Translation of mRNA into protein occurs through three main stages: initiation, elongation, and termination. During initiation, the ribosome assembles on the mRNA with the help of initiation factors. In elongation, tRNAs bring amino acids to the ribosome according to the mRNA codons and link them together to form the polypeptide chain. Termination occurs when a stop codon enters the A site, signaling the release of the complete protein. In eukaryotes, post-translational modifications such as phosphorylation, acetylation, and protein folding further process the protein to produce its active form.

1. Protein synthesis:
Translation
Post translation modification
of protein in eukaryotes

2. Translation
 In molecular biology and genetics,
Translation is the process in which
ribosomes or endoplasmic reticulum (cytosol)
synthesize proteins after the process of
transcription of DNA to RNA in the cell's
nucleus.
 During translation, a cell reads information
from a molecule called a messenger RNA
(mRNA) to build a protein, with the help of
transfer RNA (tRNA).

3. Translation
• In translation, polypeptides (long chains of
amino acids) are synthesized using:
 mRNA sequences and cellular machinery,
including tRNAs that match mRNA nucleotides.
Genetic code:
The sequence of nucleotides / the set of certain
rules in deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA) that determines the
amino acid sequence of proteins.

4. Genetic code cont..
• A codon (genetic language) is a three-
nucleotide or triplet sequence found on
mRNA that codes for a certain amino acid
during translation.
• The anticodon is a three-nucleotide
sequence found on tRNA that binds to the
corresponding mRNA sequence.

5. Genetic codes

6. Translation
tRNA molecules are not identical:
• each carries a specific amino acid on one
end.
• each has an anticodon on the other end
that base pairs with a complementary codon
on mRNA.

8. Translation
• There are 20 different amino
acids coded in DNA.
• They all have an amino group
(-NH2) group on one end, and
an acid group (-COOH) on the
other end. Attached to the
central carbon is an R group,
which differs for each of the
different amino acids.
• When polypeptides are
synthesized, the acid group of
one amino acid is attached to
the amino group of the next
amino acid, forming a peptide
bond.

9. Translation
• Translation of mRNA into protein is
accomplished by the ribosome.
• Ribosomes consist of two major
components: the small (40S) and large (60S)
ribosomal subunits.
• Each subunit consists of one or
more ribosomal RNA (rRNA) molecules and
many ribosomal proteins (RPs or r-proteins).
• The ribosomes and associated molecules
are also known as the translational
apparatus.

10. A ribosome(rRNA) has 3 binding sites
for tRNA:
• - A site: holds the tRNA carrying the next
amino acid to be added to the chain
• - P site: holds the tRNA carrying the growing
polypeptide chain
• - E site (Exit): where discharged tRNAs leave
the ribosome

13. Stages of traslation process
(1) Initiation
(2) Elongation
(3) Termination

14. What is needed for translation
initiation?
• Mature mRNA
• Ribosome (small 40s and large 60s
subunits)
• Eukaryotic initiation factors (eIFs)
• GTP and ATP as energy source
• Initiator tRNA (Met-tRNA)

15. 1.Initiation
• Initiation ("beginning"): in this stage, the
ribosome gets together with the mRNA and the
first tRNA, so translation can begin.
• Events of initiation include:
1.Initiator tRNA (Met-tRNA) gets loaded onto the
small ribosomal subunit (40S).
2.Eukaryotic initiation factors (eIFs) gets loaded as
well on the 40S ribosomal subunit.
3. A pre-initiation complex is formed containing
(40S – Met-tRNA – eIFs).

16. Cont…
• 4.The complex (40S – Met-tRNA – eIFs) binds
to the 5’ end of the mRNA.
• 5.More eIFs are bound to the 5’ end of mRNA.
• The complex (40S – Met-tRNA – eIFs) moves
along the transcript (5’ – 3’) in search for the
first codon AUG (methionine)
• 7.The process is called “scanning” of mRNA.
• 8.When first AUG is found, eIFs dissociate.

17. Cont…
• The complex (40S – Met-tRNA – eIFs)
moves along the transcript (5’ – 3’) in
search for the first codon AUG
(methionine)
• 7.The process is called “scanning” of
mRNA.
• 8.When first AUG is found, eIFs
dissociate.

18. Cont…
9.The large ribosomal subunit (60S)
assembles to the complex at the start
codon.
10. The initiator tRNA is positioned in the P
site of the ribosome and the A site is
available to receive to next charged tRNA.

19. Elongation
• In this stage, amino acids are brought to the
ribosome by tRNAs and linked together to
form a chain.
• Amino acids are added one by one.
• Each addition involves elongation factors and
occurs in 3 steps:
-Codon recognition
-Peptide bond formation
-Translocation: involves movement of on to
the next codon on the mRNA

20. Elongation
• During the elongation stage, the
ribosome continues to translate each
codon in turn.
• Each corresponding amino acid is added
to the growing chain and linked via a bond
called a peptide bond.
• Elongation continues until all of the
codons are read.

22. Termination
 Translation termination takes place
when the end of the coding sequence is
reached by the ribosome and a stop
codon (UAA, UGA, or UAG) enters the A
site.
 It requires two release factors (RFs):
 eRF1 and eRF3, which bind to the A-site.

23. What happens if no stop codon?
• Without stop codons, an organism is
unable to produce specific proteins.
• The new polypeptide (protein) chain will
just grow and grow until the cell bursts or
there are no more available amino acids
to add to it.

24. Post translation modification (PTM)
• It is a later stage in protein biosynthesis
• It refers to the reversible of irreversible
chemical changes proteins may undergo
after translation.
• This chemical modifications range from
enzymatic cleavage of peptide bond to
covalent addition of particular chemical
group to amino acids side chains.

