2. Translation
• The conversion of genetic information transcribed
on a mRNA to a sequence of amino acids in a
polypeptide chain
• Protein synthesis occurs on ribosomes.
• mRNA is translated 5’ to 3’.
• Protein is synthesized N-terminus to C-terminus.
• Amino acids bound to tRNAs are transported to
the ribosome.
3. Ribosomes:
a) Large subunit (50S): consists of 23S and 5S
rRNAs and 31 ribosomal proteins
b) Small subunit (30S): consists of 16S rRNA and
21 ribosomal proteins
(The S stands for Svedberg unit, which measures
the rate of sedimentation of a particle in a
centrifuge and so is an indicator of Size)
6. Translation-4 main steps
1. Charging of tRNA
2. Initiation
3. Elongation (3 steps)
a. Binding of the aminoacyl tRNA to the
ribosome
b. Formation of the peptide bond.
c. Translocation of the ribosome to the next
codon.
4. Termination
7. Step 1-Charging of tRNA (aminoacylation)
1. ATP and amino acid bind to aminoacyl-tRNA
synthetase, to form aminoacyl-AMP + 2
phosphate.
2. tRNA binds to aminoacyl-AMP.
3. Amino acid transfers to tRNA, displacing AMP.
4. Amino acid always is attached to adenine on 3’
end of tRNA by its carboxyl group forming
aminoacyl-tRNA.
10. Step 2-Initiation-steps (e.g., prokaryotes):
• The small ribosomal subunit binds to an IF and this
complex in turn binds to mRNA at the 5 end.
• Then other IF help in the binding of the initiator
tRNA which is charged with fMet
• The small subunit plus mRNA plus fMet -tRNA
represent the initiation complex. This complex
combines with the large subunit.
• The required energy is provided by the hydrolysis
of a molecule of GTP. Then the initiation factors are
released.
11.
12. Step 3-Elongation of a polypeptide:
1. Binding of the aminoacyl tRNA (charged tRNA) to
the ribosome.
2. Formation of the peptide bond.
3. Translocation of the ribosome to the next codon.
13. 3-1. Binding of the aminoacyl tRNA to the ribosome.
• Ribosomes have two sites, P site (5’) and A site (3’)
relative to the mRNA.
• Synthesis begins with fMet (prokaryotes) in the P
site.
• Next codon to be translated (downstream) is in the
A site.
• Incoming aminoacyl-tRNA (aa-tRNA) bound to
elongation factor EF-Tu + GTP binds to the A site.
• Hydrolysis of GTP releases EF-Tu, which is
recycled.
14. 3-2. Formation of the peptide bond.
• Two aminoacyl-tRNAs positioned in the ribosome,
one in the P site (5’) and another in the A site (3’).
• Bond is cleaved between amino acid and tRNA in
the P site.
• Peptidyl transferase forms a peptide bond between
the free amino acid in the P site and aminoacyl-
tRNA in the A site.
• tRNA in the A site now has the growing polypeptide
attached to it.
15. 3-3. Translocation of the ribosome to the next codon:
• The entire mRNA-tRNA-aa2-aa1 complex shifts one
codon towards the 3’ end resulting in transfer of the
tRNA with the polypeptide chain to occupy the P
site after release of the uncharged tRNA.
• This translocation requires several elongation
factors, as well as the energy derived from the
hydrolysis of GTP.
• The elongation process are repeated over and over.
An additional amino acid is added to the growing
polypeptide chain each time the mRNA advances
through the ribosome.
16.
17. Step 4. Polypeptide chain termination
• Signaled by a stop codon (UAA, UAG, UGA).
• Stop codons have no corresponding tRNA.
• Release factors (RFs) bind to stop codon and
assist the ribosome in terminating translation.
RF1 recognizes UAA and UAG
RF2 recognizes UAA and UGA
RF3 stimulates termination
18.
19. Polyribosomes:
As elongation of the polypeptide chain proceeds, and
the initial portion of mRNA has passed through the
ribosome, the message is free to associate with
another small ribosomal subunit to form another
initiation complex which start another polypeptide
chain. This process can be repeated several times
along a single mRNA and are called Polyribosomes
or polysomes.
