{"id":996,"date":"2022-04-20T21:03:42","date_gmt":"2022-04-20T21:03:42","guid":{"rendered":"https:\/\/pressbooks.hcfl.edu\/bio1\/chapter\/rna-processing-in-eukaryotes\/"},"modified":"2025-08-29T19:16:20","modified_gmt":"2025-08-29T19:16:20","slug":"rna-processing-in-eukaryotes","status":"publish","type":"chapter","link":"https:\/\/pressbooks.hcfl.edu\/bio1\/chapter\/rna-processing-in-eukaryotes\/","title":{"raw":"RNA Processing in Eukaryotes","rendered":"RNA Processing in Eukaryotes"},"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>Describe the different steps in RNA processing<\/li>\n \t<li>Understand the significance of exons, introns, and splicing for mRNAs<\/li>\n \t<li>Explain how tRNAs and rRNAs are processed<\/li>\n<\/ul>\n<\/div>\n<\/div>\nAfter transcription, eukaryotic pre-mRNAs must undergo several processing steps before they can be translated. Eukaryotic (and prokaryotic) tRNAs and rRNAs also undergo processing before they can function as components in the protein-synthesis machinery.\n\n<section id=\"fs-id1983488\" data-depth=\"1\">\n<h3 data-type=\"title\">mRNA Processing<\/h3>\n<p id=\"fs-id2186668\">The eukaryotic pre-mRNA undergoes extensive processing before it is ready to be translated. Eukaryotic protein-coding sequences are not continuous, as they are in prokaryotes. The coding sequences (exons) are interrupted by noncoding introns, which must be removed to make a translatable mRNA. The additional steps involved in eukaryotic mRNA maturation also create a molecule with a much longer half-life than a prokaryotic mRNA. Eukaryotic mRNAs last for several hours, whereas the typical\u00a0<em data-effect=\"italics\">E. coli<\/em>\u00a0mRNA lasts no more than five seconds.<\/p>\n<p id=\"fs-id2262076\">Pre-mRNAs are first coated in RNA-stabilizing proteins; these protect the pre-mRNA from degradation while it is processed and exported out of the nucleus. The three most important steps of pre-mRNA processing are the addition of stabilizing and signaling factors at the 5' and 3' ends of the molecule, and the removal of the introns (Figure 15.11). In rare cases, the mRNA transcript can be \u201cedited\u201d after it is transcribed.<\/p>\n\n\n[caption id=\"attachment_993\" align=\"aligncenter\" width=\"319\"]<img class=\"wp-image-993 size-full\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/Figure-15-11.png\" alt=\"Eukaryotic mRNA diagram\" width=\"319\" height=\"181\"> Figure\u00a015.11 Eukaryotic mRNA contains introns that must be spliced out. A 5' cap and 3' poly-A tail are also added.[\/caption]\n\n<div id=\"fig-ch15_03_02\" class=\"os-figure\">\n<figure data-id=\"fig-ch15_03_02\"><span id=\"fs-id2278509\" data-type=\"media\" data-alt=\"An illustration shows that in Pre- m R N A processing, there is a primary R N A transcript including regions labeled, left to right, as exon 1, intron, exon 2, intron, and exon 3. The introns are cut and exons are spliced together to produce mRNA. After R N A processing, there is a spliced R N A with these parts, left to right are a 5 prime cap, a 5 prime untranslated region, exon 1, exon 2, exon 3, a 3 prime untranslated region, and a poly a tail.\"><\/span><\/figure>\n<div class=\"os-caption-container\"><span style=\"text-align: justify;font-size: 1em\">\u00a0<\/span><\/div>\n<div>\n<h4 data-type=\"title\">5' Capping<\/h4>\n<\/div>\n<div class=\"os-caption-container\"><span style=\"text-align: justify;font-size: 1em\">While the pre-mRNA is still being synthesized, a <\/span><strong style=\"text-align: justify;font-size: 1em\">7-methylguanosine cap <\/strong><span style=\"text-align: justify;font-size: 1em\">is added to the 5' end of the growing transcript by a phosphate linkage. This functional group protects the nascent mRNA from degradation. In addition, factors involved in protein synthesis recognize the cap to help initiate translation by ribosomes.<\/span><\/div>\n<\/div>\n<div id=\"fs-id2863005\" class=\"evolution ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\"><section>\n<div class=\"os-note-body\">\n<div>\n<h4 data-type=\"title\">3' Poly-A Tail<\/h4>\nOnce elongation is complete, the pre-mRNA is cleaved by an endonuclease between an AAUAAA consensus sequence and a GU-rich sequence, leaving the AAUAAA sequence on the pre-mRNA. An enzyme called poly-A polymerase then adds a string of approximately 200 A residues, called the <strong>poly-A tail<\/strong>. This modification further protects the pre-mRNA from degradation and is also the binding site for a protein necessary for exporting the processed mRNA to the cytoplasm.