HNGU PATAN. m.sc sem 3 botany cbo - 503

1. TRANSLATION
( SYNTHESIS OF PROTEINS )
PRESENTED BY :- CHARMI J PATEL
SUB :- BOTANY
SEM :- 3
PAPER CODE :- CBO - 503
DEPARTMENT OF LIFE SCIENCES, H.N.G.U. ,
PATAN

2. • Content :-
• Introduction
• Requirements for translation
• Steps of translation
Activation of amino acid & charging of t-RNa
Initiation
Elongation
Termination
• Modification of released polypeptide
• Polysome formation

3. • Introduction :-
• Translation :- The conversion of something from one form or
medium into another.
• Translation is the mechanism by which the triplet base
sequence of a mRNA guides the linking of a specific sequence
of amino acids to form a polypeptide (protein) on ribosomes.
• The process by which amino acids are linearly arranged into
proteins through the involvement of ribosomal RNA, transfer
RNA, messenger RNA and various enzymes.

4. • Requirements for translation :-
• Ribosomes
• m-RNA
• t-RNA
• Amino acids
• Aminoacyl t-RNA synthetases
• ATP, GTP
• Enzymes of translation
• Releasing factor

5. • Steps of translation :-
1. Activation of amino acids and charging of t-RNA :-
m-RNA :-
• The DNA, that controls protein synthesis, is located in the chromosomes
within the nucleus, whereas the ribosomes, on which the protein
synthesis actually occurs, are placed in the cytoplasm.
• Therefore, some sort of agency must exist to carry instructions from the
DNA to the ribosomes. This agency does exist in the form of mRNA.
• The mRNA carries the message (information) from DNA about the
sequence of particular amino acids to be joined to form a polypeptide.
• The mRNA forms about 5% of the total RNA of a cell.

6.  t-RNA :-
• The t-RNA has many varieties. Each variety carries a specific amino
acid from the amino acid pool to the mRNA on the ribosomes to
form a polypeptide.
• A tRNA molecule has the form of a clover leaf. It has four regions :
I. Carrier End :- This is the 3’ end of the molecule. Here a specific
amino acid joints it. It in all cases has a base triplet CCA with -OH
at the tip. The -COOH of amino acid joints the -OH.
II. Enzyme Site :- It is meant for a specific charging enzyme which
catalyzes the union of a specific amino acid to tRNA molecule.

7. III. Recognition End :- It is the opposite end of the molecule.
It has 3 unpaired ribonucleotides.
The bases of these ribonucleotides have complementary
bases on the mRNA chain.
A base triplet on mRNA chain is called a codon, and its
complementary base triplet on tRNA molecule is termed an
anticodon.
IV. Ribosome Site :- It is on the other site of the molecule. It
is meant for attachment to a ribosome.
• The tRNA form about 15% of the total RNA of a cell.

9. • Activation of amino acids :-
• Amino acid reacts with ATP to form amino acid-AMP complex and
pyrophosphate.
• The reaction is catalyzed by a specific amino acid activating enzyme
called aminoacyl- tRNA synthetase in the presence of Mg ion.
• There is a separate aminoacyl – tRNA synthetase enzyme for each kind
of amino acid.
• The complex remains temporarily associated with the enzyme. The
amino acid- AMP- enzyme complex is called an activated amino acid.

10. • Charging of tRNA :-
• The amino acid –AMP – enzyme complex joins to the amino acid binding site
of its specific tRNA, where its -COOH group bonds to -OH group of the
terminal base triplet CCA.
• The reaction is catalyzed by the same aminoacyl – tRNA synthetase enzyme.
• The resulting tRNA – amino acid complex is called a charged tRNA. AMP and
enzyme are freed.
• The energy released by change of ATP to AMP is retained in the amino acid
– tRNA complex.
• This energy is later used to drive the formation of peptide bond when
amino acids link together on ribosomes.

