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Translation
Continued......
Lecture 16
Dr. Attya Bhatti

Translation occurs in three distinct
steps;
Initiation
2. Elongation
3. termination
1.
Initiation:
Figure: Initiation of Protein Synthesis. Start signals are
required for the initiation of protein synthesis in (A)
prokaryotes and (B) eukaryotes.


Amino Acids Are First Activated by ATP and then transfer
to tRNA to produce
aminoacyl-tRNA (aminoacyl-tRNA
synthetase).
The activated tRNA is bound in the P site on the ribosome.
Elongation:

Initiation of protein synthesis in E. coli requires
initiation factors IF-1, IF-2, & IF-3.
Elongation:
•First the mRNA attaches itself to a ribosome (to the small
subunit).
•A complementary tRNA molecule attached on the ribosome
in the first position P.
•Another tRNA base pairs with the other three mRNA
bases in the ribosome at position A.
•The enzyme peptidyl transferase forms a peptide bond
between the two amino acids.
•The first tRNA (without its amino acid) leaves the
ribosome.
A ribosome on the rough
endoplasmic reticulum attaches to
the mRNA molecule.
ribosome
A U G G G C U U AAA G C A G U G C A C G U U
Amino acid
tRNA molecule
A transfer RNA molecule arrives.
It brings an amino acid to the first three bases
(codon) on the mRNA.
anticodon
The three unpaired bases (anticodon) on the
tRNA link up with the codon.
UAC
A U G G G C U U AAA G C A G U G C A C G U U
Another tRNA molecule comes into place,
bringing a second amino acid.
Its anticodon links up with the second codon on
the mRNA.
UAC
A U G G G C U U AAA G C A G U G C A C G U U
Peptide bond
A peptide bond forms between the two amino
acids by the enzyme peptidyl transferase..
A U G G G C U U AAA G C A G U G C A C G U U
The first tRNA molecule releases its amino acid and
moves off into the cytoplasm.
A U G G G C U U AAA G C A G U G C A C G U U
The ribosome moves along the mRNA to the next
codon.
A U G G G C U U AAA G C A G U G C A C G U U
Another tRNA molecule brings the
next amino acid into place.
A U G G G C U U AAA G C A G U G C A C G U U
A peptide bond joins the second and third
amino acids to form a polypeptide chain.
A U G G G C U U AAA G C A G U G C A C G U U
The process continues.
The polypeptide chain gets longer.
This continues until a termination (stop)
codon is reached.
The polypeptide is then complete.
A U G G G C U U AAA G C A G U G C A C G U U
Termination:
•Chain termination requires release factors.
•When the peptidyl-tRNA is in the P site, the
release factors, in response to the chain
terminating codons, bind to the A site.
•The polypeptide is then released from the P site,
and
•The ribosomes dissociate into two subunits..
Termination:
Figure : Steps leading to termination of protein synthesis
Protein Processing or Post Translational
Modifications:
•Glycosylation (Sugars are added)
•Proteolytic processing (Signal peptidase)
•Phosphorylation
•Acetylation
•Regulated degradation (Ubiquitin)
Why are proteins modified?
1. Regulation of activity
modification may turn activity on
modification may turn activity off
modification may generate a different function
2. Protein-protein interaction
modification site may be a binding interface
3. Subcellular localization
modification site may be a targeting signal
modification may be a membrane anchor
4. Aging
modification may identify the protein for degradation
modification may target a protein to be scavenged