DR.
KENNETH DESTURA
R N A T R A N S L AT I O N
• the process in which a sequence of nucleotide triplets in a
messenger RNA give rise to a specific sequence of amino acids
during the synthesis of a polypeptide chain or protein.
R N A T R A N S L AT I O N
R N A T R A N S L AT I O N
• Ribosomes translate the genetic message of mRNA into
proteins.
• The mRNA is translated 5'→3', producing a corresponding
N-terminal → C-terminal polypeptide.
• Amino acids bound to tRNAs are inserted in the proper
sequence due to:
• Specific binding of each amino acid to its tRNA.
• Specific base pairing between the mRNA codon and
tRNA anticodon.
R N A T R A N S L AT I O N
• TRANSLATIONAL MACHINERY
• mRNAs Translate the code
written in four base
• tRNAs
alphabet (A, U, G, C)
• Aminoacyl tRNA synthetase into a second code
• Ribosome written in language
of 20 amino acids
R N A T R A N S L AT I O N
• Messenger RNA (mRNA)
• Provides an intermediate that carries the copy of DNA sequence
that represents protein
Protein coding region of
mRNA is composed of a
contiguous, non-
overlapping string of
codons called Open
Reading Frame (ORF)
R N A T R A N S L AT I O N
• Messenger RNA (mRNA)
• 3 modifications for the recruitment of ribosome to mRNA
Kozak sequence - presence of a purine 3 bp upstream of AUG and
presence of a guanine immediately downstream (5'-G/ANNAUGG-3)
Methylated Poly A tail
guanine cap
CODONS
• ordered series of three
nucleotides specific for
amino acids
R N A T R A N S L AT I O N
• Transfer RNA (tRNA)
• Acts as adaptor between codons and the amino acids they specify
• There are many types of tRNA but each is attached to a specific
amino acid and each recognize a particular codon
• 75-95 ribonucleotides in length
• Terminus is 5’-CCA-3’ which is the binding site of amino acid
• Contains unusual or modified bases- uridine, thymine, pseudouridine,
methylguanine, hypoxanthine etc.
• They are not essential for tRNA but those lacking these show reduced
rate of growth
ACCEPTOR ARM
the site of attachment of the
specific amino acid
TΨC ARM
involved in binding of the
aminoacyl-tRNA to the
ribosomal surface at the site
of protein synthesis
D ARM
recognition of a given tRNA
species by its proper
aminoacyl-tRNA synthase
ANTI-CODON LOOP
recognizes the three-letter
codon in mRNA
R N A T R A N S L AT I O N
• Transfer RNA (tRNA)
• Anti-codon loop – base pair
complementary to mRNA codons
• Acceptor stem – 5’-CC—3’ which is
the binding site of amino acids
R N A T R A N S L AT I O N
• Amino Acyl tRNA Synthetase
• Provide specificity in joining amino
acids to their RNAs
• Linkage is an endergonic process
that occurs at the expense of ATP,
which loses two phosphate groups,
becoming AMP.
ATTAC H M E N T OF
AMINO ACIDS
TO RNA
1. Adenylation - Amino
acid with the ATP to
become adenylated.
carbonyl group of amino
acid is ionized to
phosphate group of
AMP by releasing PPi
from ATP.
2. Charging - Carbonyl
group of adenylated aa
react with 3'OH of tRNA.
A high energy bond and
the release of AMP
R N A T R A N S L AT I O N
Aminocyl-tRNA synthetase attaches amino acids to their specific RNA
molecules. The charging process (aminoacylation) produces a charged tRNA
(aminoacyl-tRNA), using energy from ATP hydrolysis.
R N A T R A N S L AT I O N
There are 20 different aminoacyl-tRNA synthetase enzymes, one for each
amino acid. Some of these enzymes recognize tRNAs by their anticodon
regions.
R N A T R A N S L AT I O N
The amino acid and ATP bind to the specific aminoacyl- tRNA synthetase
enzyme. ATP loses two phosphates and the resulting AMP is bound to the
amino acid, forming aminoacyl-AMP.
R N A T R A N S L AT I O N
The tRNA binds to the enzyme, and the amino acid is transferred onto it,
displacing the AMP. The aminoacyl-tRNA is released from the enzyme.
R N A T R A N S L AT I O N
The amino acid is now covalently attached by its carboxyl group to the 3'r
end of the tRNA. Every tRNA has a 3’r adenine, and the amino acid is attached
to the 3'r-OH or 2'r-OH of this nucleotide.
R N A T R A N S L AT I O N
• Ribosome
• It directs the synthesis of proteins
• Large subunit contains the peptidyl
transferase center which is responsible
for the formation of peptide bonds
• Small subunit contains the decoding
center in which charged tRNAs read or
decode the codons of mRNA
• Both subunits undergo association and
dissociation during each cycle of
translation
R N A T R A N S L AT I O N
• Ribosome
R N A T R A N S L AT I O N
• An mRNA bearing multiple ribosome is known as
polyribosome or polysome.
