Gen511-3-14-08F

Gen511-3-14-08F - 14-1Chapter 15Messenger RNA Processing II...

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Unformatted text preview: 14-1Chapter 15Messenger RNA Processing II: Capping and Polyadenylation•Cap: G residue and methylations added to 5’ end of eukaryotic mRNAs.•Polyadenylation: String of A residues (~250) added to the 3’ end of mRNA.14-2Why the Cap and polyA?Caps serve at least four functions:–Protect mRNAs from degradation–Enhance translatability of mRNAs–Transport of mRNAs out of nucleus –Efficiency of splicing mRNAs•Poly(A) enhances both the lifetime and translatability of mRNA•Relative importance of these two effects seems to vary from one system to another•In rabbit reticulocyte extracts, poly(A) seems to enhance translatability by helping to recruit mRNA to polysomesFigure 15.3Capping: Addition of a modified base (7-methyl guanosine) to the 5’ end of mRNA. Also may have 2'-O-methylation on the first and second ribose.Figure 15.4Sequence of events in capping …Step 1. Phosphatase removes terminal phosphate from mRNAStep 2. Guanylyl transferase adds GMP (from GTP) to 5’ endStep 3. Methyl transferase adds methyl group to guanine N7Step4. Methyl transferase adds methyl group to 2' OHCapping occurs very early in transcription, before mRNA is 30 bp long.Figure 15.13Formation of mRNA 3' ends:1. RNA polymerase transcribes beyond the incipient 3' end.1.Transcript is cleaved at a site downstream of the polyA signal (AAUAAA).2.Transcript is polyadenylated: about 200 A residues added to the mRNA PolyadenylationSummary of data on 369 vertebrate polyadenylation signalsThe top figure depicts the consensus sequence, and the subscript indicates the...
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This note was uploaded on 03/31/2008 for the course GEN 511 taught by Professor Peterson during the Spring '08 term at Iowa State.

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Gen511-3-14-08F - 14-1Chapter 15Messenger RNA Processing II...

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