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Lecture 18: Transcription & mRNA processing - Coggle Diagram
Lecture 18: Transcription & mRNA processing
Purpose of Txn
transcribe DNA into RNA so functional proteins can be made (translation)
1 strand read for RNA: template
1 strand not read: coding strand (same seq as RNA)
RNA
mRNA: translated into functional proteins
tRNA: carries aa's and binds to polypeptide chain
rRNA: helps ribosome read transcript in translation
miRNA: complementary to small chunk of mRNA, can lead to degradation of unwanted mRNA
sh/siRNA: can stop translation
Initiation:
Starts at TATAAA box (inside PROMOTER)
1) 1st Basal Txn Factor Binds to TATAAA
2) Recruits more txn factors
3) Txn factors recruit RNAP (II)
No need for SSBP's bc Txn factors large (steric hindrance)
No need for helicase bc RNAP has helicase activity
For prokaryotes:
1 txn factor: sigma factor binds to promoter (no TATAAA)
Elongation
1) RNAP recognizes DNA/RNA duplex
2) Reads DNA 3'-5' and adds 5'-3'
In replication, uses NTP not DNTP
NO PROOFREADING
Termination
Extrinsic
Prokaryotes: Rho
1) Rho binds to binding site/consensus sequence
2) Rho moves 5'-3' along mRNA
3) Rho unwinds RNA/DNA duplex once it reaches RNA polymerase (rho is faster than RNAP)
freed RNA stops transcription
Intrinsic
GC rich region binds to other GC rich region in RNA sequence, creates hairpin loop in RNA
steric hindrance from hairpin causes RNAP to stall/pause
many A-U base pairs at the end of mRNA strand have weak affinity, causing the DNA/RNA duplex to be unstable
instability causes RNAP and mRNA strand to dissociate from DNA
mRNA processing
5' capping
5' carbon of nucleoside --- 5' end of mRNA (7-methyl guanosine)
5'-5' triphosphate linkage
Condensation rxn forms bond; water molecule released
provides protection from RNAses/degradation
acts as recognition site for ribosome to start translation
Splicing
process of removing introns (not useful for translation) and rebinding exons
snRP's: small nucleoRibonucleic proteins
1) U1 binds to 5' splice site of intron
2) U1 recruits U2, then U2 binds to branch site
3) U1+U2 recruit U4,5,6 bind
4) Trimer makes intron loop outwards
5) 5' splice site gets cut, end loops to create lariat (U1 and U4 dissociate)
6) 3' splice site is cut(hydrolysis); releases lariat
7) exon 1+2 ligated together; 3'OH binds to 5'Phosphate (condensation)
8) rest of snRP's (U2, 5,6) dissociate from intron
9) Floating Intron gets degraded by RNAses; loose NMP's
3' PolyA tail
protects mRNA from degradation, allows mRNA to leave nucleus
1) endonuclease cleaves mRNA 10-35nt's from AAUAAA seq, towards 3' end
2) polyApolymerase AAUAAA seq, adds many A's to the end (30-300 A's)
Leaving the nucleus:
TAP-1 nuclear export factor (NXF-1) recognizes the polyA tail and lets mRNA pass through nuclear pore into cytosol