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LECTURE 5
(Chapter 13)
Translation of
mRNA
1
INTRODUCTION

The translation of the mRNA codons into amino
acid sequences leads to the synthesis of proteins

A variety of cellular components play important
roles in translation


These include proteins, RNAs and small molecules
In this chapter we will discuss the current state of
knowledge regarding the molecular features of
mRNA translation
2
13.1 THE GENETIC BASIS FOR
PROTEIN SYNTHESIS

Proteins are the active participants in cell
structure and function

Genes that encode polypeptides are termed
structural genes


These are transcribed into messenger RNA (mRNA)
The main function of the genetic material is to
encode the production of cellular proteins

In the correct cell, at the proper time, and in suitable
amounts
3
4
Archibald Garrod


First to propose (at the beginning of the 20th
century) a relationship between genes and
protein production
Garrod studied patients who had defects in their
ability to metabolize certain compounds


Urine chemist
He was particularly interested in alkaptonuria


Patients bodies accumulate abnormal levels of
homogentisic acid (alkapton)
Disease characterized by

Black urine and bluish black discoloration of cartilage and skin
5
6
Archibald Garrod

He proposed that alkaptonuria was due to a
missing enzyme, namely homogentisic acid
oxidase

Garrod also knew that alkaptonuria follows an
autosomal recessive pattern of inheritance

He proposed that a relationship exists between
the inheritance of the trait and the inheritance
of a defective enzyme
7
Inheritance of alkaptonuria
8
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Dietary
protein
H
CH2
C
COOH
NH2
Phenylalanine
Phenylketonuria
Phenylalanine
hydroxylase
H
HO
CH2
C
COOH
NH2
Tyrosine
Tyrosine
aminotransferase
p-hydroxyphenylpyruvic
acid
Tyrosinosis
Hydroxyphenylpyruvate
oxidase
Homogentisic
acid
Alkaptonuria
Homogentisic
acid oxidase
Maleylacetoacetic
acid
Figure 13.1
Metabolic pathway of phenylalanine metabolism and related
genetic diseases
9
Beadle and Tatum’s Experiments

In the early 1940s, George Beadle and Edward
Tatum were also interested in the relationship
between genes, enzymes and traits


Experiments supported Garrod’s idea that each gene =
one enzyme
Their genetic model was Neurospora crassa (a
common bread mold)

Their studies involved the analysis of simple nutritional
requirements
10
Beadle and Tatum’s Experiments

They analyzed more than 2,000 strains that had
been irradiated to produce mutations




At this point, DNA identified as probable carrier of genetic
information
Does DNA somehow “code” for enzymes?
They analyzed enzyme pathways for synthesis of
vitamins and amino acids
Figure 13.2 shows an example of their findings on
the synthesis of the amino acid methionine
11
12
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Neurospora
growth
WT
1
WT
1
24
4
3
Minimal
WT
1
24
3
+O–acetylhomoserine
WT
24
3
+Cystathionine
WT
1
1
2 4
3
+Homocysteine
2
3
+Methionine
(a) Growth of strains on minimal and supplemented growth media
Homoserine
O–acetylhomoserine
Enzyme 1
Cystathionine
Enzyme 2
Methionine
Homocysteine
Enzyme 3
Enzyme 4
(b) Simplified pathway for methionine biosynthesis
Every mutant strain was blocked at one (and only one)
particular step in the synthesis pathway, showing that
each gene encoded one enzyme
Figure 13.2
13

Beadle and Tatum’s conclusion: A single “gene” in
DNA controls the synthesis of a single enzyme

This was referred to as the one gene–one enzyme
hypothesis
14

In later decades, this theory was progressively
modified by new research


1. Enzymes are only one category of proteins
2. Some proteins are composed of two or more different
polypeptides





The term polypeptide denotes structure
The term protein denotes function
So it is more accurate to say a structural gene encodes a
polypeptide
In eukaryotes, alternative splicing means that a structural gene
can encode many different polypeptides
3. Many genes have been identified that do not encode
polypeptides

For instance, functional RNA molecules (tRNA, rRNA, etc.)
15

Degenerate: (Adj) Having declined or become
less specialized

Adaptor (Noun) A device that converts attributes
of one device or system to those of an otherwise
incompatible device or system.