\n<h4 data-type=\"title\">Pre-mRNA Splicing<\/h4>\nEukaryotic genes are composed of <strong>exons<\/strong>, which correspond to protein-coding sequences (<em>ex-<\/em>on signifies that they are <em>ex<\/em>pressed), and <em>int<\/em>ervening sequences called <strong>introns <\/strong>(<em>int-<\/em>ron denotes their <em>int<\/em>ervening role), which may be involved in gene regulation but are removed from the pre-mRNA during processing. Intron sequences in mRNA do not encode functional <span style=\"font-size: 1em\">proteins.<\/span>\n\n<\/div>\nThe discovery of introns came as a surprise to researchers in the 1970s who expected that pre-mRNAs would specify protein sequences without further processing, as they had observed in prokaryotes. The genes of higher eukaryotes very often contain one or more introns. These regions may correspond to regulatory sequences; however, the biological significance of having many introns or having very long introns in a gene is unclear. It is possible that introns slow down gene expression because it takes longer to transcribe pre-mRNAs with lots of introns. Alternatively, introns may be nonfunctional sequence remnants left over from the fusion of ancient genes throughout the course of evolution. This is supported by the fact that separate exons often encode separate protein subunits or domains. For the most part, the sequences of introns can be mutated without ultimately affecting the protein product.\n\nAll of a pre-mRNA\u2019s introns must be completely and precisely removed before protein synthesis. If the process errs by even a single nucleotide, the reading frame of the rejoined exons would shift, and the resulting protein would be dysfunctional. The process of removing introns and reconnecting exons is called <strong>splicing <\/strong>(Figure 15.13). Introns are removed and degraded while the pre-mRNA is still in the nucleus. Splicing occurs by a sequence-specific mechanism that ensures introns will be removed and exons rejoined with the accuracy and precision of a single nucleotide. Although the intron itself is noncoding, the beginning and end of each intron is marked with specific nucleotides: GU at the 5' end and AG at the 3' end of the intron. The splicing of pre-mRNAs is conducted by complexes of proteins and RNA molecules called spliceosomes.\n\n<\/div>\n<div class=\"textbox\">\n<h4 id=\"5\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Visual Connection<\/span><\/h4>\n<div id=\"fig-ch15_04_02\" class=\"os-figure\">\n<figure data-id=\"fig-ch15_04_02\"><span id=\"fs-id1480619\" data-type=\"media\" data-alt=\"Illustration shows a spliceosome bound to m R N A. An intron is wrapped around small R N As associated with the spliceosome. When the splice is complete, the exons on either side of the intron are fused together, and the intron forms a ring structure.\"><\/span><\/figure>\n<p class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/p>\n\n\n[caption id=\"attachment_994\" align=\"aligncenter\" width=\"900\"]<img class=\"wp-image-994 size-full\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/83.png\" alt=\"Illustration shows a spliceosome bound to m R N A. An intron is wrapped around small R N As associated with the spliceosome. When the splice is complete, the exons on either side of the intron are fused together, and the intron forms a ring structure.\" width=\"900\" height=\"675\"> Figure\u00a015.13\u00a0Pre-mRNA splicing involves the precise removal of introns from the primary RNA transcript. The splicing process is catalyzed by protein complexes called spliceosomes that are composed of proteins and RNA molecules called small nuclear RNAs (snRNAs). Spliceosomes recognize sequences at the 5' and 3' end of the intron. Rao, A. and Ryan, K. Department of Biology, Texas A&amp;M University.[\/caption]\n\n&nbsp;\n\n<span style=\"font-size: 1rem\">Errors in splicing are implicated in cancers and other human diseases. What kinds of mutations might lead to splicing errors? Think of different possible outcomes if splicing errors occur.