LECTURE 5 (Chapter 13) Translation of mRNA 1. INTRODUCTION The translation of the mRNA codons into...

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

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

13.1 THE GENETIC BASIS FOR PROTEIN SYNTHESIS

3

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

Archibald Garrod

5

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

Archibald Garrod

7

8

Inheritance of alkaptonuria

Metabolic pathway of phenylalanine metabolism and related genetic diseases

Figure 13.1

Dietaryprotein

CH2

NH2

Phenylalanine

Tyrosine

Phenylalaninehydroxylase

Tyrosineaminotransferase

Hydroxyphenylpyruvateoxidase

Homogentisicacid oxidase

p-hydroxyphenylpyruvicacid

Homogentisicacid

Maleylacetoaceticacid

Phenylketonuria

Tyrosinosis

Alkaptonuria

COOHC

CH2HO COOHC

H

H

NH2

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

9

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

Beadle and Tatum’s Experiments

10

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

Beadle and Tatum’s Experiments

11

Figure 13.2

Every mutant strain was blocked at one (and only one) particular step in the synthesis pathway, showing that each gene encoded one enzyme

1

3

4

1

3

1

3

1

3

1

2

3


Neurosporagrowth

WT WT WT WT WT

2

Minimal +O–acetylhomoserine +Cystathionine +Homocysteine +Methionine

(a) Growth of strains on minimal and supplemented growth media

(b) Simplified pathway for methionine biosynthesis

Homoserine O–acetylhomoserine Cystathionine Homocysteine MethionineEnzyme 1 Enzyme 2 Enzyme 3 Enzyme 4

4 2 4 2 4 2 4

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

Translation involves an interpretation of one language into another In genetics, the nucleotide language of mRNA is

translated into the amino acid language of proteins

Translation relies on the genetic code Refer to Table 13.1

The genetic information is coded within mRNA in groups of three nucleotides known as codons

The Genetic Code (first slide quiz 8, Sec 7)

17

Triplet codons correspond to a specific amino acid

Multiple codons may encode the same amino acid.

These are known as synonymous codons

Three codons do not encode an amino acid. These are read

as STOP signals for translation

18

Special codons: AUG (which specifies methionine) = start codon

This defines the reading frame for all following codons AUG specifies additional methionines within the coding sequence

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

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 13.319

Figure 13.3

Figure 13.3 provides an overview of gene expression

Note that the start codon sets the reading frame for all remaining codons

5′

Template strand

Coding strand

Transcription

3′

Translation

DNA

mRNA

tRNAPolypeptide

5′ − untranslated region

3′ − untranslated region

Startcodon

Codons Anticodons

3′

3′

5′

5′

A C T G C C C A T G G G G C TC G A CA G GC G G G A A T A A C C G T C G A G G

G G C A G C T C C

C C G U C G A G G

T T GC A C

T G A C G G G T A C C C C G AG C T GT C CG C C C T T A T TA A CG T G

5′ 3′A C U G C C C A U G G G G C UC G A CA G GC G G G A A U A AU U GC A C

Met Gly LeuSer Asp Gly GluHis Leu

Stopcodon

UAC CCC GAGUCG CUG CCC CUUGUG A AC


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

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 Said to be N-terminal or amino terminal end

The last amino acid has an exposed carboxyl group Said to be C-terminal or carboxy terminal end

Refer to Figure 13.6

A Polypeptide Chain Has Directionality

22

Figure 13.6

(a) Attachment of an amino acid to a peptide chain

(b) Directionality in a polypeptide and mRNA

H H H H H

H3N+ H3N+

H3N+

H3N+

C C C CN C C C+

+

N

R1 R2O O

O– O–

R3 R4O

C

O

H H H H H H

Last peptide bond formed in thegrowing chain of amino acids

H O–

O–

H2OC C C CN C CN C CN

R1 R2O O R3 R4O O

H HO

H3C

Aminoterminalend

Carboxylterminalend

Methionine Serine

Peptide bonds

Sequence in mRNA

Valine

CH2

CH3

CH3

CH2

CH2

OH

CH

S

C C CN

H

O

C CN C

H O H

Cysteine

CH2

SH

CN

H

O

C

Tyrosine

CH2

OH

H

CN C

H O

H

5′ 3′A U G A G C GU U U A C U G C


H

23

Figure 13.7

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

Nonpolar amino acids are hydrophobic

They are often buried within the interior of a folded protein or in a cell membrane

