Download - Two Stage Processes of Gene Expression
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Two Stage Processes of Gene Expression
Transcription:- mRNA- tRNA- rRNA
Translation:- mRNA translated to Protein- tRNA & rRNA provide other components of the apparatus for protein synthesis
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Prokaryotic Translation
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Life Cycle of mRNA
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Fig5.19 Two stage process ofbacterial mRNA Degradation
Endonucleolytic cleavagefrom 5’ to 3’ behide ribosome
Exonuclease degrade from 3’ to 5’
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tRNA - ‘adapter’- small mol., only 75-95 base long
Aminoacyl-tRNA; charged tRNA with the aa. corresponding to its anticodon
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Four major arms of tRNA + an extra arm
Invariant or Conserved,>90-95%of t RNAspecific bases.
Semi-invariant or Semiconserved, restrict to one type of base.
Extra arm, the most variable feater of tRNA
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Tertiary Structure of tRNA
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Ribosome
-Ribonucleoprotein particles-Contain more RNA than Protein-Ribosomal protein, ‘r-protein’
-Ribosome in a cell are identical-Consist of two subunits, each contain; *a major rRNA *a number of small proteins
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Fig.6.4 The ribosome has two sides for bindingcharged tRNA.
P site holds peptidyl-tRNA
A site, entered by aminoacyl-tRNA
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Elongation Factor T loads aminoacyl-tRNA into A-site
EF-Ts for replacement of GDP by GTP
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Fig.6.6Three stages of Protein Synthesis
Initiation
Elongation
Termination
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Fig 6.7Initiation requires;-free ribosome subunit-initiation factors,IF, only on 30S sub.
Two steps of reaction;1.Recognition of mRNA2.Large sub. joins
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Bacteria use IF-1 IF-2 IF-3
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A Special Initiator tRNA
-Protein synthesis start with the same aa, METHIONINE-Initiation codon, AUG, in Bacteria, GUG or UUG also used
Two types of tRNA carry METHIONINE- one for initiation- other for recognizing AUG codons during elongation
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*N-formyl-methionyl-tRNA; The initiator tRNA carries a methionine residue with formulating
on amino gr., Known as tRNAfMet
A Special Initiator tRNA
The name of the aminoacyl-tRNA, fMet-tRNAf
-This tRNA is used only for Initiation.-Recognize AUG or GUG (occasionally UUG)
tRNAmMet for recognizing AUG
codons in internal locations, no methionine formylation
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Two sites on Ribosome
P-site for fMet-tRNAf
A-site for amioacyl-tRNA,aa-tRNA
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fMet-tRNAf
The Initiator tRNA
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Initiation Factors:
IF-2 ,control the initiator tRNA entry to the ribosomeIF-1, bind 30S to complete initiation complexIF-3, need for 30S to bind initiation site
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Fig6.15 Initiation occurs independently at each cistronin a polycistronic mRNA
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Termination Reaction:- release the completed polypeptides-expel tRNAfrom ribosome-dissociate the ribosome tRNA
Termination codons;UAA,most commonUGA > UAGUGA, more error reading
Release Factors,- catalyze termination, A-site-RF-1 for UAA, UAG-RF-2 for UGA, UAA-RF-3 for release RF-1&RF-2
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Ribosome Recycling Factors, RRF
- act together with EF-G causing dissociation of 50S & 30S
- IF-3 also reequire, remove deacylated tRNA from 30S , and prevent reassociation
- RRF act on 50S