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RNA Pol II requireATP hydrolysis (TF IIE) & helicase activity (TF IIH) for Pol. movement
A model of phosphorylationto release RNA Pol II from TF for elongation
222 5 3Fig. . Overview: Addition to the ’ and ’ ends,SSSSSSSS SS SSS SSS SSSSSSS
Two stages of splicing:
SSSSSS SSSSS. ; 5 ’ , SSSSSS -SSSS S SSS.
SSSS S SSSSSS 2 3nd st.; cut at ’ splice site,
release free intron, right exon ligated to
the left exon
SPLICING REACTION: by transesterification reaction;breaking and making of bonds
Cis-splicing trans-splicing reaction
tRNA splicing is different mechanism:
separate and ligate
tRNA splicing depends on a secondary structure rather than sequence of the intron
1st step: phosphodiester bond cleavage by endonuclease
2nd step: require ATP, bond formation by RNA ligase
Group I intron: Self-splicing by transesterification SSSSSSSS :
- a monovalent cation, a divalent cation,- a guanine nucleotide cofactor (only G), - 3musthave a ’ OHgroup
FIG.23.2 Self-splicing OR Autosplicing of Group I introns
Some introns code for Proteins:- Endonuclease- Reverse transcriptase- Muturase: for splicing
Fig. 23.11 Endonuclease:- make a double-strand break in DNA- duplicated-intron inserted at the break
Fig.23.12 Endonuclease / Reverse transcriptase
5’ END of eukaryotic mRNA is capped:- start with nucleoside triphosphate (purine A or G)- first nucleotide retains 5’ triphosphate group- addition of the 5’ terminal G catalyzed by a nuclear enz., guanylyl transferase- SSS SSS , SS SSSSSSS SSSSSSSSSSS , called CAP
3 ’ ENDS OF EUKARYOTIC mRNA- RNA Pol I & Pol III terminate like bact. RNA Pol- RNA Pol I; recognize of an 18 base termination seq.- RNA Pol III; recognize at the second U within a run of 4U bases- RNA Pol II; loosely specified by cleavage / polyadenylation - seq. AAUAAA locate from 11 to 30 nts upstream of poly A site / provide the signal for both cleavage / polyadenylation