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