<\/span>\n\n<\/div>\n<\/div>\nNote that more than 70 individual introns can be present, and each has to undergo the process of splicing\u2014in addition to 5' capping and the addition of a poly-A tail\u2014just to generate a single, translatable mRNA molecule.\n<div class=\"os-note-body\">\n<div class=\"textbox\">\n<h4 id=\"7\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Link to Learning<\/span><\/h4>\n<p id=\"fs-id1425631\">See how introns are removed during RNA splicing\u00a0<a href=\"http:\/\/openstax.org\/l\/RNA_splicing\" target=\"_blank\" rel=\"noopener nofollow\">at this website<\/a>.<\/p>\nWatch <a href=\"https:\/\/www.youtube.com\/watch?v=UteY3Iah88Q\">this video<\/a> to learn about eukaryotic RNA processing and modifications.\n\n<\/div>\n<h3 data-type=\"title\">Processing of tRNAs and rRNAs<\/h3>\n<span style=\"font-size: 1em\">The tRNAs and rRNAs are structural molecules that have roles in protein synthesis; however, these RNAs are not themselves translated. Pre-rRNAs are transcribed, processed, and assembled into ribosomes in the nucleolus. Pre-tRNAs are transcribed and processed in the nucleus and then released into the cytoplasm, where they are linked to free amino acids for protein synthesis.<\/span>\n\n<\/div>\n<\/section><\/div>\n<\/section><section id=\"fs-id1650543\" data-depth=\"1\">\n<p id=\"fs-id2595368\">Most of the tRNAs and rRNAs in eukaryotes and prokaryotes are first transcribed as a long precursor molecule that spans multiple rRNAs or tRNAs. Enzymes then cleave the precursors into subunits corresponding to each structural RNA. Some of the bases of pre-rRNAs are\u00a0<em data-effect=\"italics\">methylated<\/em>; that is, a \u2013CH<sub>3<\/sub>\u00a0methyl functional group is added for stability. Pre-tRNA molecules also undergo methylation. As with pre-mRNAs, subunit excision occurs in eukaryotic pre-RNAs destined to become tRNAs or rRNAs.<\/p>\n<p id=\"fs-id1450254\">Mature rRNAs make up approximately 50 percent of each ribosome. Some of a ribosome\u2019s RNA molecules are purely structural, whereas others have catalytic or binding activities. Mature tRNAs take on a three-dimensional structure through local regions of base pairing stabilized by intramolecular hydrogen bonding. The tRNA folds to position the amino acid binding site at one end and the\u00a0<strong><span id=\"term584\" data-type=\"term\">anticodon<\/span>\u00a0<\/strong>at the other end (Figure 15.14). The anticodon is a three-nucleotide sequence in a tRNA that interacts with an mRNA codon through complementary base pairing.<\/p>\n\n<figure data-id=\"fig-ch15_04_03\"><span id=\"fs-id1440481\" data-type=\"media\" data-alt=\"The molecular model of phenylalanine t R N A is L shaped. At one end is the anticodon A A G. At the other end is the attachment site for the amino acid phenylalanine.\"><\/span><\/figure>\n<div class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/div>\n<div>\n\n[caption id=\"attachment_995\" align=\"aligncenter\" width=\"544\"]<img class=\"wp-image-995 size-full\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_725_Image_0001.jpg\" alt=\"The molecular model of phenylalanine t R N A is L shaped. At one end is the anticodon A A G. At the other end is the attachment site for the amino acid phenylalanine.\" width=\"544\" height=\"718\"> Figure\u00a015.14\u00a0This is a space-filling model of a tRNA molecule that adds the amino acid phenylalanine to a growing polypeptide chain. The anticodon AAG binds the Codon UUC on the mRNA. The amino acid phenylalanine is attached to the other end of the tRNA.[\/caption]\n\n<\/div>\n<\/section>","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>Describe the different steps in RNA processing<\/li>\n<li>Understand the significance of exons, introns, and splicing for mRNAs<\/li>\n<li>Explain how tRNAs and rRNAs are processed<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<p>After transcription, eukaryotic pre-mRNAs must undergo several processing steps before they can be translated. Eukaryotic (and prokaryotic) tRNAs and rRNAs also undergo processing before they can function as components in the protein-synthesis machinery.