Chapter 11 - Transcription and Translation

Role in initiating translation 3 tail helps prevent

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Unformatted text preview: revent digestion by exonucleases mRNA Splicing o o Removal of introns from a pre-mRNA breaks are introduced at the 5 and 3 ends (splice sites); exons on either side must be covalently joined (ligated) It must be done precisely addition or loss of a single nucleotide would result in mistranslation (frame shift) Nucleotide sequences at the splice sites of pre mRNAs 5 o Sequences between exon-intron boundaries are highly conserved allows the same basic splicing machinaery to work on thousands of different mRNAs G/GU at the 5 end and AG/G at the 3 end mRNA Splicing y y RNA splicing is thought to have evolved from self splicing RNAs An example of a self splicing intron is the group II intron, discovered in various organisms (example: variety of bacteria) The Structure of Self Splicing of Group II introns o o o o The 2 OH on an adenosine within the intron carries out a nucleophillic attack on the 5 splice site The A forms a phosphodiester bond with 1st nucleotide of intron 3 OH of the exon attacks the 3 splice site, which cleaves RNA Intron is released, 3 and 5 ends of exons are ligated Eukaryotic mRNA processing o o o o o o o o o In eukaryotes the pre-mRNA is not capable of self-splicing, and requires small nuclear RNAs enzymatic process similar to self splicing group II introns As each hnRNA is transcribed, it becomes associated with a hnRNP Processing occurs as each intron becomes associated with a complex called a splicesosome The splicesosome consists of small nuclear ri...
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