Charge (Verb) To give a task to something or
someone (Last slide Quiz 6, Sec 7)
16
The Genetic Code (first slide quiz
8, Sec 7)

Translation involves an interpretation of one
language into another


Translation relies on the genetic code


In genetics, the nucleotide language of mRNA is
translated into the amino acid language of proteins
Refer to Table 13.1
The genetic information is coded within mRNA in
groups of three nucleotides known as codons
17
Three codons do not encode an
amino acid. These are read
as STOP signals for
translation
Triplet codons correspond to a
specific amino acid
Multiple codons may encode
the same amino acid.
These are known as
synonymous codons
18

Special codons:

AUG (which specifies methionine) = start codon




UAA, UAG and UGA = termination, or stop, codons
The code is degenerate

More than one codon can specify the same amino acid


For example: GGU, GGC, GGA and GGG all code for glycine
In most instances, the third base is the variable base


This defines the reading frame for all following codons
AUG specifies additional methionines within the coding sequence
It is sometime referred to as the wobble base
The code is nearly universal

Only a few rare exceptions have been noted

Refer to Table 13.3
19

Figure 13.3 provides an overview of gene expression
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Coding strand
DNA
Transcription
5′
3′
A C T G C C C A T G A G C G A C C A C T T G G G G C T C G G G G A A T A AC C G T C G A G G
T G A CG GG T A CT C G CT G G TG A A CC CC G A G CC CC T TA T TGGC AGC T C C
3′
5′
Template strand
5′
mRNA
A C UG C C C A UG A G C G AC C A CU UG G G G C U CG G G G A A UA A C C G UC G A G G
5′ − untranslated Start
region
codon
Codons
3′
Stop
3′ − untranslated
codon
region
Anticodons
Translation
UAC UCG CUG GUG A AC CCC GAG CCC CUU
Polypeptide
tRNA
5′
3′
Figure 13.3
Met
Ser
Asp
His
Leu
Gly
Leu
Gly
Note that the start codon sets the
reading frame for all remaining
codons
Glu
20
Sample Problem
(only one answer is correct)
A tRNA has the anticodon 5’-CAU-3’. What amino acid does it carry?
a.
Histidine
b.
Methionine
c.
Phenyalanine
d.
Valine
e.
None of the above
21
A Polypeptide Chain Has Directionality



Polypeptide synthesis has a directionality that
parallels the 5’ to 3’ orientation of mRNA
During each cycle of elongation, a peptide bond is
formed between the carboxyl group of the last amino
acid in the polypeptide chain and the amino group in
the amino acid being added
The first amino acid has an exposed amino group


The last amino acid has an exposed carboxyl group


Said to be N-terminal or amino terminal end
Said to be C-terminal or carboxy terminal end
Refer to Figure 13.6
22
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R1 O
H3N+
C
C
H
R2 O
N
C
H
H
R1 O
H3N+
C
C
H
R3 O
C
C
H
H
+ H3N+
N
C
C
H
H
O–
R2 O
N
R4 O
R3 O
C
N
C
H
H
C
C
C
H
O–
R4 O
N
C
C + H2O
H
H
O–
Last peptide bond formed in the
growing chain of amino acids
(a) Attachment of an amino acid to a peptide chain
OH
CH3
S
CH2
OH
CH2
CH2
H 3C
H
+
H3N
Amino
terminal
end
C
C
H
O
Methionine
N
H
C
C
H
O
Serine
N
SH
CH3
CH
CH2
H
C
C
H
O
Valine
N
CH2
H
C
C
H
O
Tyrosine
N
C
C
H
O
O– Carboxyl
terminal
end
Cysteine
Peptide bonds
5′
AUG
AGC
GU U
UAC
UGC
3′
Sequence in mRNA
(b) Directionality in a polypeptide and mRNA
Figure 13.6
23