<\/p>\n<section id=\"fs-id1983488\" data-depth=\"1\">\n<h3 data-type=\"title\">mRNA Processing<\/h3>\n<p id=\"fs-id2186668\">The eukaryotic pre-mRNA undergoes extensive processing before it is ready to be translated. Eukaryotic protein-coding sequences are not continuous, as they are in prokaryotes. The coding sequences (exons) are interrupted by noncoding introns, which must be removed to make a translatable mRNA. The additional steps involved in eukaryotic mRNA maturation also create a molecule with a much longer half-life than a prokaryotic mRNA. Eukaryotic mRNAs last for several hours, whereas the typical\u00a0<em data-effect=\"italics\">E. coli<\/em>\u00a0mRNA lasts no more than five seconds.<\/p>\n<p id=\"fs-id2262076\">Pre-mRNAs are first coated in RNA-stabilizing proteins; these protect the pre-mRNA from degradation while it is processed and exported out of the nucleus. The three most important steps of pre-mRNA processing are the addition of stabilizing and signaling factors at the 5&#8242; and 3&#8242; ends of the molecule, and the removal of the introns (Figure 15.11). In rare cases, the mRNA transcript can be \u201cedited\u201d after it is transcribed.<\/p>\n<figure id=\"attachment_993\" aria-describedby=\"caption-attachment-993\" style=\"width: 319px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-993 size-full\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/Figure-15-11.png\" alt=\"Eukaryotic mRNA diagram\" width=\"319\" height=\"181\" srcset=\"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/Figure-15-11.png 319w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/Figure-15-11-300x170.png 300w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/Figure-15-11-65x37.png 65w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2022\/04\/Figure-15-11-225x128.png 225w\" sizes=\"auto, (max-width: 319px) 100vw, 319px\" \/><figcaption id=\"caption-attachment-993\" class=\"wp-caption-text\">Figure\u00a015.11 Eukaryotic mRNA contains introns that must be spliced out. A 5&#8242; cap and 3&#8242; poly-A tail are also added.<\/figcaption><\/figure>\n<div id=\"fig-ch15_03_02\" class=\"os-figure\">\n<figure data-id=\"fig-ch15_03_02\"><span id=\"fs-id2278509\" data-type=\"media\" data-alt=\"An illustration shows that in Pre- m R N A processing, there is a primary R N A transcript including regions labeled, left to right, as exon 1, intron, exon 2, intron, and exon 3. The introns are cut and exons are spliced together to produce mRNA. After R N A processing, there is a spliced R N A with these parts, left to right are a 5 prime cap, a 5 prime untranslated region, exon 1, exon 2, exon 3, a 3 prime untranslated region, and a poly a tail.\"><\/span><\/figure>\n<div class=\"os-caption-container\"><span style=\"text-align: justify;font-size: 1em\">\u00a0<\/span><\/div>\n<div>\n<h4 data-type=\"title\">5&#8242; Capping<\/h4>\n<\/div>\n<div class=\"os-caption-container\"><span style=\"text-align: justify;font-size: 1em\">While the pre-mRNA is still being synthesized, a <\/span><strong style=\"text-align: justify;font-size: 1em\">7-methylguanosine cap <\/strong><span style=\"text-align: justify;font-size: 1em\">is added to the 5&#8242; end of the growing transcript by a phosphate linkage. This functional group protects the nascent mRNA from degradation. In addition, factors involved in protein synthesis recognize the cap to help initiate translation by ribosomes.<\/span><\/div>\n<\/div>\n<div id=\"fs-id2863005\" class=\"evolution ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<section>\n<div class=\"os-note-body\">\n<div>\n<h4 data-type=\"title\">3&#8242; Poly-A Tail<\/h4>\n<p>Once elongation is complete, the pre-mRNA is cleaved by an endonuclease between an AAUAAA consensus sequence and a GU-rich sequence, leaving the AAUAAA sequence on the pre-mRNA. An enzyme called poly-A polymerase then adds a string of approximately 200 A residues, called the <strong>poly-A tail<\/strong>. This modification further protects the pre-mRNA from degradation and is also the binding site for a protein necessary for exporting the processed mRNA to the cytoplasm.<\/p>\n<h4 data-type=\"title\">Pre-mRNA Splicing<\/h4>\n<p>Eukaryotic genes are composed of <strong>exons<\/strong>, which correspond to protein-coding sequences (<em>ex-<\/em>on signifies that they are <em>ex<\/em>pressed), and <em>int<\/em>ervening sequences called <strong>introns <\/strong>(<em>int-<\/em>ron denotes their <em>int<\/em>ervening role), which may be involved in gene regulation but are removed from the pre-mRNA during processing. Intron sequences in mRNA do not encode functional <span style=\"font-size: 1em\">proteins.