Contents

Animations of Translation
Introduction
Translations in Prokaryotes
Translation in Eukaryotes
MCQs
References
 Introduction to Translation
Translation: The biosynthesis of a protein or a
polypeptide inside a living cell.

In process of translation the language of nucleotides

sequence on mRNA is translated in to the language of
amino acid sequence.

It occur in cytoplasm where ribosomes are located

It is a universal process
Cont……
In translation ,messenger RNA is decoded to produce
a specific polypeptide

This uses mRNA sequence as a template to guide the

synthesis of a chain of amino acid that form protein

Many types of transcribed RNA, such

tRNA,rRNA,snRNA are not necessarily translated to
amino acid sequence
Steps of Translation
Translation proceeds in four phases
- Activation
- Initiation
- Elongation and
- Termination

Most studies do not include activation as a step of

translation
 Activation
In activation, the correct amino acid is covalently
bonded to the correct tRNA

While technically this not a step in translation, it is

required for translation to proceed

The amino acid is joined by its carboxyl group to the 3`

OH of tRNA by an ester bond with help of ATP

When tRNA has an amino acid linked to it, it is

termed as “charged”
Cont…..
 Initiation
Prokaryotes initiation require the large and small
subunits, the mRNA, the initiator tRNA and three
initiation factors (IF-1, IF-2, IF-3) and GTP.

IF-3 binds to the free 30s subunit, this help to prevent

large subunit binding to it without mRNA and
forming an inactive ribosome

IF-2 complexed with GTP and IF-1 binds to small

subunit . It will assist the charged initiator tRNA to
bind
Cont…..
The 30s subunit attached to a mRNA molecule
making use of the ribosomal binding site on mRNA

The initiator tRNA can then bind to the complex by

base pairing of its anticodon with AUG codon on
mRNA

At this point,IF3 can be released, as its role in keeping

the subunits apart are complete

This complex is called 30s initiation complex

Cont….
The 50s subunit can now bind, which displace IF1 and
IF2, and the GTP is hydrolysed in this energy
consuming step

This complex is called 70s initiation complex

Cont…..
The assembled ribosome has two tRNA binding sites
These are called the A and P sites , for amino acyl
and peptidyl sites and one site is E (exit site) for
empty tRNA

The A site is where incoming amino acyl tRNA

molecule bind, and the p site is where the growing
polypeptide chain is usually found

The sites are in cleft of small subunit and contain

adjacent codon that are being translated
Cont……
One major of initiation is the placement of initiator
tRNA in the P site

It is the only tRNA that does this , as all other must

enter the A site
 Elongation
With the formation of 70s initiation complex the
elongation cycle can begin

It involves three elongation factors EF-Tu, EF-Ts and

EF-G, GTP, charged tRNA and the 70s initiation
complex
longation is divided into 3 steps
1. Amino acyl tRNA delivery.

• EF-TU is required to deliver the amino acyl tRNA to A site

and energy is consumed in this step by hydrolysis of GTP
• The released EF-Tu GDP complex is regenerated with the
help of EF-TS

• In the EF-Tu EF-Ts exchange cycle EF-Ts displaces the GDP

and replace itself by GTP
• The resultant EF-Tu.GTP complex is now available to bind
another amino acyl tRNA and deliver it to ribosome
Cont….
All amino acyl tRNA can form this complex with EF-Tu
except the initiator tRNA
Cont…..
2 .Peptide bond formation.
• After aminoacyl-tRNA delivery ,the A and P sites are
both occupied and the two amino acids that are to be
joined are close to each other

• The peptidyl transferase activity of the 50s subunit can

now form a peptide bond between the two amino acids
3. Translocation
A complex of EF-G(translocase) and GTP binds to the
ribosome and ,is an energy consuming step, the
discharged tRNA is ejected from the P site, the peptidyl-
tRNA is moved from A site to P site

The mRNA moves by one codon relative to one codon to

the ribosome

GDP and EF-G are released . A new codon is now present

in the vacant site
 Termination
Termination of translation happens when the A site of
the ribosome faces a stop codon (UUA,UGA or UGA)

When this happens , no tRNA can recognize it, but a

releasing factor can recognize the stop codons and
causes the release of polypeptide chain

