Translation and
post translational
modifications
BC10.4 - Describe in brief the major steps
involved in Replication, Transcription, and
translation.
Content
Translation
• Definition
• Requirements
• Steps
• Post-translational modifications
• Inhibitors
• Summary
Definition
• It is the process of converting or translating
the information in mRNA into a sequence of
amino acids that makes up proteins.
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Tools Required
• Ribosomes
• RNAs – mRNA , tRNA rRNA
• Translation factors
• Energy in the form of ATP and GTP
• Aminoacids
Site of protein synthesis -- RIBOSOMES
A site - gives attachment to incoming t-RNA carrying an amino acid
P site - attachment to t-RNA carrying a polypeptide chain
E site - From where t-RNA exits after delivering amino acid
•The mammalian ribosome has sedimentation coefficients of
80 S unit. It has larger 60 S subunit and another smaller 40S
subunit.
•The bacterial ribosome has sedimentation coefficients of
70 S unit. It has larger 50 S subunit and another smaller
30S subunit.
mRNA
• Polycistronic mRNA (Prokaryotes) :
MORE THAN ONE POLYPEPTIDE synthesized
from single mRNA.
• Monocistronic mRNA (eukaryotes) :
ONE POLYPEPTIDE synthesized from single
mRNA.
mRNA
• Nucleotide sequences
present in mRNA
concerned with protein
synthesis is known as
GENETIC CODE.
• A series of three adjacent
bases in genetic code
forms a CODON
• Each CODON codes for
single aminoacid. mRNA
Genetic Code
• Using the 4 bases A,U,G
and C There are 43, or 64
codons are possible.
• AUG acts as the initiator
codon.
• 3 Termination codons
(UGA, UAA , UAG)
Genetic Code
AUG codes for methionine in eukaryotes,but
codes for N-formyl methionine in prokaryotes.
Genetic code - Wobbling phenomenon
The reduced stringency between the
• 3rd base of the codon and
• The complimentary nucleotide in the
anticodon
The pairing of codon-anticodon can wobble at
the 3rd letter.
Eg: GGU, GGC, GGA will pair with CCI of Glycine-
tRNA
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tRNA
• It transfers amino acid to the site of
protein synthesis.
• A triplet of nucleotides in tRNA that
is complementary to the codon in
mRNA—called an anticodon. Amino acid
• Anticodons are present in tRNA
molecule
Anticodon
tRNA
Clover –leaf
3’ End – Acceptor arm- accepts
the amino acid
Anti –codon arm – recognizes the
Pseudo uridine
complementary codon on mRNA
TψC arm – attachment of charged
t-RNA to the ribosomes.
D arm – recognition of t-RNA by
the enzyme that transfers amino
acid to it
Translation factors
The PROTEIN FACTORS required for protein
synthesis are...
• Initiation factors (IF)
• Elongation factors (EF)
• Releasing factors (RF)
Steps of protein synthesis
• Activation of amino acid.
• Initiation.
• Elongation.
• Termination.
• Post translational modification.
• mRNA is translated from 5’ to 3’ end.
• Protein is synthesized from N terminal to C
terminal.
ACTIVATION OF AMINO ACIDS
Enzyme - Aminoacyl tRNA synthetase
At least one Aminoacyl-tRNA synthetase exists for each
aminoacid.
Recognizes the tRNA and the correct amino acid.
Charged t-RNA
RECOGNITION OF INITIATION CODON
• The marker sequence for the identification of
INITIATION CODON (AUG ) in the mRNA is
done by RIBOSOMES
• KOZAK CONSENSUS sequence in eukaryotes.