There are 20 amino acids that may be found in polypeptides
 Each contains a different side chain, or R group
 Each R group has its own particular chemical properties
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CH3
H
H3N
CH3 CH3
CH
CH3
+
COO–
C
+
H3N
H
Glycine (Gly) G
C
COO–
H
Alanine (Ala) A
+
H3N
C
CH3
CH3 CH3
CH
CH2
COO–
H
Valine (Val) V
+
H3N
C
CH2
CH3
COO–
H
Leucine (Leu) L
S
+
H3N
CH
C
SH
CH2
CH2 CH2
COO–
H
Isoleucine (Ile) I
+
H2N
C
COO–
H
Proline (Pro) P
CH2
CH2
CH2
+
H3N
C
COO–
+
H3N
C
COO–
H
H
Cysteine (Cys) C Methionine (Met) M
(a) Nonpolar, aliphatic amino acids
H
OH

N
CH2
+
H3N
C
CH2
COO–
+
H3N
C

CH2
COO–
H
H
Phenylalanine (Phe) F Tyrosine (Tyr) Y
+
H3N
C
Nonpolar amino acids are
hydrophobic
COO–
H
Tryptophan (Trp) W
They are often buried
within the interior of a
folded protein or in a cell
membrane
(b) Aromatic amino acids
Figure 13.7
24

Polar and charged amino acids are hydrophilic

They are more likely to be on the surface of a protein
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O
O
OH
HCOH
CH2
+
H3N
COO–
C
H
Serine (Ser) S
+
H3N
CH2
CH2
COO–
C
NH2
C
CH3
H
Threonine (Thr) T
+
H3N
C
NH2
C
CH2
COO–
+
H3N
C
COO–
H
H
Asparagine (Asn) N Glutamine (Gln) Q
(c) Polar, neutral amino acids
NH2
O–
O
C
O–
O
C
H3N
C
+
NH
CH2
CH2
+
HN
CH2
COO–
+
H3N
C
CH2
COO–
H
H
Aspartic acid (Asp) D Glutamic acid (Glu) E
(d) Polar, acidic amino acids
+
H3N
C
NH3
C
CH2
NH
CH2
CH2
NH
CH2
CH2
CH2
CH2
COO–
H
Histidine (His) H
+
H3N
N
+
+
C
CH3
+
H3N
C
C
O
CH2
CH2
COO–
H
Lysine (Lys) K
(e) Polar, basic amino acids
NH2
COO–
H
Arginine (Arg) R
SeH
CH2
CH2
+
H3N
C
CH2
COO–
+
H3N
C
COO–
H
H
Selenocysteine (Sec) Pyrrolysine (Pyl)
(f) Nonstandard amino acids
Figure 13.7
25
Levels of Structure in Proteins

There are four levels of structure in proteins





1.
2.
3.
4.
Primary
Secondary
Tertiary
Quaternary
A protein’s primary structure is its amino acid
sequence

Refer to Figure 13.8
26
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Arg Cys Glu
Phe Gly
Leu
1
Val
Lys

10
Ala
Ala
Ala
NH3+
Met
Lys
20
Gly Arg
His
Tyr Asn
Asp Leu Gly
Tyr
Ser
Leu
Arg
Gly
30
Asn
The amino acid
sequence of
the enzyme
lysozyme
Ala Ala
Trp Val Cys
Lys Phe Glu
Ser
Asn
Phe
Asn Arg Asn
Thr
Thr
Ala
Asp
40 Asn
Within the cell, the
protein will not be
found in this linear
state
 Rather, it will adopt
a compact 3-D
structure
Gin Thr
Gly
50
Ser