H

HGlycine (Gly) G

(a) Nonpolar, aliphatic amino acids

H3N C COO–

CH3 CH3

CH

HAlanine (Ala) A

H3N COO–

CH3 CH3

CH

CH2

HValine (Val) V

H3N C COO–+

CH2CH2

CH2

HProline (Pro) P

H2N C COO–+

CH2

CH3

CH3 CH

HLeucine (Leu) L Methionine (Met) M

H3N C COO–+

Cysteine (Cys) C

+

CH2

SH

H

H3N C COO–

CH2

CH2

CH3

S

H

H3N C COO–+

HIsoleucine (Ile) I

H3N C COO–+

(b) Aromatic amino acids

Phenylalanine (Phe) F Tyrosine (Tyr) YH

H3N C COO–+

CH2

H

H3N C COO–+

CH2

OH

Tryptophan (Trp) WH

H3N C COO–+

CH2

N

H


+

CH3

C+

24

Figure 13.7

Polar and charged amino acids are hydrophilic They are more likely to be on the surface of a protein

(c) Polar, neutral amino acids

Serine (Ser) S Threonine (Thr) TH

H3N C COO–+

CH2

OH

H

HCOH

H3N C

CH3

COO–+

HGlutamine (Gln) Q

H3N C COO–+

CH2

C

O NH2

HAsparagine (Asn) N

H3N C COO–+

CH2

CH2

C

O NH2

HGlutamic acid (Glu) E

H3N C COO–+

CH2

C

O O–

HAspartic acid (Asp) D

H3N C COO–+

CH2

CH2

C

O O–

(d) Polar, acidic amino acids (e) Polar, basic amino acids

Histidine (His) HH

H3N C COO–+

+

+ +

CH2

NHHN

Lysine (Lys) KH

H3N C COO–+

CH2

CH2

CH2

CH2

NH3

Arginine (Arg) RH

H3N C COO–+

CH2

CH2

CH2

C

NH

NH2

NH2

(f) Nonstandard amino acids

Selenocysteine (Sec)H

H3N C COO–+

CH2

SeH

N

CH3

Pyrrolysine (Pyl)H

H3N C COO–+

CH2

CH2

CH2

CH2

NH

C O


25

There are four levels of structure in proteins 1. Primary 2. Secondary 3. Tertiary 4. Quaternary

A protein’s primary structure is its amino acid sequence Refer to Figure 13.8

Levels of Structure in Proteins

26

Lys

NH3+

110

20

30

40

5060

70

8090

100

110

120

129

ValPhe Gly Arg Cys Glu

LeuAla

AlaAla

Met

Lys

Arg

HisGlyLeuAspAsnTyrArgGlyTyr

Ser

Thr

AspTyr Gly

Leu

Asn

SerGluPheLysAlaAlaCysValTrpAsn

Leu

Gly

Phe

Asn

ThrGinAla

ThrAsnArgAsnThr

Asp

Gly

Ser

lleGln

lleAsn

SerArg Trp

Trp

Cys

Asn

AspGlyArgThrProGlySerArgAsnLeu

Cys

Asn

lle

Pro

CysSer Ala Leu

LeuSer

SerAsp

lleThr

Arg AsnArg Cys

Lys

Gly

Thr

Asp

AlaTrp ValAlaAsn

Met

GlyAsp

GlyAsp Ser Val lle Lys Lys Ala Cys

AsnVal

Ser

Ala

ValGlnAlaTrplleArgGlyCysArg

Leu

Trp

COO–


Figure 13.8

The amino acid sequence of the enzyme lysozyme

129 amino acids long

Within the cell, the protein will not be found in this linear state Rather, it will adopt

a compact 3-D structure

Indeed, this folding can begin during translation

The progression from the primary structure to the 3-D structure is dictated by the amino acid sequence within the polypeptide

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The primary structure of a protein folds to form regular, repeating shapes known as secondary structures

There are two types of secondary structures helix 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


Levels of Structures in Proteins

28

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

Levels of Structures in Proteins

29

Figure 13.9Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

α helix

β sheet

Primarystructure

Secondarystructure

Quaternarystructure

Tertiarystructure

Proteinsubunit

Ala C

O

C

C

C

C

O

O

Val

Phe

Glu

Tyr

Leu

Iso

Ala

H

NNH3

+

NH3+

COO–

COO–

NH3+

COO–

H

N

CC

CC O

O

HH

NN

H

N

CC

C

CC

CO

OC

OH

H

N

NN

Depending onthe amino acidsequence,some regionsmay fold intoan α helix orβ sheet.

