Chapter 11 - Transcription and Translation

Revealed that exons average about 150 nucleotides

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Unformatted text preview: leotides while introns average about 3500 nucleotides which is why hnRNA > mRNA Eukaryotic mRNA processing o o o o o The Processing of Eukaryotic Messenger RNAs RNA transcripts become associated with ribonucleoproteins as they are synthesized during processing, a 5 methylguanosine cap and 3 poly A tails are added intervening sequences are removed and exons are connected by RNA splicing Pre-mRNA transcripts are processed cotranscriptionally as mRNA strand is being built it is also edited Steps in Addition of 5 methylguanosine cap o o o o 5 ends of all RNAs initially posses a triphosphate several enzymes act on this end One phosphate is removed and guanylyltransferase adds a guanine residue in a reverse orientation so it links 5 to 5 end Methyltransferases then added methyl groups to the terminal guanosine cap and to the ribose of the nucleotide that had been at the end of the nascent RNA This process happens early in synthesis and very quickly Steps in the Addition of a 3 polyA tail o o o o o mRNAs contain a string of A residues at the 3 end poly A tail begins 20 base pairs downstream of AAUAAA on transcript AAUAAA serves as recognition site for polyA processing complex (CBC) First endonucleases cleave RNA transcript creating new 3 end polyA polymerase adds adenosine without a DNA template 5 methylguanosine cap and 3 polyA tails o o 5 cap: helps prevent digestion by exonucleases, transport of mRNA out of nucleus, important role in initiating translation 3 tail: helps p...
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