60
Thr
Asp
Tyr Gly
lle
Leu
Asn
lle
Gln
Ser
Arg Trp Trp
Cys
Asn
70
Leu Asn Arg Ser
Gly Pro
Thr
Cys
Asp
Gly
Arg
Indeed, this folding
can begin during
translation
Asn
lle
129 amino acids
long
Pro
80
Cys
Leu
Ser Ala
Leu
Ser
Ser
Asp
lle
Thr
Ser
Asp
Gly
Gly
Met
Asn
Asp Ser
Val
lle
Lys Lys Ala Cys
Arg Asn
Arg
Cys
Lys
129
Gly
Leu
Arg
COO–
Figure 13.8
Asn
100
Ala Trp Val Ala Trp
110
Cys Gly Arg
lle
90
Ala
Trp Ala Gln
120
Thr
Val

The progression from
the primary structure
to the 3-D structure is
dictated by the amino
acid sequence within
the polypeptide
Val Asp
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27
Levels of Structures in Proteins


The primary structure of a protein folds to form
regular, repeating shapes known as secondary
structures
There are two types of secondary structures

a helix
b sheet

Certain amino acids are good candidates for each structure

These secondary structures are stabilized by the
formation of hydrogen bonds between atoms located in
the polypeptide backbone
Refer to Figure 13.9


28
Levels of Structures in Proteins

The short regions of secondary structure in a protein
fold into a three-dimensional tertiary structure




Refer to Figure 13.9
This is the final conformation of proteins that are
composed of a single polypeptide
Structure determined by hydrophobic and ionic interactions as well as
hydrogen bonds and Van der Waals interactions
Proteins made up of two or more polypeptides have
a quaternary structure


This is formed when the various polypeptides associate
with one another to make a functional protein
Refer to Figure 13.9
29
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Primary
structure
Tertiary
structure
Secondary
structure
Quaternary
structure
C
Phe
Glu
C
O
H
N
O
H
H
N
C
C
C
H
Leu
N
Iso
N
C
HO
O N
Tyr
H
C
C
C
C
O
H
C
C
H
C
N
Two or more
polypeptides
may associate
with each other.
NH3+
COO–
N
C
O
C
Regions of
NH3+
secondary
structure and
irregularly shaped
regions fold into a
–
three-dimensional COO
conformation.
NH3+
Val
Depending on
the amino acid
sequence,
some regions
may fold into
an α helix or
β sheet.
COO–
Ala
(c)
C
N
Protein
subunit
O
O
α helix
Ala
(d)
(a)
β sheet
(b)
Figure 13.9
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30
Functions of Proteins

To a great extent, the characteristics of a cell depend on the
types of proteins its makes

Proteins can perform a variety of functions
 Refer to Table 13.5

A key category of proteins are enzymes


Accelerate chemical reactions within a cell
Can be divided into two main categories
 Anabolic enzymes  Synthesize molecules and macromolecules
 Catabolic enzymes  Break down large molecules into small ones

Important in generating cellular energy
31
32
13-39
13.2 STRUCTURE AND
FUNCTION OF tRNA

In the 1950s, Francis Crick and Mahon Hoagland
proposed the adaptor hypothesis


tRNAs play a direct role in the recognition of codons in
the mRNA
In particular, the hypothesis proposed that tRNA
has two functions


1. Recognizing a 3-base codon in mRNA
2. Carrying an amino acid that is specific for that codon
33
Recognition Between tRNA and mRNA

During mRNA-tRNA recognition, the anticodon in
tRNA binds to a complementary codon in mRNA
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Phenylalanine
Proline
tRNAs are named
according to the
amino acid they bear
tRNAPhe
tRNAPro
A A G
The anticodon is
anti-parallel
to the codon
G GC
Phenylalanine
anticodon
Proline
anticodon
U UC
C C G
5′
3′ mRNA
Phenylalanine
codon
Figure 13.10
Proline
codon
34
tRNAs Share Common Structural
Features

The secondary structure of tRNAs exhibits a
cloverleaf pattern

It contains



Three stem-loop structures
A few variable sites
An acceptor stem with a 3’ single strand region