Two or morepolypeptidesmay associatewith each other.

Regions ofsecondarystructure andirregularly shapedregions fold into athree-dimensionalconformation.

C

C

C C

O

H

H

N

NN

C

C

C

C CC

O

O

H

H

NC C C

O

NC C C

O

NC

O

HC

C

C

O

O

H

H

NCHC

C

O

H

NO

CC

HC

C

O

H

CC

O

H

C CO

H

(a)

(b)

(c)

(d)

H

COO C

HH

H

O C


30

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

Functions of Proteins

31

13-3932

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

13.2 STRUCTURE AND FUNCTION OF tRNA

33

During mRNA-tRNA recognition, the anticodon in tRNA binds to a complementary codon in mRNA

Recognition Between tRNA and mRNA

Figure 13.10

tRNAs are named according to the

amino acid they bear

The anticodon is anti-parallel to the codon

Phenylalanine

tRNAPhetRNAPro

Phenylalanineanticodon

Phenylalaninecodon

Prolinecodon

A G

Proline

Prolineanticodon

U C

3′ mRNA5′


G CA G

U C C G

34

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

tRNAs Share Common Structural Features

35

Anticodon

U

GG

C

GA

A

UH2

UH2 UH2

30

10

19

40

60

70

Acceptor stem

50

U

I C

mI

P

G

PO4

OH

UU

A

G

CPT

m2G

A

C

C

3′

5′

A

C

C

NH3+

C RC O

H

O CovalentbondbetweentRNAand anaminoacid

U


Stem–loop

Structure of tRNAFigure 13.12

Found in all tRNAs

Not found in all tRNAsOther variable sites are

shown in blue as well

The modified bases are:I = inosinemI = methylinosineT = ribothymidineUH2 = dihydrouridinem2G = dimethylguanosine= pseudouridine

36

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


Charging of tRNAs

37

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

Charging of tRNAs

38

Figure 13.13

The amino acid is attached to the 3’ end of the tRNA by an ester bond

P

P P

P PPyrophosphate

Specificamino acid

Aminoacyl-tRNAsynthetase

A

PA

PA

3′

3′

5′

3′5′

5′

AMP

ATP An amino acid and ATP bind tothe enzyme. AMP is covalentlybound to the amino acid, andpyrophosphate is released.

The correct tRNA binds to theenzyme. The amino acidbecomes covalently attached tothe 3′ end of the tRNA. AMP isreleased.

The “charged” tRNA isreleased.

tRNA


39

40

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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

tRNAs and the Wobble Rule

41

U

3′5′

5′

Wobbleposition

Nucleotide of tRNA anticodon

Third nucleotideof mRNA codon

GCAUI

xm5s2Uxm5Um

C, UGU, C, G, (A)A, U, G, (C)U, C, A

A, (G)

U, A, GA

(a) Location of wobble position

(b) Revised wobble rules

Phenylalanine


3′

Umxm5U

xo5Uk2C

A A G

U U

Wobble position and base pairing rulesFigure 13.14

tRNAs that can recognize the same codon are termed isoacceptor tRNAs

Recognized very poorly

by the tRNA

5-methyl-2-thiouridine

inosine

5-methyl-2’-O-methyluridine

5-methyluridine

lysidine

2’-O-methyluridine

5-hydroxyuridine

42

You don’t need to memorize these rules

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

13.3 RIBOSOME STRUCTURE AND ASSEMBLY

43

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

13.3 RIBOSOME STRUCTURE AND ASSEMBLY

44

During bacterial translation, the mRNA lies on the surface of the 30S subunit As a polypeptide is being synthesized, it exits through a

channel within the 50S subunit

Ribosomes contain three discrete sites Peptidyl site (P site) Aminoacyl site (A site) Exit site (E site)

Ribosomal structure is shown in Figure 13.15

Functional Sites of Ribosomes

46

Figure 13.15

(c) Model for ribosome structure

Polypeptide

30S

50S

35

tRNA

mRNA

E P A


47

Translation can be viewed as occurring in three stages Initiation Elongation Termination

Refer to 13.16 for an overview of translation

13.4 STAGES OF TRANSLATION

48

mRNA

UACAnticodon

InitiatortRNA – tRNAwith firstamino acid

AUGStart codon

AUGStart codon

UAGStop codon

UAGStop codon

Completedpolypeptide

Termination

Elongation(This stepoccurs manytimes.)

