n<\/div>\n<p>Given the different numbers of &#8220;letters&#8221; in the mRNA and protein &#8220;alphabets,&#8221; scientists theorized that combinations of nucleotides corresponded to single amino acids. Scientists theorized that amino acids were encoded by <strong>nucleotide triplets<\/strong> and that the genetic code was\u00a0<strong>degenerate<\/strong>. In other words, a given amino acid could be encoded by more than one nucleotide triplet.\u00a0 These nucleotide triplets are called <strong>codons<\/strong>. Scientists painstakingly solved the <strong>genetic code<\/strong> by translating synthetic mRNAs in vitro and sequencing the proteins they specified (Figure 1).<\/p>\n<div id=\"attachment_1446\" style=\"width: 562px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1446\" class=\"wp-image-1446 size-full\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02185922\/Figure_15_01_04.jpg\" alt=\"Figure shows all 64 codons. Sixty-two of these code for amino acids, and three are stop codons.\" width=\"552\" height=\"473\" \/><\/p>\n<p id=\"caption-attachment-1446\" class=\"wp-caption-text\">Figure 1. This figure shows the genetic code for translating each nucleotide triplet in mRNA into an amino acid or a termination signal in a nascent protein. (credit: modification of work by NIH)<\/p>\n<\/div>\n<p>In addition to instructing the addition of a specific amino acid to a polypeptide chain, three (<strong>UAA, UAG, UGA<\/strong>) of the 64 codons terminate protein synthesis and release the polypeptide from the translation machinery. These triplets are called <strong>stop codons, or nonsense codons<\/strong>. Another codon, <strong>AUG<\/strong>, also has a special function. In addition to specifying the amino acid methionine, it also serves as the <strong>start codon<\/strong> to initiate translation. The reading frame for translation is set by the AUG start codon near the 5&#8242; end of the mRNA.<\/p>\n<p>The genetic code is <strong>universal<\/strong>. With a few exceptions, virtually all species use the same genetic code for protein synthesis. Conservation of codons means that a purified mRNA encoding the globin protein in horses could be transferred to a tulip cell, and the tulip would synthesize horse globin. That there is only one genetic code is powerful evidence that all of life on Earth shares a common origin, especially considering that there are about 1084 possible combinations of 20 amino acids and 64 triplet codons.<\/p>\n<div class=\"textbox shaded\">Transcribe a gene and translate it to protein using complementary pairing and the genetic code at this\u00a0<a href=\"https:\/\/learn.genetics.utah.edu\/content\/basics\/transcribe\/\" target=\"_blank\" rel=\"noopener\">site<\/a>.<\/div>\n<p>Degeneracy is believed to be a cellular mechanism to reduce the negative impact of random mutations. Codons that specify the same amino acid typically only differ by one nucleotide. In addition, amino acids with chemically similar side chains are encoded by similar codons. This nuance of the genetic code ensures that a single-nucleotide substitution mutation might either specify the same amino acid but have no effect or specify a similar amino acid, preventing the protein from being rendered completely nonfunctional.<\/p>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_02f33400-22f9-496b-87a1-83cea5e5bdb5\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/02f33400-22f9-496b-87a1-83cea5e5bdb5?iframe_resize_id=assessment_practice_id_02f33400-22f9-496b-87a1-83cea5e5bdb5\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe><\/p>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-2651\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":7,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology\",\"author\":\"\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\"}]","CANDELA_OUTCOMES_GUID":"4b4adefc-1b80-4871-8d3c-3ffb55f34504, 88bcd34f-0838-40d2-b366-2f79055831dd","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-2651","chapter","type-chapter","status-publish","hentry"],"part":316,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/pressbooks\/v2\/chapters\/2651","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":14,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/pressbooks\/v2\/chapters\/2651\/revisions"}],"predecessor-version":[{"id":6841,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/pressbooks\/v2\/chapters\/2651\/revisions\/6841"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/pressbooks\/v2\/parts\/316"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/pressbooks\/v2\/chapters\/2651\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/wp\/v2\/media?parent=2651"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/pressbooks\/v2\/chapter-type?post=2651"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/wp\/v2\/contributor?post=2651"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-nmbiology1\/wp-json\/wp\/v2\/license?post=2651"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}