{"id":2646,"date":"2016-06-06T20:39:06","date_gmt":"2016-06-06T20:39:06","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/biologyxwaymakerxmaster\/?post_type=chapter&p=2646"},"modified":"2017-08-02T16:07:39","modified_gmt":"2017-08-02T16:07:39","slug":"reading-pre-rna-and-mrna","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/hccs-waymakerbiology1\/chapter\/reading-pre-rna-and-mrna\/","title":{"raw":"pre-RNA and mRNA","rendered":"pre-RNA and mRNA"},"content":{"raw":"
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Learning Outcomes<\/h3>\r\nUnderstand the difference between pre-RNA and mRNA\r\n\r\n<\/div>\r\nAfter transcription, eukaryotic pre-mRNA<\/strong>s 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.\r\n

mRNA Processing<\/h2>\r\nThe eukaryotic pre-mRNA undergoes extensive processing before it is ready to be translated. The additional steps involved in eukaryotic mRNA maturation create a molecule with a much longer half-life than a prokaryotic mRNA. Eukaryotic mRNAs last for several hours, whereas the typical\u00a0E. coli<\/em> mRNA lasts no more than five seconds.\r\n\r\nThe three most important steps of pre-mRNA processing are the addition of stabilizing and signaling factors at the 5\u2032 and 3\u2032 ends of the molecule, and the removal of intervening sequences that do not specify the appropriate amino acids.\r\n

5\u2032 Capping<\/h3>\r\n

A cap<\/strong> is added to the 5\u2032 end of the growing transcript by a phosphate linkage. This addition protects the mRNA from degradation. In addition, factors involved in protein synthesis recognize the cap to help initiate translation by ribosomes.<\/p>\r\n\r\n

3\u2032 Poly-A Tail<\/h3>\r\n

Once elongation is complete, an enzyme called poly-A polymerase adds a string of approximately 200 A residues, called the\u00a0poly-A tail <\/strong>to the pre-mRNA. This modification further protects the pre-mRNA from degradation and signals the export of the cellular factors that the transcript needs to the cytoplasm.<\/p>\r\n\r\n

Pre-mRNA Splicing<\/h3>\r\n

Eukaryotic genes are composed of\u00a0exons<\/strong>, which correspond to protein-coding sequences (ex-<\/em>on signifies that they are ex<\/em>pressed), and int<\/em>ervening sequences called introns<\/strong> (int<\/em>ron denotes their int<\/em>ervening role), which are removed from the pre-mRNA during processing. Intron sequences in mRNA do not encode functional proteins.<\/p>\r\n

All of a pre-mRNA's 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\u00a0splicing<\/strong> (Figure 1).<\/p>\r\n\r\n

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Practice Question<\/h3>\r\n[caption id=\"attachment_4556\" align=\"aligncenter\" width=\"884\"]\"Illustration Figure 1. Pre-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 snRNAs. Spliceosomes recognize sequences at the 5\u2032 and 3\u2032 end of the intron.[\/caption]\r\n\r\n \r\n\r\nErrors in splicing are implicated in cancers and other human diseases. What kinds of mutations might lead to splicing errors?\r\n\r\n[practice-area rows=\"2\"][\/practice-area]\r\n[reveal-answer q=\"454729\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"454729\"]Think of different possible outcomes if splicing errors occur. Mutations in the spliceosome recognition sequence at each end of the intron, or in the proteins and RNAs that make up the spliceosome, may impair splicing. Mutations may also add new spliceosome recognition sites. Splicing errors could lead to introns being retained in spliced RNA, exons being excised, or changes in the location of the splice site.[\/hidden-answer]\r\n\r\n<\/div>\r\n
See how introns are removed during RNA splicing\u00a0at this website.<\/a><\/div>","rendered":"
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Learning Outcomes<\/h3>\n

Understand the difference between pre-RNA and mRNA<\/p>\n<\/div>\n

After transcription, eukaryotic pre-mRNA<\/strong>s 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

mRNA Processing<\/h2>\n

The eukaryotic pre-mRNA undergoes extensive processing before it is ready to be translated. The additional steps involved in eukaryotic mRNA maturation create a molecule with a much longer half-life than a prokaryotic mRNA. Eukaryotic mRNAs last for several hours, whereas the typical\u00a0E. coli<\/em> mRNA lasts no more than five seconds.<\/p>\n

The three most important steps of pre-mRNA processing are the addition of stabilizing and signaling factors at the 5\u2032 and 3\u2032 ends of the molecule, and the removal of intervening sequences that do not specify the appropriate amino acids.<\/p>\n

5\u2032 Capping<\/h3>\n

A cap<\/strong> is added to the 5\u2032 end of the growing transcript by a phosphate linkage. This addition protects the mRNA from degradation. In addition, factors involved in protein synthesis recognize the cap to help initiate translation by ribosomes.<\/p>\n

3\u2032 Poly-A Tail<\/h3>\n

Once elongation is complete, an enzyme called poly-A polymerase adds a string of approximately 200 A residues, called the\u00a0poly-A tail <\/strong>to the pre-mRNA. This modification further protects the pre-mRNA from degradation and signals the export of the cellular factors that the transcript needs to the cytoplasm.<\/p>\n

Pre-mRNA Splicing<\/h3>\n

Eukaryotic genes are composed of\u00a0exons<\/strong>, which correspond to protein-coding sequences (ex-<\/em>on signifies that they are ex<\/em>pressed), and int<\/em>ervening sequences called introns<\/strong> (int<\/em>ron denotes their int<\/em>ervening role), which are removed from the pre-mRNA during processing. Intron sequences in mRNA do not encode functional proteins.<\/p>\n

All of a pre-mRNA’s 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\u00a0splicing<\/strong> (Figure 1).<\/p>\n

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Practice Question<\/h3>\n
\"Illustration<\/p>\n

Figure 1. Pre-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 snRNAs. Spliceosomes recognize sequences at the 5\u2032 and 3\u2032 end of the intron.<\/p>\n<\/div>\n

 <\/p>\n

Errors in splicing are implicated in cancers and other human diseases. What kinds of mutations might lead to splicing errors?<\/p>\n