<\/span><\/p>\n<\/div>\n<p>The discovery of introns came as a surprise to researchers in the 1970s who expected that pre-mRNAs would specify protein sequences without further processing, as they had observed in prokaryotes. The genes of higher eukaryotes very often contain one or more introns. These regions may correspond to regulatory sequences; however, the biological significance of having many introns or having very long introns in a gene is unclear. It is possible that introns slow down gene expression because it takes longer to transcribe pre-mRNAs with lots of introns. Alternatively, introns may be nonfunctional sequence remnants left over from the fusion of ancient genes throughout the course of evolution. This is supported by the fact that separate exons often encode separate protein subunits or domains. For the most part, the sequences of introns can be mutated without ultimately affecting the protein product.<\/p>\n<p>All of a pre-mRNA\u2019s introns must be completely and precisely removed before protein synthesis. If the process errs by even a single nucleotide, the reading frame of the rejoined exons would shift, and the resulting protein would be dysfunctional. The process of removing introns and reconnecting exons is called <strong>splicing <\/strong>(Figure 15.13). Introns are removed and degraded while the pre-mRNA is still in the nucleus. Splicing occurs by a sequence-specific mechanism that ensures introns will be removed and exons rejoined with the accuracy and precision of a single nucleotide. Although the intron itself is noncoding, the beginning and end of each intron is marked with specific nucleotides: GU at the 5&#8242; end and AG at the 3&#8242; end of the intron. The splicing of pre-mRNAs is conducted by complexes of proteins and RNA molecules called spliceosomes.<\/p>\n<\/div>\n<div class=\"textbox\">\n<h4 id=\"5\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Visual Connection<\/span><\/h4>\n<div id=\"fig-ch15_04_02\" class=\"os-figure\">\n<figure data-id=\"fig-ch15_04_02\"><span id=\"fs-id1480619\" data-type=\"media\" data-alt=\"Illustration shows a spliceosome bound to m R N A. An intron is wrapped around small R N As associated with the spliceosome. When the splice is complete, the exons on either side of the intron are fused together, and the intron forms a ring structure.\"><\/span><\/figure>\n<p class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/p>\n<figure id=\"attachment_994\" aria-describedby=\"caption-attachment-994\" style=\"width: 900px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-994 size-full\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/83.png\" alt=\"Illustration shows a spliceosome bound to m R N A. An intron is wrapped around small R N As associated with the spliceosome. When the splice is complete, the exons on either side of the intron are fused together, and the intron forms a ring structure.\" width=\"900\" height=\"675\" srcset=\"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/83.png 900w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/83-300x225.png 300w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/83-768x576.png 768w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/83-65x49.png 65w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/83-225x169.png 225w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/83-350x263.png 350w\" sizes=\"auto, (max-width: 900px) 100vw, 900px\" \/><figcaption id=\"caption-attachment-994\" class=\"wp-caption-text\">Figure\u00a015.13\u00a0Pre-mRNA splicing involves the precise removal of introns from the primary RNA transcript. The splicing process is catalyzed by protein complexes called spliceosomes that are composed of proteins and RNA molecules called small nuclear RNAs (snRNAs). Spliceosomes recognize sequences at the 5&#8242; and 3&#8242; end of the intron. Rao, A. and Ryan, K. Department of Biology, Texas A&amp;M University.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p><span style=\"font-size: 1rem\">Errors in splicing are implicated in cancers and other human diseases. What kinds of mutations might lead to splicing errors? Think of different possible outcomes if splicing errors occur.