The actual three-dimensional or tertiary structure
involves additional folding

In addition to the normal A, U, G and C nucleotides,
tRNAs commonly contain modified nucleotides

More than 80 of these can occur
35
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NH3+
H C R
C O
3′
A
C
C
Found in all tRNAs
5′
OH
O
Covalent
bond
between
tRNA
and an
amino
acid
A
C
C
Acceptor stem
PO4
70
Stem–loop
60
U
UH2 G
C
A
U
U
10
A
G
T
50
G
Not found in all tRNAs
Other variable sites are
shown in blue as well
C
P
m 2G
A
G UH2
19
UH2
40
30
U
P
U
mI
I
G
C
The modified bases are:

I = inosine

mI = methylinosine

T = ribothymidine

UH2 = dihydrouridine

m2G = dimethylguanosine

y = pseudouridine
Anticodon
Figure 13.12 Structure of tRNA
36
Charging of tRNAs

The enzymes that attach amino acids to tRNAs are
known as aminoacyl-tRNA synthetases

There are 20 types


One for each amino acid
Aminoacyl-tRNA synthetases catalyze a two-step
reaction involving three different molecules

Amino acid, tRNA and ATP

Refer to Figure 13.13
37
Charging of tRNAs

The aminoacyl-tRNA synthetases are responsible
for the “second genetic code”




The selection of the correct amino acid must be highly
accurate or the polypeptides may be nonfunctional
Error rate is less than one in every 100,000
Sequences throughout the tRNA including but not limited
to the anticodon are used as recognition sites
Modified bases may affect
 translation rates
 recognition by aminoacyl-tRNA synthetases
 Codon-anticodon recognition
38
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Aminoacyl-tRNA
synthetase
Specific
amino acid
A
P
P
P
ATP
An amino acid and ATP bind to
the enzyme. AMP is covalently
bound to the amino acid, and
pyrophosphate is released.
A
P
P P
Pyrophosphate
The correct tRNA binds to the
enzyme. The amino acid
becomes covalently attached to
the 3′ end of the tRNA. AMP is
released.
tRNA
3′
5′
5′
3′
A
The amino acid is
attached to the 3’
end of the tRNA
by an ester bond
P
AMP
The “charged” tRNA is
released.
5′
Figure 13.13
3′
39
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40
tRNAs and the Wobble Rule

As mentioned earlier, the genetic code is degenerate


With the exception of serine, arginine and leucine, this
degeneracy always occurs at the codon’s third position
To explain this pattern of degeneracy, Francis Crick
proposed in 1966 the wobble hypothesis


In the codon-anticodon recognition process, the first two
positions pair strictly according to the A – U /G – C rule
However, the third position can actually “wobble” or move a
bit

Thus tolerating certain types of mismatches
41
Phenylalanine
tRNAs that can recognize the same codon are
termed isoacceptor tRNAs
5′
3′
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Nucleotide of
tRNA anticodon
A A G
U U U
Wobble
position
3′
5′

G
C
A
U
inosine I
5-methyl-2-thiouridine xm5s2U

5-methyl-2’-O-methyluridine xm5Um

2’-O-methyluridine Um
position

5-methyluridine xm5U
Third nucleotide
of mRNA codon
C, U
G
U, C, G, (A)
A, U, G, (C)
U, C, A

(a) Location of wobble
You don’t need to
memorize these
rules

5-hydroxyuridine xo5U

lysidine k2C
A, (G)
U, A, G
A
(b) Revised wobble rules
Figure 13.14 Wobble position and base pairing rules
Recognized
very poorly
by the
tRNA 42
13.3 RIBOSOME STRUCTURE
AND ASSEMBLY

Translation occurs on the surface of a large
macromolecular complex termed the ribosome

Bacterial cells have one type of ribosome


Found in their cytoplasm
Eukaryotic cells have two types of ribosomes


One type is found in the cytoplasm
The other is found in organelles

Mitochondria ; Chloroplasts
43
13.3 RIBOSOME STRUCTURE
AND ASSEMBLY

Unless otherwise noted the term eukaryotic
ribosome refers to the ribosomes in the cytosol