12. 2. Initiation :-
• Ribosomes :-
• Ribosomes serve as the site for protein synthesis.
• The small and large subunits of ribosomes occur separately when not
involved in protein synthesis.
• The two subunits form association when protein synthesis starts, and
undergo dissociation when protein synthesis stops.
• The rRNA forms the small and large subunits of the ribosomes.

15. • The mRNA chain has at its 5’ end an “initiator” or “start”
codon (AUG) that signals the start of polypeptide
formation.
• This codon lies close to the P site of the ribosome. The
amino acid formyl- methionine initiates the process.
• It is carried by tRNA having UAC anticodon which bonds
to AUG initiator codon of mRNA by hydrogen bonds.

16. • Initiation factors (IF 1, IF 2 and IF 3) and GTP promote the
initiation process. The large ribosomal subunit now joins the
small subunit to complete the ribosome. At this stage, GTP
is hydrolyzed to GDP.
• The ribosome has formyl methionine-bearing tRNA at the P
site. Later, the formyl methionine is changed to normal
methionine by the enzyme deformylase.
• If not required, methionine is later separated from the
polypeptide chain by a proteolytic enzyme amino peptidase.
• Initiation factors are used again to start new chains.

18. • Elongation :-
• Binding of new aminoacyl – tRNA :-
• Codon in mRNA determines the incoming aminoacyl – tRNA
complex.
• EF 1 and GTP reqd.
• Peptide bond formation :-
• α- NH2 group of incoming amino acid in A site forms peptide
bond with -COOH group of amino acid in P site.
• Enzyme is peptidyl transferase. No need for energy as amino
acid is activated.

19. • Translocation :-
• when peptide bond formed Met from P site shifted to A site. tRNA is
released from P site, now P site is free.
• Ribosome moves forward by 1 codon. Peptidyl tRNA translocated to P
site. Now A site empty.
• New aminoacyl tRNA will come only to A site. It requires EF 2 and GTP.
New aminoacyl tRNA can come in now. tRNA with next anticodon binds
to mRNA.
• Amino acids are bonded together. Ribosome moves one codon on mRNA.
tRNA is released.

21. • Termination :-
• Two conditions are necessary for termination of protein synthesis.
• One is the presence of a stop codon that signals the chain
elongation to terminate, and the other is the presence of release
factors (RF) which recognise the chain terminating signal.
• There are three terminating codons, UAA, UGA, and UAG for which
tRNAs do not exist.
• Release of the peptidyl tRNA from the ribosome is promoted by
three specific release factors, RF 1, RF 2, RF 3. RF 1 recognizes
triplets UAA and UAG, while RF 2 recognizes UAA and UGA.

22. • The third factor RF 3 does not possess any release
activity of its own, but it binds to OTP and stimulates
the binding of RF 1 and RF 2 with the ribosome.
• Subsequently dissociation of 30S and 50S ribosome
subunits takes place with concomitant binding of IF3 to
30S subunit to prevent reassembly in the absence of
mRNA and fMet – tRNA.

24. • Modification of released polypeptide :-
• The just released polypeptide has primary structure. It is a
straight, linear molecule. It is often called an nascent
polypeptide.
• It may lose some amino acids from the end with the help of
an exopeptidase enzyme, and then coil and fold on itself to
secondary and tertiary structure.
• It may combine with other polypeptides to have quaternary
structure.

25. • Polysome formation :-
• When the ribosome has moved sufficiently down the mRNA
chain towards 3’ end, another ribosome takes up position at
the initiator codon of mRNA, and starts synthesis of a second
copy of the same polypeptide chain.
• A row of ribosomes joined to the mRNA molecule, is called a
polyribosome, or simply a polysome.
• Synthesis of many molecules of the same polypeptide
simultaneously from the one mRNA molecule by a polysome is
called translation amplification.

27. • References :-
• Cell biology, genetics, molecular biology, evolution and ecology
By :- P.S.Verma and V.K.Agarwal
• W.W.W.google.com

28. Thank you…