This  is a process by which the genetic code contained within a messenger RNA (mRNA) molecule is decoded to produce a specific sequence of amino acids in a polypeptide chain.

1. Translation
Dr. M.Sankareswaran
Assistant professor, Department of Microbiology.

2. Central Dogma
• There are two main stages of gene expression, transcription and
translation
Transcription the process of converting DNA into messenger RNA
Translation the process of synthesizing a specific polypeptide as
coded for by messenger RNA

5. Translation
 This is a process by which the genetic code contained within a messenger
RNA (mRNA) molecule is decoded to produce a specific sequence of
amino acids in a polypeptide chain.
 It occurs in ribosome which are present in cytoplasm.
 It begins after mRNA enters in cytoplasm.
 It uses tRNA as the interpreter of mRNA

7. Components required for
Translation
 Amino acids
 Transfer RNA
 Messenger RNA
 Aminoacyl t RNA synthetase
 Functionally competent ribosomes
 Protein factors
 ATP and GTP as a source of energy

8. Role of Ribosomes in Protein
synthesis:
 Ribosome is a macromolecular structure that directs
the synthesis of proteins.
 A ribosome is a multicomponent, compact,
ribonucleoprotein particle which contains rRNA, many
proteins and enzymes needed for protein synthesis.
 Ribosome is a nucleoprotein particle having two
subunits. These two subunits lie separately but come
together for the synthesis of polypeptide chain.
 In E. coli ribosome is a 70S particle having two subunits
of 30S and 50S. Their association and dissociation
depends a upon the concentration of magnesium.

9. Units of Ribosomes
 Large subunit contains peptidyl
transferase centre, which forms
the peptide bonds between
successive amino acids of the
newly synthesized peptide chain.
 Small subunit of ribosome
contains the decoding centre in
which charged tRNAs decode of
the codons of mRNA.

10. Steps of Protein Synthesis
 The process of protein synthesis is divided into 3 stages
 Initiation
 Elongation
 Termination

13. Activation of aminoacids
 The activation of aminoacids takes place in cytosol.
 The activation of aminoacids is catalyzed by their aminoacyl
tRNA synthetases.
 All the 20 aminoacids are activated and bound to 3’ end of
their specific tRNA in the presence of ATP and Mg++.
 The N-formylated methionine is chain initiating aminoacid in
bacteria whereas methionine is chain initiating aminoacid in
eukaryotes.
 Methionine is activated by methionyl-tRNA synthetase.
 For N-formylmethionine two types of tRNA are used ie. tRNAmet
and tRNAfmet.

14. Initiation
 In the first step, initiation factor-3 (IF-3) binds to 30S ribosomal unit.
 Then mRNA binds to 30S ribosomal subunit in such a way that AUG
codon lie on the peptidyl (P) site and the second codon lies on
aminoacyl (A) site.
 The tRNA carrying formylated methionine ie. FMet–tRNAFMet is palced
at P-site. This specificity is induced by IF-2 with utilization of GTP. The IF-1
prevent binding of FMet–tRNAFMet is in A-site.
 Shinedalgrno sequence in the mRNA guide correct positioning of AUG
codon at P-site of 30S ribosome.
 After binding of FMet–tRNAFMet on P-site, IF-3, IF-2 and IF-1 are
released so that 50S ribosomal unit bind with 30S forming 70S
sibosome. The exit site is located in 50S

17. Elongation
Binding of AA-tRNA at A-site:
 The 2nd tRNA carrying next aminoacid comes into A-site and
recognizes the codon on mRNA. This binding is facilitated by EF-TU
and utilizes GTP.
 After binding, GTP is hydrolysed and EF-TU-GDP is releasd
 EF=TU-GDP then and enter into EF-TS cycle.
Peptide bond formation:
 The aminoacid present in t-RNA of P-site ie Fmet is transferred to t-
RNA of A-site forming peptide bond. This reaction is catalyzed by
peptidyltransferase.
 Now, the t-RNA at P-site become uncharged

20. Ribosome translocation:
 The peptidyl tRNA carrying two amino acids present at “A” site
is now translocated to”P” site. This movement is called
translocation.
 Elongation factor called EF-G control translocation.
 This factor G is called translocase.
 Hydrolysis of GTP provides energy for translocation and release
of deacylated tRNA (free of amino acid).
 The uncharged tRNA exit from ribosome and enter to cytosol.
 The ribosomal translocation requires EF-G-GTP (translocase
enzyme) which change the 3D structure of ribosome and
catalyze 5’-3’ movement.
 The codon on A-site is now recognized by other aminoacyl-
tRNA as in previous.

21. Termination
 The peptide bond formation and elongation of polypeptide continues
until stop codon appear on A-site.
 If stop codon appear on A-site it is not recognized by t-RNA carrying
aminoacids because stop codon donot have anticodon on mRNA.
 The stop codon are recognized by next protein called release factor
(Rf-1, RF-2 and RF-3) which hydrolyses and cause release of all
component ie 30s, 50S, mRNA and polypeptide separates.
 RF-1 recognisaes UAA and UAg while RF-2 recognises UAA and UGA
while RF-3 dissociate 30S and 50S subunits.
 In case of eukaryotes only one release actor eRF causes dissociation

23. tRNA Binding
Sites on the
Ribosome
(Ribosme moves
like an enzyme)

24. Stages of Translation
• Polypeptide Chain Initiation
• Chain Elongation
• Chain Termination

25. Translation Initiation
• 30S subunit of the ribosome
• Initiator tRNA (tRNAMet)
• mRNA
• Initiation Factors IF-1, IF-2, and IF-3
• One molecule of GTP
• 50S subunit of the ribosome

28. Polypeptide Chain
Elongation
• An aminoacyl-tRNA binds to the A site of the
ribosome.
• The growing polypeptide chain is transferred
from the tRNA in the P site to the tRNA in the
A site by the formation of a new peptide
bond.
• The ribosome translocates along the mRNA
to position the next codon in the A site. At
the same time,
– The nascent polypeptide-tRNA is translocated
from the A site to the P site.
– The uncharged tRNA is translocated from the P
site to the E site.

34. Elongation of
Polypeptides (A mRNA can have
multiple Ribosomes

35. Polypeptide Chain
Termination
 Polypeptide chain termination occurs
when a chain-termination codon (stop
codon) enters the A site of the ribosome.
 The stop codons are UAA, UAG, and UGA.
 When a stop codon is encountered, a
release factor binds to the A site.
 A water molecule is added to the
carboxyl terminus of the nascent
polypeptide, causing termination.

36. No tRNA exists for stop codons!

38. Post translation modification
The newly formed polypeptide may not be biologiy functional so it undergoes
several folding and processing known as post translation modification.
Amino terminal and carboxyl terminal modification:
 The N-formylmethionine in case of bacteria is removed from polypeptide chain
and some carboxyl terminal are also removed by enzymatic action to make
functional protein.
 In case of eukaryotic protein, amino terminal is N- acetylated.
Loss of signal sequences:
 In some protein the amino terminal end is cleaved by specific peptidase so that
protein loss its signaling property.
Modification of individual aminoacids:
 The aminoacids may be phosphorylated, acetylated for modification
Attachment of carbohydrate side chain:
 Carbohydrate side chain is added to make protein functional. Eg, glycoprotein.
Lipoprotein
Addition of isoprenyl group:
 In some protein, isoprenyl group is added so to make protein active.