• SHINE-DALGARNO sequence in prokaryotes
Steps of Initiation
1.Ribosomal Dissociation
2.Formation of 43S pre-initiation complex
3.Formation of the 48S Initiation Complex
4.Formation of the 80S Initiation Complex
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Ribosomal Dissociation
E-IF 1 & 3 bind with 40 S
and prevent its
reassociation with 60 S
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Formation of 43S pre-initiation
complex
Ternary complex binds
with 40 S unit to form
43 pre-initiation
Binary complex complex
binds with Met-t-
RNA to form
ternary complex
Eukaryotic initiation
factor -2 binds with GTP
to form binary complex
(IF-2 + GTP)
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Formation of the 48S Initiation
Complex
Initiation factor 4F ( composed of
4A,4G,4E) binds to cap of mRNA
PAB protein & other initiating factors
help in activation of mRNA
Activated mRNA binds to 43S
Pre-initiation complex
48S Initiation Complex
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Formation of the 80S Initiation
Complex
48 S initiation complex binds
to 60S ribosomal subunit to
form 80S initiation complex
All the initiation factors are
released
Met- t-RNA is in the P-site
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Steps of Elongation
1.Binding of Aminoacyl-tRNA to the A Site
2.Peptide Bond Formation
3.Translocation
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Binding of Aminoacyl-tRNA to the A Site
Elongation factor (GTP bound EF1)
Directs the incoming aminoacyl t-RNA
to the A site
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Peptide Bond Formation
Peptide bond is formed between the amino
group of new aminoacyl t-RNA at the A site
and the carboxyl group of aminoacyl t-RNA
at the P site.
Catalyzed by Peptidyl transferase
Deacylated t-RNA vacates
the p site
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Translocation
Deacylated t-RNA vacates
the P site.
Whole ribosome moves over the mRNA
through the distance of 1 codon
t-RNA with two amino acids (Peptidyl t-RNA)
at the A site moves to the P site with the help
of EF2
A site is empty
New incoming aminoacyl t-RNA attaches and
the process continues
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Termination of
translation
Stop codon
appears at
the A site
No t-RNA
Releasing
factors and
GTP cleaves
the bond
between
polypeptide
and t-RNA
Termination of
translation-cont...
POLYPEPTIDE chain
is released from the P
site
m-RNA is freed from 80 S ribosome disassociates
ribosome in to 60S and 40S
POST TRANSLATIONAL MODIFICATIONS
• Proteins synthesised in a few cases may already
be a functionaly active.
• In other cases , proteins synthesised undergo
post translational modification before attaining
biological activity.
• Modifications in proteins occur during their
storage in Golgibodies.
POST TRANSLATIONAL MODIFICATIONS
• PROTEOLYTIC CLEAVAGE- Proinsulin to insulin.
• HYDROXYLATION- of Pro & Lys as in collagen.
• GLYCOSYLATION- Addition of carbohydrates to
glycoprotein.
• GAMMA CARBOXYLATION- of Glutamic acid residues
in clotting factors
• PHOSPHORYLATION – of certain serine & threonine
residues in casein.
• Formation of S-S bond between two cysteine
residues.
Inhibitors of protein synthesis in prokaryotes.
Tetracycline Inhibit tRNA binding to
ribosomes.
Chloramphenicol Inhibits the Peptidyl
transferase activity of the
50s ribosomal subunit.
Streptomycin misreading of codon
Erythromycin Binds to the 50s subunit &
inhibits translocation.
Inhibitors of protein synthesis in
Prokaryotes and Eukaryotes
Puromycin It has structural
resemblance to tyrosinyl
tRNA.
Cycloheximide Inhibits Peptidyl
transferase in 60s subunit.
Diphtheria toxin Inactivation of EF-2
Summary
Definition - Translation is the process of converting or translating
the information in mRNA into a sequence of amino acids that makes
up proteins.
Requirements – Ribosomes, RNAs (mRNA , tRNA, rRNA),
Translation factors, ATP, GTP and Amino acids.
Steps –
• Activation of amino acid
• Initiation
• Elongation
• Termination
• Post translational modifications (Proteolytic cleavage,
hydroxylation, glycosylation, gamma carboxylation,
phosphorylation, S-S bond)
Inhibitors – Tetracycline, Chloramphenicol, Streptomycin,
Erythromycin, Puromycin, Cycloheximide, Diphtheria toxin.
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