R N A T R A N S L AT I O N
• Ribosome
Peptidyl tRNA
A-site
-binding site for the first
aminocylated tRNA
R N A T R A N S L AT I O N
• Ribosome
Peptidyl tRNA
P-site
-binding site for the first
peptidyl tRNA
R N A T R A N S L AT I O N
• Ribosome
Peptidyl tRNA
E-site
-binding site for the first
uncharged tRNA
R N A T R A N S L AT I O N
• Ribosome
• Small subunit of ribosome has
two narrow tunnels
• Entry channel for mRNA
• Exit channel for mRNA
• Large subunit has an exit
channel for newly synthesized
polypeptide chain
CODONS
• ordered series of three
nucleotides specific for
amino acids
S TA R T
CODON 5’-AUG-3'
• First codon of an ORF present at the 5’ end
STOP
CODON 5'-UAG-3'
5'-UGA-3'
5'-UAA-3’
• last codon of an ORF at the
3’end which define the signal
termination of protein synthesis
THE GENETIC
CODE
THE GENETIC CODE
THE GENETIC CODE
Features of the
Genetic Code
Degenerate
Unambiguous
Non-overlapping
Not punctuated
Universal
THE GENETIC CODE
• Degenerate
• Multiple codons must decode the same amino acid
• In general, the third nucleotide in a codon is less important than the first two in
determining the specific amino acid to be incorporated
THE GENETIC CODE
• Unambiguous
• Given a specific codon, only a single amino acid is indicated
• AUG Met
• CUG Leu
• AUC Ile
THE GENETIC CODE
• For a given codon in the mRNA,
only a single species of tRNA
molecule possesses the proper
anticodon.
• Since each tRNA molecule can
be charged with only one
specific amino acid, each codon
therefore specifies only one
amino acid.
THE GENETIC CODE
• Non-overlapping
• The reading of the genetic code during the process of protein synthesis does
not involve any overlap of codons
THE GENETIC CODE
• No Punctuation
• Once the reading is commenced at a specific codon, the message is read in a
continuing sequence of nucleotide triplets until a stop codon is reached.
THE GENETIC CODE
• Universal
• The code is the same in all organisms from viruses and bacteria
to humans with few exceptions
THE GENETIC CODE
• Wobble Base Pairs
• Two nucleotides that does not following Watson-Crick base pair rules
• One tRNA molecule can recognize and bind to more than one codon due to the
less precise base pairs between the 3rd base of the codon and the base at the
1st position on the anticodon
THE GENETIC CODE
THE GENETIC CODE
R N A T R A N S L AT I O N
Initiation Elongation Termination
I N I T I AT I O N
• Involves 4 general steps:
a. Binding of tRNA precedes binding of
mRNA
b. mRNA is recruited separately
c. Small subunit bound to itRNA scans
mRNA for AUG
d. Large subunit is recruited after itRNA
base pairs with the start codon
• The initiator tRNA is charged with with
methionine
I N I T I AT I O N
• Binding of iTRNA to the P site
• eIF, eIFA, eIF2, eIF5 bind to small subunit
• iRNA is escorted by GTP binding protein –eIF2 to form the ternary complex
• eIFe positions itRNA to P site
I N I T I AT I O N
• Binding of mRNA to initiation complex
• 4 initiation Factors involved are elF4E, elF4G, elF4B, elF4A
• Recognition by 5' cap by a 3 subunit complex elF4E
• elF4G binds to elF4E and mRNA, to which binds elF4A
= elFUE & mRNA elF4A
elF46
=
I N I T I AT I O N
• Binding of mRNA to initiation complex
• It is then joined by elF4B which activates an RNA helicase activity of elF4A that
unwinds any secondary structure
• This elF4F-elF4B complex is then recruits the 43S preinitiation complex to the
mRNA by interactions between elF4F and elF3.
• This 43S complex with mRNA is called 48S preinitiation complex.
I N I T I AT I O N
• Scanning for AUG
• After assembly at the 5'end of mRNA, the small subunit scans the mRNA for the
start codon in 5' - 3' direction in a ATP dependent process
• Correct base pairing between the initiator tRNA and start codon releases elF3
and elF which allows the large subunit to bind to the small subunit.
• Binding of large subunit leads to loss of elF5B by GTP hydrolysis and binding
of initiator tRNA to P site and formation of 80S complex
I N I T I AT I O N
• Association of small and large subunit
• Correct base pairing changes the conformation of 48S complex leading to the
release of elF and a change in conformation of elF5
• Both these events hydrolyze elF2GTP into elF GDP
• Loss of elF2GDP stimulates the loss of elF5B which stimulate the correct base
pairing of large and small subunit of ribosome
I N I T I AT I O N
• The small subunit attaches near
the start codon (AUG)
• The initiator tRNA binds to the
start codon (AUG)
• The large subunit joins the
complex
• The start codon sets the reading
frame; from there the codons are
read 3 at a time
E L O N G AT I O N
• Three key events for correct addition of each amino acids:
• Correct aminoacyl-tRNA is loaded to the A site of the
ribosome as dictated by the A site codon
• Peptidyl transferase reaction
• Translocation of peptidyl tRNA from A site of ribosome
to the P site of ribosome
E L O N G AT I O N
E L O N G AT I O N
E L O N G AT I O N
• Once the correctly charged tRNA has been placed in the A site and has rotated in
the peptidyl transferase center, peptide bond formation takes place
E L O N G AT I O N
T E R M I N AT I O N
• Stop codons are recognized by proteins called release
factors(RF)
• These activate the hydrolysis of of polypeptie from the
peptidyl tRNA.
• There are 2 classes of RF:
• Class 1 RFs - recognize stop codon and trigger the
hydrolysis of peptide chain from the peptidyl tRNA
• Class 2 RFs - stimulate the dissociation of class 1 RFs
from the ribosome after the release of polypeptide
T E R M I N AT I O N
1. After RFs bind to the A site and recognize stop codon, there
is a conformational change in RF which releases the
polypeptide.
2. RF3 GDP binds on class 1 RFs after release of polypeptide.
3. Change in conformation of ribosome and RFs stimulates RF3
to exchange its bound GDP to GTP.
4. This RF GTP forms a high affinity interaction with ribosome
that displaces class 1 RFs and concurrent hydrolysis of GTP
into GDP.
5. Now this RF- GDP has a low affinity for ribosome and is
released.
T E R M I N AT I O N
