Eukaryotic transcripts are modied extensively

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Unformatted text preview: in eukaryotes are colinear within exons, but are interrupted by noncoding sequences Introns vary in length, and are longer, on average, than exons Gene length in eukaryotes is largely dependent on intron size mRNA Structure Both have a 5’ untranslated region (5’ UTR), start codon, stop codon, and 3’ UTR Prokaryotic messages also have a consensus ribosome-binding site (Shine-Delgarno box) Eukaryotic messages are capped and polyadenylated. Eukaryotic transcripts are modified extensively Eukaryotic transcripts undergo several types of post-transcriptional modification: Capping Polyadenylation Splicing Editing Capping The addition of a single modified nucleotide to the 5’ end of the transcript Provides a ribosome-binding site dont need shine delgarno Promotes stability of the message (prevents degradation) Enhances splicing Capping • During transcription, a 7-methylguanosine (7-mG) cap is added to the 5’ end of the nascent RNA 1. Removal of one of the 5’ phosphate groups 2. Guanine is added using a 5’-to’-5’ linkage not phosphodiester Capping • During transcription, a 7-methylguanosine (7-mG) cap is added to the 5’ end of the nascent RNA 3. Methyl groups are added to position 7 of the guanine nucleotide, and often to the 2’ position of the sugar in the next two nucleotides Polyadenylation Addition of 50 - 250 (or even more) adenine residues to the 3’ end of a message Increases mRNA stability (prevents exonuclease degradation) Facilitates ribosome binding Alternative to a specific transcription termination signal Polyadenylation Polyadenylation is coupled to transcription Consensus sequence AAUAAA signals cleavage of the premRNA Cleavage occurs 10-35 bases downstream of the signal sequence Polyadenylation The Poly-A comple...
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