<\/span><\/p>\n<\/div>\n<\/div>\n<p>Note that more than 70 individual introns can be present, and each has to undergo the process of splicing\u2014in addition to 5&#8242; capping and the addition of a poly-A tail\u2014just to generate a single, translatable mRNA molecule.<\/p>\n<div class=\"os-note-body\">\n<div class=\"textbox\">\n<h4 id=\"7\" class=\"os-subtitle\" data-type=\"title\"><span class=\"os-subtitle-label\">Link to Learning<\/span><\/h4>\n<p id=\"fs-id1425631\">See how introns are removed during RNA splicing\u00a0<a href=\"http:\/\/openstax.org\/l\/RNA_splicing\" target=\"_blank\" rel=\"noopener nofollow\">at this website<\/a>.<\/p>\n<p>Watch <a href=\"https:\/\/www.youtube.com\/watch?v=UteY3Iah88Q\">this video<\/a> to learn about eukaryotic RNA processing and modifications.<\/p>\n<\/div>\n<h3 data-type=\"title\">Processing of tRNAs and rRNAs<\/h3>\n<p><span style=\"font-size: 1em\">The tRNAs and rRNAs are structural molecules that have roles in protein synthesis; however, these RNAs are not themselves translated. Pre-rRNAs are transcribed, processed, and assembled into ribosomes in the nucleolus. Pre-tRNAs are transcribed and processed in the nucleus and then released into the cytoplasm, where they are linked to free amino acids for protein synthesis.<\/span><\/p>\n<\/div>\n<\/section>\n<\/div>\n<\/section>\n<section id=\"fs-id1650543\" data-depth=\"1\">\n<p id=\"fs-id2595368\">Most of the tRNAs and rRNAs in eukaryotes and prokaryotes are first transcribed as a long precursor molecule that spans multiple rRNAs or tRNAs. Enzymes then cleave the precursors into subunits corresponding to each structural RNA. Some of the bases of pre-rRNAs are\u00a0<em data-effect=\"italics\">methylated<\/em>; that is, a \u2013CH<sub>3<\/sub>\u00a0methyl functional group is added for stability. Pre-tRNA molecules also undergo methylation. As with pre-mRNAs, subunit excision occurs in eukaryotic pre-RNAs destined to become tRNAs or rRNAs.<\/p>\n<p id=\"fs-id1450254\">Mature rRNAs make up approximately 50 percent of each ribosome. Some of a ribosome\u2019s RNA molecules are purely structural, whereas others have catalytic or binding activities. Mature tRNAs take on a three-dimensional structure through local regions of base pairing stabilized by intramolecular hydrogen bonding. The tRNA folds to position the amino acid binding site at one end and the\u00a0<strong><span id=\"term584\" data-type=\"term\">anticodon<\/span>\u00a0<\/strong>at the other end (Figure 15.14). The anticodon is a three-nucleotide sequence in a tRNA that interacts with an mRNA codon through complementary base pairing.<\/p>\n<figure data-id=\"fig-ch15_04_03\"><span id=\"fs-id1440481\" data-type=\"media\" data-alt=\"The molecular model of phenylalanine t R N A is L shaped. At one end is the anticodon A A G. At the other end is the attachment site for the amino acid phenylalanine.\"><\/span><\/figure>\n<div class=\"os-caption-container\"><span class=\"os-caption\">\u00a0<\/span><\/div>\n<div>\n<figure id=\"attachment_995\" aria-describedby=\"caption-attachment-995\" style=\"width: 544px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-995 size-full\" src=\"http:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_725_Image_0001.jpg\" alt=\"The molecular model of phenylalanine t R N A is L shaped. At one end is the anticodon A A G. At the other end is the attachment site for the amino acid phenylalanine.\" width=\"544\" height=\"718\" srcset=\"https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_725_Image_0001.jpg 544w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_725_Image_0001-227x300.jpg 227w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_725_Image_0001-65x86.jpg 65w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_725_Image_0001-225x297.jpg 225w, https:\/\/pressbooks.hcfl.edu\/bio1\/wp-content\/uploads\/sites\/106\/2025\/08\/General-Biology-I-Lecture-Lab-1657046460_Page_725_Image_0001-350x462.jpg 350w\" sizes=\"auto, (max-width: 544px) 100vw, 544px\" \/><figcaption id=\"caption-attachment-995\" class=\"wp-caption-text\">Figure\u00a015.14\u00a0This is a space-filling model of a tRNA molecule that adds the amino acid phenylalanine to a growing polypeptide chain. The anticodon AAG binds the Codon UUC on the mRNA. 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