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

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14-1 Chapter 15 Messenger 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.
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14-2 Why 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 polysomes
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Figure 15.3 Capping: 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.
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Figure 15.4 Sequence of events in capping … Step 1. Phosphatase removes terminal phosphate from mRNA Step 2. Guanylyl transferase adds GMP (from GTP) to 5’ end Step 3. Methyl transferase adds methyl group to guanine N7 Step4. Methyl transferase adds methyl group to 2' OH Capping occurs very early in transcription, before mRNA is 30 bp long.
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Figure 15.13 Formation 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 Polyadenylation
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Summary of data on 369 vertebrate polyadenylation signals The top figure depicts the
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