The information for the proteins found in a cell is encoded in genes of the genome of the cell. A protein- coding gene is expressed by the process of transcription to produce an mRNA, followed by translation of the mRNA. Translation involves the conversion of the base sequence of the mRNA into the amino acid sequence of a polypeptide.

1. TRANSLATION
Prepared By:
Dr. Asit Prasad Dash
Assistant Professor
DEPARTMENT OF PLANT BREEDING AND GENETICS
INSTITUTE OF AGRICULTURAL SCIENCES
SIKSHA ‘O’ ANUSANDHAN (DEEMED TO BE UNIVERSITY), BHUBANESWAR,
751029

2. INTRODUCTION
v A protein consists of of smaller building blocks: the amino acids.
v Amino acids are joined with each other by peptide bond.
v The base sequence of mRNA is converted into the amino acid
sequence of the protein; this process is known as translation.

4. THE GENETIC CODE
v The number and the sequence of bases in mRNA specifying an amino acid is
known as codon.
v The set of bases in a tRNA that base-pairs with a codon of an mRNA is known as
anticodon.
v The sequence of bases in an anticodon is exactly the opposite of and
complementary to that present in the codon.
v The set of all the codons that specify the 20 amino acids is termed as the genetic
code, genetic language or coding dictionary.

6. Characteristics of the Genetic Code
v The code is a triplet code.
v The code is comma free; that is, it is continuous.
v The code is nonoverlapping.
v The code is almost universal.
v The code is “degenerate.” i.e. more than one codon occurs for each
amino acid; (the exceptions are AUG, which alone codes for
methionine, and UGG, which alone codes for tryptophan.) This
multiple coding is called the degeneracy or redundancy of the code.
v Ambiguity in the Genetic Code: (Ambiguity denotes that a single
codon may code for more than one amino acid.) There is no
evidence that the genetic code is ambiguous in vivo.
(The only exception appears to be the AUG codon in prokaryotes, and
in chloroplasts and mitochondria; it codes for formylmethionine at the
initiation site, while at other positions it specifies methionine.)

7. v The code has start and stop signals.
v In both eukaryotes and prokaryotes, AUG (which codes for
methionine) is almost always the start codon for protein
synthesis.
v Only 61 of the 64 codons code for 20 amino acids; these codons
are called sense codons.
v The other three codons—UAG, UAA, and UGA do not specify an
amino acid, hence these three codons are called the stop codons/
nonsense codons/ chain-terminating codons.
v The base at the 5’ end of the anticodon can pair with more than one
type of base at the 3’ end of the codon in other words, the 5’-base of
the anticodon can wobble.
v According to the wobble hypothesis proposed by Crick, the
complete set of 61 sense codons can be read by fewer than 61
distinct tRNAs, because of pairing properties of the bases in the
anticodon.

10. The process of translation requires the
following major components:
(1)mRNA,
(2)tRNA,
(3)ribosome (containing rRNA) and
(4)many translation factors.

11. MESSENGER RNA
v All mRNA molecules have a translation initiation site (AUG) close
to their 5'- end and a chain termination site (UAA/ UAG/ UGA)
towards the 3'-end.
v In prokaryotes, ribosomes attach to the 5'-end of mRNA. The
group of ribosomes together with the single mRNA molecule they are
translating is called polysome.
v The 5' leader of bacterial mRNAs has a consensus sequence, called
Shine-Dalgarno sequence, located -7 bases upstream of the AUG
(initiation) codon; the sequence of this consensus is 5' AGGAGG 3'.

12. Diagram of a polysome—a number of ribosomes, each translating the same mRNA
sequentially.

13. RIBOSOMES AND RIBOSOMAL RNA
v Prokaryotic (70S) ribosome consist of a 50S and a 30S subunit.
v The 30 S subunit of prokaryotic ribosomes has a single 16S rRNA
molecule which is associated with 21 different proteins.
v The bacterial 16 S rRNA has near it 3'-end, a hexamer sequence 3'
UCCUCC 5’, which is complementary to and base-pairs with the
Shine-Dalgarno sequence of mRNA.
v This base pairing allows the smaller subunit of ribosomes to bind
mRNA during translation.
Functional Sites on Ribosomes
(1) an aminoacyl attachment site (site A): for the attachment
of aminoacyl-tRNAs (amino acid carrying tRNA),
(2) a peptidyl site (site P) and
(3) an exit site (site E).