A ribosome is composed of structures called the
large and small subunits

Each subunit is formed from the assembly of



Proteins
rRNA
Table 13.6 presents the composition of bacterial and
eukaryotic ribosomes
44
45
Functional Sites of Ribosomes

During bacterial translation, the mRNA lies on the
surface of the 30S subunit


Ribosomes contain three discrete sites




As a polypeptide is being synthesized, it exits through a
channel within the 50S subunit
Peptidyl site (P site)
Aminoacyl site (A site)
Exit site (E site)
Ribosomal structure is shown in Figure 13.15
46
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Polypeptide
tRNA
E
P
A
50S
30S
mRNA
5
3
(c) Model for ribosome structure
Figure 13.15
47
13.4 STAGES OF
TRANSLATION

Translation can be viewed as occurring in three
stages




Initiation
Elongation
Termination
Refer to 13.16 for an overview of translation
48
Initiator tRNA
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aa1
aa1
Initiator
tRNA – tRNA
with first
amino acid
Large
E
Ribosomal
subunits
UAC
Anticodon
A
Initiation
Small
AUG
Start codon
mRNA
UAG
Stop codon
5′
P
3′
5′
3′
AUG
Start codon
Elongation
(This step
occurs many
times.)
aa1
aa2
aa3
aa4
Recycling of translational
components
Release
factor
Completed
polypeptide
E
P
aa5
E
A
P
A
Termination
UAG
Stop codon
5′
Figure 13.16
3′
5′
3′
49
The Translation Initiation Stage

The mRNA, initiator tRNA, and ribosomal subunits
associate to form an initiation complex


This process requires three Initiation Factors
The initiator tRNA recognizes the start codon in
mRNA

In bacteria, this tRNA is designated tRNAfmet


It carries a methionine that has been covalently modified to
N-formylmethionine
The start codon is AUG, but in some cases GUG or UUG

In all three cases, the first amino acid is N-formylmethionine
50

The binding of mRNA to the 30S subunit is facilitated by a
ribosomal-binding site or Shine-Dalgarno sequence

This is complementary to a sequence in the 16S rRNA
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Component of the
30S subunit
3′
5′
16S rRNA
Hydrogen bonding
mRNA
A UCU AGU A AGGAGGUUGU A UGGUU C AGCGC A CG
Figure 13.18

A UUCC UC C A
Shine-Dalgarno
sequence
CAG
3′
Start
codon
Figure 13.17 outlines the steps that occur during
translational initiation in bacteria
51
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IF1 and IF3 bind to the 30S subunit.
IF3
IF1
The mRNA binds to the 30S subunit.
The Shine-Dalgarno sequence is
complementary to a portion of the
16S rRNA.
Portion of
16S rRNA
IF3
5′
30S subunit
IF1
Start
Shinecodon
Dalgarno
sequence
(actually 9
nucleotides long)
3′
IF2, which uses GTP, promotes
the binding of the initiator tRNA
to the start codon in the P site.
Figure 13.17
52
tRNAfMet
Initiator tRNA
GTP
IF2
IF1
IF3
3′
5′
IF1 and IF3 are released.
IF2 hydrolyzes its GTP and is released.
The 50S subunit associates.
tRNAfMet
70S initiation
complex
E
Figure 13.17
5′
P
A
70S
initiation
complex
This marks the
end of the
initiation
stage
3′
53
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54
The Translation Initiation Stage

In eukaryotes, the assembly of the initiation complex
is similar to that in bacteria

However, additional factors are required



Note that eukaryotic Initiation Factors are denoted eIF
Refer to Table 13.7
The initiator tRNA is designated tRNAmet

It carries a methionine rather than a formylmethionine
55

The start codon for eukaryotic translation is AUG


Ribosome scans from the 5’ end of mRNA until it finds
the AUG start codon (not all AUGs can act as a start)
The consensus sequence for optimal start codon
recognition is show here
Most important positions for codon selection


