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LS12RNAprocessing

LS12RNAprocessing - mRNA processing 1 5 CAP addition 2 3...

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mRNA processing 1. 5' CAP addition 2. 3' polyadenylation 3. How is mRNA spliced? 4. RNA editing
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Eukaryotic Gene Structure and Expression is More Complex  than in Prokaryotes  Prokaryotes mRNA is exact replica of gene Transcription and translation occur in the same compartment Transcription and translation of mRNA are coupled Eukaryotes mRNA is not replica of gene Transcription takes place in nucleus, Translation in cytoplasm Nuclear mRNA precursors (pre-mRNA) undergo RNA  processing(capping, splicing, polyA addition) -> mRNAs  that are  transported to cytoplasm for translation Transcription & mRNA processing events are coupled
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From Lodish et al (2004) Molecular Cell Biology.  W.H. Freeman & Co.
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Most RNAs in Eukaryotes are made from larger precursors by  RNA processing Types and amounts of RNA in eukaryotic cells cytoplasmic RNAs: ribosomal RNA (rRNA)  80% of total transfer RNA (tRNA)  15% of total messengerRNA (mRNA) ~3% of total nuclear RNA: precursor RNAs, small RNAs ~2% of total All types of RNA in higher eukaryotic cells are processed from larger  precursors RNA processing directed by specific sequences in precursor RNAs
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Discovery of mRNA Processing Early experiments indicated that mRNA was synthesized as a longer  precursor in the nucleus This larger class of nuclear RNAs referred to as heterogenous nuclear  RNA (hnRNA) has many different sizes (from 5,000 - 50,000 nt  long)  mRNA associated with cytoplasmic polyribosomes was smaller than  hnRNA (mostly <5000 nt long) mRNA and hnRNA found to contain 5’ caps and 3’ poly A = unique  tags marking mRNA & its precursors (now called pre-mRNA) Discovered in early 1970s: J. Darnell A. Shatkin R. Perry
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    Pulse/chase expt: Pulse/chase expt:           hnRNA -> mRNA hnRNA -> mRNA       1. 30 min pulse label 1. 30 min pulse label       2. actinomycin D to  2. actinomycin D to                block transcription block transcription         3. 4 hr chase 3. 4 hr chase         4. Isolate & analyze 4. Isolate & analyze                 hnRNA (30 min) & hnRNA (30 min) &                 cytoplasmic mRNA cytoplasmic mRNA                 (4 hr chase) in  (4 hr chase) in                    sucrose gradients sucrose gradients
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Eukaryotic mRNA is synthesized in the nucleus as the primary transcript (e.g. pre-mRNA), which will be processed in the nucleus and then transported to the cytosol to be used for translation. The pre-mRNA is generally processed in 3 steps: capping, polyadenylation, and intron splicing: The cap and polyA are found only in mRNAs.
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