tailieunhanh - Lecture Molecular biology: Chapter 15 - Robert F. Weaver

Besides splicing, eukaryotic cells perform several other kinds of processing on their RNAs. Messenger RNAs are subject to two kinds of processing, known as capping and polyadenylation. In capping, a special blocking nucleotide (a cap) is added to the 59-end of a pre-mRNA. In polyadenylation, a string of AMPs (poly[A]) is added to the 39-end of the pre-mRNA. These steps are essential for the proper function of mRNAs and will be topics in this chapter. | Molecular Biology Fourth Edition Chapter 15 Messenger RNA Processing II: Capping and Polyadenylation Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Capping By 1974, mRNA from a variety of eukaryotic species and viruses were found to be methylated A significant amount of this methylation was clustered at the 5’-end of mRNA This methylation cluster formed a structure we call a cap 15- Cap Structure Early study used viral mRNA as they are easier to purify and investigate The b-phosphate of a nucleoside triphosphate remains only in the first nucleotide in an RNA Cap is at the 5’-terminus of RNA The cap is made of 7-methylguanosine, m7G Linkage is a triphosphate Charge on the cap area is near -5 15- Reovirus Cap Structure The m7G contributes a positive charge Triphosphate linkage contributes 3 negative charges Phosphodiester bond contributes 1 negative charge Terminal phosphate . | Molecular Biology Fourth Edition Chapter 15 Messenger RNA Processing II: Capping and Polyadenylation Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Capping By 1974, mRNA from a variety of eukaryotic species and viruses were found to be methylated A significant amount of this methylation was clustered at the 5’-end of mRNA This methylation cluster formed a structure we call a cap 15- Cap Structure Early study used viral mRNA as they are easier to purify and investigate The b-phosphate of a nucleoside triphosphate remains only in the first nucleotide in an RNA Cap is at the 5’-terminus of RNA The cap is made of 7-methylguanosine, m7G Linkage is a triphosphate Charge on the cap area is near -5 15- Reovirus Cap Structure The m7G contributes a positive charge Triphosphate linkage contributes 3 negative charges Phosphodiester bond contributes 1 negative charge Terminal phosphate contributes 2 negative charges 15- Cap Synthesis First step RNA triphosphatase removes terminal phosphate from pre-mRNA Then, guanylyl transferase adds capping GMP from GTP Next, 2 methyl transferases methylate N7 of capping guanosine and 2’-O-methyl group of penultimate nucleotide This occurs early in transcription, before chain is 30 nt long 15- Functions of Caps Caps serve at least four functions: Protect mRNAs from degradation Enhance translatability of mRNAs Transport of mRNAs out of nucleus Efficiency of splicing mRNAs 15- Polyadenylation The process of adding poly(A) to RNA is called polyadenylation A long chain of AMP residues is called poly (A) Heterogeneous nuclear mRNA is a precursor to mRNA 15- Poly(A) Most eukaryotic mRNAs and their precursors have a chain of AMP residues about 250 nt long at their 3’-ends Poly(A) is added posttranscriptionally by poly(A) polymerase 15- Functions of Poly(A) Poly(A) enhances both the lifetime and translatability of mRNA

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