Download tRNA

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
bacterial mRNA Degradation
Endonucleolytic cleavage
from 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 RNA
specific 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 binding
charged 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
Initiation
Fig 6.7 requires;
-free ribosome
subunit
-initiation factors,IF,
only on 30S sub.
Two steps of reaction;
1.Recognition
of mRNA
2.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
A Special Initiator tRNA
*N-formyl-methionyl-tRNA; The initiator tRNA
carries a methionine residue with formulating
on amino gr., Known as tRNAfMet
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
The Initiator tRNA
fMet-tRNAf
Initiation Factors:
IF-2 ,control the initiator
tRNA entry to the ribosome
IF-1, bind 30S to complete
initiation complex
IF-3, need for 30S to bind
initiation site
Fig6.15 Initiation occurs independently at each cistron
in a polycistronic mRNA
Termination Reaction:
- release the completed polypeptides
-expel tRNAfrom ribosome
-dissociate the ribosome tRNA
Termination codons;
UAA,most common
UGA > UAG
UGA, 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