2. Case Scenario
Ali complains of a fever and cough. A sputum
culture confirms that he has pneumonia, a
respiratory infection caused by Streptococcus
pneumonia bacteria. He has started oral
erythromycin therapy. How can erythromycin
help him?
3. Definition:
-The synthesis of protein in the cytoplasm using mRNA as the
template
-The mRNA carries the instructions to make protein in the process of
translation (carries the genetic code)
Translation
4. • It is the sequence of three adjacent nucleotides in the 5´-
3´direction on mRNA. i.e. triplet codon.
• One genetic codon codes for one amino acid.
What is the Genetic codon??
5. Characters of genetic codon:
Each amino acid has specific codons (1 or
more)
Each codon consists of 3 nucleotides
There are 20 aa however
There are (4)3 = 4×4×4= 64 codons
➢61 = sense codons codes for aa
➢3 = non sense codons or termination
codons (UAA, UAG, UGA)
AUG (which encode for methionine) act as
initiation codon for translation
• One codon for start
signal: AUG.
• It also codes for
methionine.
• Three codons for
stop signal: UAA,
UAG, UGA.
• 61 codons for 20
amino acids.
6. • During the process of translation, specific codons on mRNA
are recognized by tRNA molecules (containing anti-codons
complementary to codons).
• Each tRNA carries a specific amino acid.
5’-End 3’-End
7. Characteristics of genetic code
1- Linear: bases of codon in mRNA is read from 5' to 3' end .
2- Triplet code: on mRNA that specify certain a.a.
3- Specific: a specific codon always codes for the specific a.a.
4- Universal: It is the same for all species i.e. plants, animal.
5- Degenerate: a given a.a may have >one codon that specify the same a.a. They
are different in the 3rd base.
6- Non-overlapping & commaless (without interruption): read from a fixed starting
point continuously without spacing or overlapping.
9. 1. A ribosome: protein synthesizing machinery.
2. mRNA: the template that carries information needed for arranging the amino
acids in the proper order of the specific protein.
3. tRNA: carries the amino acids to the proper place in the polypeptide
chain.
4. Amino acids: the building units of the protein.
5. Aminoacyl-tRNA synthetase enzyme: connects the amino acids to the
specific carrier tRNA.
6. Protein factors: as initiation factors (IF), elongation factors (EF) and
releasing factors (RF).
7. A source of energy: in the form of ATP and GTP .
Requirements for protein synthesis
10. 4 Steps for Translation
1. Activation of aa: synthesis of aminoacyl-tRNA
2. Initiation: formation of the initiation complex
3. Elongation: polypeptide chain synthesis
4. Termination: release of the polypeptide chain
➢ The polypeptide chain produced may be modified by post- translational
modification.
11. Steps of translation
1- Activation of amino acids (Synthesis of aminoacyl-
tRNA):
▪ Charging of tRNA by its specific aa on the 3 oH
terminus end of the acceptor arm
▪ The enzyme responsible for charging of tRNA by its
specific A.A is known as aminoacyl tRNA synthetase.
▪ 2 high energy bonds (from 1 ATP) are needed for
a.a activation
12. 2- Initiation
• Assembly of the translation machinery:
1- Ribosome:
Two pieces, large and small
Two RNA binding site (Peptidyl site& Aminoacyl
site)
2- An mRNA with instructions for the protein we'll
build
3. An "initiator" tRNA carrying the first amino
acid in the protein, which is almost always
methionine (Met)
4. Initiation factors
13. 3-Elongation
• Elongation is a cyclic process involving several steps and is
catalyzed by elongation factors.
• Step (1) Binding of the new amino acyl-tRNA to A site:
– tRNA brings the correct new amino acid to A-Site. 1 GTP is needed
• Step (2) peptide bond formation
– Formation of peptide bond between the old and the newly added amino
acid catalyzed by peptidyl transferase enzyme present inside the 60s
subunit.
• Step (3) translocation
– the ribosome moves 3 nucleotides towards the 3ʹ end of mRNA. This
requires EF2 and GTP.
14. 1 GTP is needed for binding of amino acyl tRNA to A site
Binding of new amino acyl-tRNA to the A site:
17. After translocation:
▪ Release of uncharged tRNA from E (exit)site.
▪ Transfer of the newly formed peptidyl-tRNA from A site to Psite.
▪ The A site becomes free (can be occupied by another new amino
acyl tRNA according to the codon anticodon recognition).
19. - For each new peptide bond formed 4 high energy
phosphate bond are cleaved
• 2 high energy bonds (from 1 ATP) for activation
• 1 GTP for binding of new aminoacyl tRNA to A site
• 1 GTP for translocation
Energy required for formation of one peptide bond
(for adding new a.a to the polypeptide chain)
20. 4-Termination
• Elongation of polypeptide chain continues until the A site is
occupied by one of the 3 termination codons (UAA, UAG, UGA).
• Releasing factors (RF) appear in A site and can recognize all
three termination codons.
• RFs + GTP + peptidyl transferase promotes the hydrolysis of
bond between polypeptide chain and tRNA occupying the P site.
• This hydrolysis leads to:
– Release of both polypeptide and tRNA.
– Dissociation of 80s ribosomes into 40s and 60s subunit.
23. Inhibitors of translation
Protein synthesis is a primary target for antibiotics
Streptomycin: Interfere with initiation step
Tetracycline: Block A site to prevent binding of aminoacyl t-
RNA
Erythromycin: Inhibits translocation step
24. Case Report and Clinical Correlates
• Ali Mohamed’s infection was treated
with erythromycin.
• Erythromycin binds to the 50S
ribosomal subunit of bacteria.
• It prevents the translocation step.
25. The anticodon CAG is complementary to this codon?
A. CTU
B. GUC
C. GCG
D. ATA
Question 1
Questions
26. Question 2
Which of the following is not true to the nature of
the genetic code?
a) Codon is triplet
b) Codons are commaless
c) Codons are overlapping
d) Codons are universal
27. Question 3
Which type of RNA functions as the blueprint of the
genetic code?
A. rRNA
B. tRNA
C. nRNA
D. mRNA
28. ❑ Describe the characters of the genetic code.
❑ Calculate the energy required for one
peptide bond formation.
SEQs