Recycling of translationalcomponents

Releasefactor

Small

Large

Ribosomalsubunits

EEA

EAP

aa1

aa2

aa3

aa4

aa5

aa1

aa1

3′3′ 5′5′

3′5′

3′5′


P P A

Figure 13.16

Initiator tRNA

Initiation

49

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

The Translation Initiation Stage

50

Shine-Dalgarnosequence

mRNA

5′ 3′A U C U A G U A A G U U C A GG G U CG A GU C A C G C A GU G GG U A

3′

Startcodon

A U U C C C A C A GC 16S rRNAU


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

Figure 13.17 outlines the steps that occur during translational initiation in bacteria

Figure 13.18

Hydrogen bonding

Component of the 30S subunit

51

Figure 13.17

IF2, which uses GTP, promotesthe binding of the initiator tRNAto the start codon in the P site.

Portion of16S rRNA

The mRNA binds to the 30S subunit.The Shine-Dalgarno sequence iscomplementary to a portion of the16S rRNA.

IF1 and IF3 bind to the 30S subunit.

3′5′

30S subunit

Shine-Dalgarnosequence(actually 9nucleotides long)

Startcodon

IF3 IF1

IF1IF3


52

Figure 13.17

70S initiation complex

This marks the end of the initiation

stage

IF1 and IF3 are released.

IF2 hydrolyzes its GTP and is released.

The 50S subunit associates.

tRNAfMet

IF2GTP

E AP

3′5′

3′5′

70Sinitiationcomplex

IF1IF3

Initiator tRNA

tRNAfMet

53

54


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

The Translation Initiation Stage

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

Start codon

G C C (A/G) C C A U G G

-6 -5 -4 -3 -2 -1 +1 +2 +3 +4

Most important positions for codon selection

These rules are called Kozak’s rules 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

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

The Translation Elongation Stage

58

Figure 13.19

The 23S rRNA (a component of the large subunit) is the

actual peptidyl transferase

Thus, the ribosome is a ribozyme!


3′

P site

Codon 3

Codon 4

mRNA

E siteA site

aa1

aa2

aa3Ribosome

aa1

aa2

aa3

EAP

aa4

A charged tRNA bindsto the A site. EF-Tufacilitates tRNA bindingand hydrolyzes GTP.

Peptidyltransferase, whichis a component of the 50Ssubunit, catalyzes peptidebond formation between thepolypeptide and the aminoacid in the A site.Thepolypeptide is transferredto the A site.

5′

5′

3′

59

Figure 13.19

tRNAs at the P and A sites move into

the E and P sites, respectively

Codon 4

Codon 5Codon 3

3′5′

aa1

aa2

aa3

aa4

aa1aa2

aa3

E A

A

Codon 4

Codon 5Codon 33′

5′

aa1aa2aa3

aa4

EA

P

P

aa4

This process is repeated, again andagain, until a stop codon is reached.

An unchargedtRNA is releasedfrom the E site.

The ribosome translocates1 codon to the right. Thistranslocation is promotedby EF-G, which hydrolyzesGTP.

5′3′

EP


60

61


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

The Translation Termination Stage

62

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

The Translation Termination Stage

63

Figure 13.20

3′5′

Stop codonin A site

tRNA in Psite carriescompletedpolypeptide

E A

3′5′

E A

mRNAA release factor (RF) binds to the A site.

The polypeptide is cleaved from the tRNAin the P site. The tRNA is then released.

The ribosomal subunits, mRNA, andrelease factor dissociate.

Releasefactor

3′

+

3′

5′

5′

50S subunit 30S subunit

mRNA


P

P

64

65


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

Bacterial Translation Can Begin Before Transcription Is Completed

67

Figure 13.21

Coupling between transcription and translation in bacteria

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LECTURE 5 (Chapter 13) Translation of mRNA 1. INTRODUCTION The translation of the mRNA codons into...

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