two stage processes of gene expression
DESCRIPTION
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. Prokaryotic Translation. Life Cycle of mRNA. Fig5.19 Two stage process of - PowerPoint PPT PresentationTRANSCRIPT
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
Prokaryotic Translation
Life Cycle of mRNA
Fig5.19 Two stage process ofbacterial mRNA Degradation
Endonucleolytic cleavagefrom 5’ to 3’ behide ribosome
Exonuclease degrade from 3’ to 5’
tRNA - ‘adapter’- small mol., only 75-95 base long
Aminoacyl-tRNA; charged tRNA with the aa. corresponding to its anticodon
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
Tertiary Structure of tRNA
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
Fig.6.4 The ribosome has two sides for bindingcharged tRNA.
P site holds peptidyl-tRNA
A site, entered by aminoacyl-tRNA
Elongation Factor T loads aminoacyl-tRNA into A-site
EF-Ts for replacement of GDP by GTP
Fig.6.6Three stages of Protein Synthesis
Initiation
Elongation
Termination
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
Bacteria use IF-1 IF-2 IF-3
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
*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
Two sites on Ribosome
P-site for fMet-tRNAf
A-site for amioacyl-tRNA,aa-tRNA
fMet-tRNAf
The Initiator tRNA
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
Fig6.15 Initiation occurs independently at each cistronin a polycistronic mRNA
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
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