25. Post translation modification
• Most of the PTMs occur in order to
produce a fully functioning protein in
different body cells .
• Therefore the addition of groups or
deletion of parts makes a finished protein.
• PTM play an important part in modifying
the end product of expression and
contribute towards biological processes
and diseased conditions.

26. Types of post translational
modification in eukaryotes
1. Trimming
2. Covalent attachments
3. Protein folding and
4. Protein degradation

27. A. Trimming
• It involves removal of part of protein to
convert it into a three dimentional and fully
active form
• Example
 Insulin, is synthesized in inactive form that
it can not perform its function of regulating the
high glucose level in the blood.
• Removal of the signal peptide and
intervening C-chain, produces a biologically
active Insulin molecule.

28. Covalent attachments
i. Phosphorylation
• This is the addition of phosphate group to
amino acids residue usually
serine,threonine,and tyrosine on the
polypeptide chain.
• This reaction is catalyzed by the Protein
kinases enzyme.
• Addition of a phosphate group increases
hydrophilic character of that protein.

29. Covalent attachments cont..
iii. Lipidation
• Addition of a lipid group to side chains of
amino acids on a polypeptide chain.
• Targets protein that are destinated to be in
membrane organelles example endoplasmic
reticulum, mitochondria, golgi apparatus and
plasma membrane.
• Lipidation increases protein binding affinity to
biological membranes, folding and stability,
and association with other protein.
• It mostly occur on side chains of serine,
cysteine and glycine amino acids

30. Covalent attachments cont..
iv. Carboxylation
• Addition of carboxyl
group to proteins
involved in the blood
clotting process
particularly
prothrombin.
• Prothrombin is needed
to stop bleeding, the
addition of carboxyl
group on glutamine
amino acid found on
prothrombin increases
its activity.
v. Methylation
• Involves addition of
methyl groups to
protein (amino acid side
chain).
• It occurs in the cytosol.
• It eliminates the
negative charge and
increases protein
hydrophobicity.
• It usually occurs in
lysine, arginine and
histidine.

31. Covalent attachments cont..
vi. Acetylation
• Involves the addition
of acetyl
group(CH3CO) on the
protein substrate.
• Protein can be
acetylated in the N-
terminal amino acid of
amino.
• Acetylation modifies
DNA binding
properties ,protein
stability and protein –
vii. Sulfation
• Involves addition of
sulphate molecule in
protein particularly
tyrosine residues.
• Sulfation is necessary
for bioactivity of some
protein also it
increases their ability
to interact with other
proteins.

32. C. Protein degradation
• It is also called proteolysis.
• It is the lysis or breakdown of protein into smaller amino
acids
• Protein degradation involves two pathways:
• Ubiquitin proteasomes and lysosomal proteolysis
pathways.
• Lysosomal pathway: occurs in lysosome, an organelle
in many enzymes.
• Involves formation of vesicles that prevent degradation
of other cell contents .
• Lysosomes degrade cytosolic protein in selective
manner as a response to cellular starvation in order to
provide energy and amino acids.

33. The Ubiquitin Proteasome Pathway (UPP).
• The protein gets attached to the ubiquitin which acts
as a signal for degradation by series of enzymes that
include E1,E2,E3 .
• After its attachment it forms multiubiquitin chain, which
is catalysed by 26s Proteasome.
• Ubiquitin is then released from the chain under the
influence of ATP, and the misfolded and foreign protein
will be degraded.
 Protein degradation is important aspect of cell
regulation and help in eliminating the consequences of
mistakes made during protein synthesis .

34. D. Protein folding
• Protein folding is the process in which a
polypeptide chain folds to become active protein
in it is native three dimensional structure.
• It occurs in endoplasmic reticulum.
• It requires other proteins called chaperones to
fold properly within the organism.
• Chaperones increases the efficiency of the
overall process of protein folding by reducing
the probability of competing reaction such as
aggregation .
• Molecular chaperones require ATP energy for it
is effective functioning.

35. • Protein folding occurs in four steps which include
primary ,secondary ,tertiary and quaternary structure
as shown in the diagram

36. Importance of post translation
modification of protein
 Post translation modification aids in proper protein
folding.
 Protein folds in order to attain its native structure and
to be able to perform it is function correctly, due to
presence of lectin molecules called calnexin which
assist the folding of protein.

37. • It protects protein against being cleaved by
proteolytic enzyme (peptidase ), by blocking of the
cleavage sites so that the enzyme will not be able to
cleave the terminals of protein.
• Act as important component of cell signalling, for
example when they convert the inactive insulin to
active insulin and when they convert prohormones to
hormones

38. • Plays important role on the function of haemostatic
protein and are critical in settling diesease.
• Haemostatic protein include tissue factor, fibrinogen
in which these protein are influenced by PTMs .
• For example increase phosphorylation of tissue factor
results in increased affinity of platelets to vessel
endothelium

39. • It helps to increase the hydrophobicity of proteins as
the result cause changes in structure of protein .Hence
leads the stability of protein and the binding affinity
to their co factors ( metal atoms ,vitamins ).

40. References
• Allison, A .L. (2007). Fundamental Molecular
Biology. Blackwell publishing. UK
• Lewin , B.(2004). Genes VIII.Oxford.UK.
• Primrose , S .B ., Twyman, R.M.& Old,
R.W.(1995).Principles of Gene Manipulation.(5th ed).
Blackwell Publishing Company.UK.
• Snustan ,D .P.& Simmons ,M .J.(1997). Principles of
Genetics( 5thed).USA.