C C A U G G
-2 -1 +1 +2 +3 +4
These rules are called Kozak’s rules


G C C (A/G)
-6 -5 -4
-3
Start codon
After Marilyn Kozak who first proposed them
With that in mind, the start codon for eukaryotic
translation is usually the first AUG after the 5’ Cap!
56

Translational initiation in eukaryotes can be
summarized as such:






An initiation factor protein complex (eIF4) binds to the 5’
cap in mRNA
These are joined by a complex consisting of the 40S
subunit, tRNAmet, and other initiation factors
The entire assembly moves along the mRNA scanning
for the right start codon
Once it finds this AUG, the 40S subunit binds to it
The 60S subunit joins
This forms the 80S initiation complex
57
The Translation Elongation Stage

During this stage, amino acids are added to the
polypeptide chain, one at a time

The addition of each amino acid occurs via a series
of steps outlined in Figure 13.19

This process, though complex, can occur at a
remarkable rate


In bacteria  15-20 amino acids per second
In eukaryotes  2-6 amino acids per second
58
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aa1
aa2
Ribosome
aa3
E site
A site
P site
5′
Codon 4
Codon 3
aa1
aa2
aa3
3′
aa4
mRNA
E
A charged tRNA binds
to the A site. EF-Tu
facilitates tRNA binding
and hydrolyzes GTP.
The 23S rRNA (a component of
the large subunit) is the
actual peptidyl transferase
Thus, the ribosome
is a ribozyme!
Figure 13.19
P
A
3′
5′
Peptidyltransferase, which
is a component of the 50S
subunit, catalyzes peptide
bond formation between the
polypeptide and the amino
acid in the A site.The
polypeptide is transferred
to the A site.
59
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aa1
aa2
tRNAs at the P and A
sites move into
the E and P
sites, respectively
aa3
aa4
E
P
A
aa1
aa2
aa3
3′
5′
aa4
The ribosome translocates
1 codon to the right. This
translocation is promoted
by EF-G, which hydrolyzes
GTP.
aa3
aa4
aa2
E
P
A
Codon 3
aa1
Codon 5
5′
E
P
Codon 4
3′
An uncharged
tRNA is released
from the E site.
A
Codon 3
5′
Codon 5
Codon 4
3′
This process is repeated, again and
again, until a stop codon is reached.
Figure 13.19
60
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61
The Translation Termination Stage

The final stage occurs when a stop codon is
reached in the mRNA

In most species there are three stop or nonsense codons




UAG
UAA
UGA
These codons are not recognized by tRNAs, but by
proteins called release factors

Indeed, the 3-D structure of release factors mimics that of tRNAs
62
The Translation Termination Stage

Bacteria have three release factors



RF1, which recognizes UAA and UAG
RF2, which recognizes UAA and UGA
RF3, which does not recognize any of the three codons


It binds GTP and helps facilitate the termination process
Eukaryotes only have one release factor

eRF, which recognizes all three stop codons
63
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tRNA in P
site carries
completed
polypeptide
E
5′
P
A
Stop codon
in A site
3′
mRNA
A release factor (RF) binds to the A site.
E
P
A
Release
factor
3′
5′
The polypeptide is cleaved from the tRNA
in the P site. The tRNA is then released.
3′
5′
The ribosomal subunits, mRNA, and
release factor dissociate.
+
50S subunit
mRNA
Figure 13.20
5′
30S subunit
3′
64
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65
66
Bacterial Translation Can Begin
Before Transcription Is Completed

Bacteria lack a nucleus


Therefore, both transcription and translation occur in the cytoplasm
As soon an mRNA strand is long enough, a ribosome will
attach to its 5’ end



So translation begins before transcription ends
This phenomenon is termed coupling
Refer to Figure 13.21
67
Coupling between transcription and translation in bacteria
Figure 13.21
68
69