Chapter 6
Expression of
Biological
Information
(Part IV)
By :
Biology Unit
Kedah Matriculation College
Concept review…
Expression of Biological Information
DNA &
genetic
information
DNA structure
DNA as a carrier
DNA
replication
Protein
synthesis
Gene
regulation &
expression
concept
Components
Gene concept
Mechanism
Transcription
Definition
Models of DNA replication
DNA replication process
Enzyme involved
Translation
Learning outcome
 Explain translation in protein synthesis
 Describe the stages involved:
i. initiation
ii. elongation (codon recognition, peptide bond
formation and translocation)
iii. termination
Translation
Translation
Definition

Translation
- a process
- whereby the genetic information in
mRNA strand
- is translated into sequence of amino
acids to form polypeptide/protein

Occurs in cytoplasm
Translation

RNA direct the synthesis of polypeptide.


cell interprets a genetic message and
builds a protein accordingly.
The message is a series of codons along
an mRNA molecule and interpreter is
transfer RNA
Structures that involve in
translation

Ribosome
tRNA

triplet bases on tRNA: anticodon
mRNA
 Structures
that involve in
translation:
1. tRNA
 tRNA transfer amino acid
from cytoplasm to
ribosome

tRNA can be used
repeatedly.

Each type of tRNA
molecule associates a
particular mRNA codon
with a particular amino
acid.
 Shape:
2dimensional –
cloverleaf shape.
 tRNA
includes the
anticodon, the
specialized base
triplet that binds to
a specific mRNA
codon.
 e.g.: mRNA codon
UUU has AAA as its
anticodon.

From the other
end of the Lshaped tRNA
molecule
protrudes its
3’end; the
attachment site for
amino acid.


tRNA with amino
acid; aminoacyl
tRNA.
2. Aminoacyl-tRNA synthetase


Catalyze the matching up of amino acid with
the correct tRNA and using energy of ATP
(activation of amino acids)
Catalyzes the covalent attachment of amino
acid to the 3’- OH of tRNA.

Resulting aminoacyl tRNA
Activation of aminoacyl tRNA
3. Ribosomes
Facilitate the specific coupling of tRNA
anticodons with mRNA codons during protein
synthesis.

A large and small subunit join to form a
functional ribosome only when they attach to
mRNA molecule.


Each ribosomal subunit is consist proteins and
ribosomal RNA (rRNA).
Ribosome has TWO binding site for mRNA:
P site (peptidyl-tRNA site)

Holds the tRNA carrying the growing
polypeptide chain.
A site (aminoacyl-tRNA site)

Holds the tRNA carrying the next amino
acid to be added to the chain.
Translation process
Overall Process…
3 steps of translation process,
synthesis of a polypeptide chain :
1.
2.
3.
Initiation
Elongation
Termination
1. Initiation


Bring together mRNA and tRNA bearing the first
amino acid of polypeptide and two subunits of
ribosome.
Small ribosomal subunit binds to mRNA that being
read from 5’ to 3’.
Initiation

Then initiator tRNA base-pairs with the
start codon

Arrival of a large subunit completes the
initiation complex
Initiation

initiation factor: required to bring all the

translation components together
Initiator tRNA site in the P site of ribosome and
vacant A site is ready for next tRNA.

initiation complex of translation
Ribosome
(large)
tRNA
mRNA
Ribosome
(small)
2. Elongation

Amino acid are added one by one to the initial
amino acid. Each addition, occurs in 3 steps
cycle.
i.
ii.
iii.
Codon recognition
Formation od peptide bond
Translocation
i. Codon recognition

mRNA codon in A
site forms
hydrogen bonds
with the anticodon
of an incoming
molecule of tRNA
carrying its
appropriate amino
acid.

Elongation factors
lead the tRNA into
A site.
hydrogen
bonds
ii. Peptide bond formation

Polypeptide detaches from tRNA (in P site) to
which it was bond and is link to amino acid
carried by the tRNA (in A site)

large ribosomal subunit catalyzes the formation
of peptide bond between polypeptide extending
from P site and newly arrived amino acid in A
site.
2. Peptide bond Formation
3. Translocation
iii. Translocation

tRNA in P site dissociates from ribosome.

tRNA in A site now attached to the
growing polypeptide, is translocated to
the P site
iii. Translocation

as the tRNA changes sites, its anticodon
remains hydrogen bonded to the mRNA codon,
allowing the mRNA and tRNA molecules to move
as a unit.

This movement, in turn, brings the next codon to
be translated into A site.

Elongation is repeated as each amino acid is
added to the chain until the polypeptide is
completed.
Peptide bond formation

3. Termination
Elongation continues until A site reaches
termination codon.

do not code for amino acids and stop
translation.
- No aminoacyl tRNA recognize
Termination
Protein called ‘release factor’ binds to the
termination codon in A site.
- freeing the polypeptide from the ribosome.
- mRNA detaches
- ribosome separates
Initiation
Large Unit
Met
Initiation codon
AUG
P
A
UAC
5’
UUG
UGG
UAC
AUG
GAA
GAG
Small Unit
Initiation Complex
Small Unit
CAU
… UGA
3’
ACU
CUU
Met
UUU
UAC
5’
UUG
UGG
UAC
AUG
GAA
GAG
Elongation
CUC
CAA
….UGA 3’
ACU
GUG
5’
UUG
UGG
UAC
Termination
AUG
GAA
GAG
STOP codon
UAA
UAG
UGA
CAU
UGA
3’
Messenger RNA (mRNA)
start
codon
mRNA
A U G G G C U C C A U C G G C G C A U A A
codon 1
protein methionine
codon 2
codon 3
glycine
serine
codon 4
isoleucine
codon 5
codon 6
glycine
alanine
codon 7
stop
codon
Primary structure of a protein
aa1
aa2
aa3
aa4
aa5
aa6
peptide bonds
copyright cmassengale
41
mRNA U C C A U C A U G G G C U C C A U C G G A
Codon 1
methionine
Polyribosomes
Polyribosomes/polysome
A group of several
ribosomes attached to,
and translating the same
messenger RNA
simultaneously
 Increase the rate of same
protein synthesis/protein
produced rapidly
 more of same protein can be
made simultaneously
Animation
RNA polymerase
DNA
Can you tell
the story?
amino
acids
exon
intron
tRNA
pre-mRNA
5' GTP cap
mature mRNA
large ribosomal
subunit
5'
small ribosomal
subunit
poly-A tail
aminoacyl tRNA
synthetase
3'
polypeptide
tRNA
E P A
ribosome