15. TRANSFER RNA
It is a class of RNA of small size,
generally having 76 - 95 bases,
which brings amino acids to
ribosomes. Extensive pairing
among the nucleotides produces a
clover leaf secondary structure,
which has:
(1)Amino acid acceptor
region CCA at 3'-end
(2)A thymine loop
(3)Anticodon loop
(4)DHU loop (toward 5'-
end).
(5)An extra loop between
thymine and anticodon
loops.

16. Charging of tRNA:
v It refers to the attachment of amino acids to specific tRNAs.
v This is comprised of two reactions catalysed by the enzyme
aminoacyl tRNA synthetase.
First Step. It consists of amino acid activation, in which the amino
acid molecule reacts with an ATP molecule to yield an aminoacyl -
AMP (aminoacyl adenylate) molecule.
Amino acid + ATP Aminoacyl-AMP + 2 Pi.
Second Step. The amino acid from aminoacyl-AMP molecule is then
transferred to a tRNA molecule that is specific for the amino acid,
and AMP is released.
Aminoacyl - AMP + tRNA Aminoacyl - tRNA + AMP.

17. Aminoacylation (charging) of a tRNA molecule by aminoacyl–tRNA synthetase to
produce an aminoacyl–tRNA (charged tRNA).

18. THE TRANSLATION PROCESS
vTranslation begins near the 5'-end of mRNA and
progresses toward its 3'-end.
vThe process of translation may be divided into the
following 3 steps:
(1) Initiation
(2) Elongation
(3) Termination

19. Initiation
Initiation comprises all the events that precede the formation of the
first peptide bond.
It includes the following events:
(1) binding of the smaller subunit (30S) of ribosome to mRNA and
(2) binding of the first or initiator aminoacyl-tRNA (fmet-tRNAf) to
the P site of ribosome. Only fmet-tRNAf can enter the P site; all
other tRNAs can not gain a direct access to the P site.
(3) The 50S subunit of ribosome now joins the 30S subunit. The
initiation complex is now complete with an active ribosome in
which A-site is vacant, while the P site is occupied by fmet -
tRNAf.

20. Initiation of protein synthesis in
bacteria. A 30S ribosomal subunit,
mRNA, initiator f Met–tRNA, and
initiation factors form a 30S initiation
complex. Next, the 50S ribosomal
subunit binds, forming a 70S initiation
complex. During this event, the
initiation factors are released and GTP
is hydrolyzed.

21. Elongation
It includes
(i) binding of the Aminoacyl–tRNA (charged tRNA) to the
ribosome in the A site
(ii) formation of peptide bond between the amino acids and
(iii) translocation of the ribosome along the mRNA
v Because the ribosome has moved, the uncharged tRNA moves from
the P site and then binds transiently to the E site and finally
released from the ribosome.
v The peptidyl–tRNA is now located in the P site (hence the name
peptidyl site).
v After the completion of translocation, the A site is vacant. An
aminoacyl–tRNA with the correct anticodon binds to the newly
exposed codon in the A site, reiterating the process already
described.
v The whole process is repeated until translation terminates at a stop
codon.

22. The formation of a peptide bond between the first two amino acids (fMet and Ser)
of a polypeptide chain is catalyzed on the ribosome by peptidyl transferase.

23. Elongation stage of translation
(in bacteria)

25. Termination
When the A site of a ribosome reaches a nonsense codon (UAA,
UAG, UGA), the specific release factor enters A site.
As a result of this, the following three simultaneous events occur:
(1) the polypeptide chain detaches from the tRNA located at P
site,
(2) immediate release of tRNA from the P site, and
(3) the release of ribosome from the mRNA. The two subunits of
ribosomes may dissociate after their release.

27. Thank You