In prokaryotes once a stop codon occupies the A site,

three termination or release factor (RF1,RF2,RF3)
contribute to the hydrolysis of peptidyl-tRNA bond
Cont…..
Release the free polypeptide and last uncharged tRNA
from P site
The dissociation of the 70 s into 30s and 50s subunits
RF1binds A site and release the polypeptide and
uncharged tRNA
RF2 releases the RF1 from A site and release itself as
well from translation binding site(present on large
subunit)
RF3 function unknown
Another factor called Ribosomal releasing factor
causes the dissociation of 70s complex
Cont….
Translation in Eukaryotes
Eukaryotic Translation
In prokaryotic cell, transcription and translation are
coupled, that is, translation begins while the mRNA is still
being synthesized. In a eukaryotic cell, transcription occur
in the nucleus , and translation occur in the cytoplasm.

Translation process in eukaryotes involve

- Activation ( not essentially the step of translation. This
occur the same way as in prokaryotes)
- Initiation
- Elongation and
- termination
1. Initiation
The initiation of translation in eukaryotes is complex,
involving at least 10 initiation factors (eIFs) and is
divided into three steps :

a) Formation of 43s preinitiation complex.

b) Formation of 48s initiation complex.
c) Formation of 80s initiation complex.
a. Formation of 43s preinitiation complex

A ternary complex containing met-tRNA and eIF-2

bound to GTP attaches to 40s ribosomal subunit to
form 43s preinitiation complex.

The presence of eIF-3 and eIF-1A stabilizes this

complex.
b.Formation of 48s initiation complex

The binding of mRNA to 43s preinitiation complex

results in formation of 48s initiation complex.

eIF-4f is formed by the association of eIF-4G, eIF-4A

with eIF-4E

The eIF-4F(referred to as cap binding protein ) binds

to the cap of mRNA.
Cont…..
Then eIF-4A and eIF-B binds to mRNA and reduces
its complex structure.

This mRNA is then transferred to 43s complex

The ribosomal initiation complex scans the mRNA for

identification of appropriate initiation codon

5’-AUG is the initiation codon

c.Formation of 80s initiation complex

48s initiation complex binds to 60s ribosomal subunit to

form 80s initiation complex

The binding involves hydrolysis of GTP(bound to eIF-2)

This step is facilitated by the involvement of eIF-5

As the 80s complex is formed, the initiation factors

bound to 43s initiation complex are released and recycled
Cont…..
2. Elongation
Ribosomes elongate the polypeptide chain by
sequential addition of amino acids
The amino acid sequence is determined by the order of
the codons in the specific mRNA
Elongation, a cyclic process involving certain
elongation factors(EFs)
Elongation may be divided into three steps
a. Binding of aminoacyl-tRNA to A-site
b. Peptide bond formation
c. translocation
a.Binding of Aminoacyl t-RNA to A- site

The 80s initiation complex contains met tRNA in the

P-site and A- site is free
Another aminoacyl-tRNA is placed in the A site

This require proper codon recognition on mRNA and

involvement of EF-1a and supply of energy by GTP
The aminoacyl –tRNA is placed in the A-site ,EF-1a
and GDP are recycled to bring another aminoacyl-
tRNA
b. Peptide bond formation
The peptidyl transferase catalyzes the formation of
peptide bond

Net result of peptide bond formation is the

attachment off the growing peptide chain to the tRNA
in A-site
c. Translocation
The ribosome moves to the next codon of mRNA
(towards 3’ end)
This process is called translocation, involves the
movement of growing peptide chain from A-site to P-
site
Translocation require EF-2 and GTP
GTP get hydrolyzed and supplies energy to move
mRNA
EF-2 and GTP complex recycles for translocation
Cont….
3. Termination
One of the stop signals (UGA,UAA and UGA)
terminates the growing polypeptide

When the ribosome encounter s a stop codon, there is

no tRNA available to bind to the A site of ribosome

Instead a release factor binds to it

In eukaryotes eRF1 recognizes all the three stop

codons, and eRF3 stimulate the termination events
Cont…
Once the release factor binds, the ribosome unit falls
apart
- releasing the large and small subunits
- the tRNA carrying the polypeptide is also released,
freeing up the polypeptide product.

-
Cont….