chapter 17

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

PowerPoint TextEdit Art Slides for Biology, Seventh Edition

Neil Campbell and Jane Reece

Chapter 17Chapter 17

From Gene to Protein


Figure 17.1 A ribosome, part of the protein synthesis machinery


Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information (layer 1)

TRANSCRIPTION DNA

Prokaryotic cell. In a cell lacking a nucleus, mRNAproduced by transcription is immediately translatedwithout additional processing.

(a)

Eukaryotic cell. The nucleus provides a separatecompartment for transcription. The original RNAtranscript, called pre-mRNA, is processed in various ways before leaving the nucleus as mRNA.

(b)



TRANSLATION

TRANSCRIPTION DNA

mRNA

Ribosome

PolypeptideProkaryotic cell. In a cell lacking a nucleus, mRNAproduced by transcription is immediately translatedwithout additional processing.

(a)


(b)



TRANSLATION

TRANSCRIPTION

TRANSCRIPTION

DNA

mRNA

DNA

Ribosome

Polypeptide

Nuclearenvelope


(a)


(b)



TRANSLATION

TRANSCRIPTION

TRANSCRIPTION

RNA PROCESSING

DNA

mRNA

mRNA

DNA

Pre-mRNA

Ribosome

Polypeptide

Nuclearenvelope


(a)


(b)



TRANSLATION

TRANSCRIPTION

TRANSCRIPTION

RNA PROCESSING

TRANSLATION

DNA

mRNA

mRNA

DNA

Pre-mRNA

Ribosome

Polypeptide

Polypeptide

Ribosome

Nuclearenvelope


(a)


(b)


Figure 17.4 The triplet code

DNAmolecule

Gene 1

Gene 2

Gene 3

DNA strand(template)

TRANSCRIPTION

mRNA

Protein

TRANSLATION

Amino acid

A C C A A A C C G A G T

U G G U U U G G C U C A

Trp Phe Gly Ser

Codon

3 5

35


Figure 17.5 The dictionary of the genetic codeSecond mRNA base

U C A G

U

C

A

G

UUU

UUC

UUA

UUG

CUU

CUC

CUA

CUG

AUU

AUC

AUA

AUG

GUU

GUC

GUA

GUG

Met orstart

Phe

Leu

Leu

lle

Val

UCU

UCC

UCA

UCG

CCU

CCC

CCA

CCG

ACU

ACC

ACA

ACG

GCU

GCC

GCA

GCG

Ser

Pro

Thr

Ala

UAU

UAC

UGU

UGCTyr Cys

CAU

CAC

CAA

CAG

CGU

CGC

CGA

CGG

AAU

AAC

AAA

AAG

AGU

AGC

AGA

AGG

GAU

GAC

GAA

GAG

GGU

GGC

GGA

GGG

UGG

UAA

UAG Stop

Stop UGA Stop

Trp

His

Gln

Asn

Lys

Asp

Arg

Ser

Arg

Gly

U

C

A

G

U

C

A

G

U

C

A

G

U

C

A

G

Fir

st m

RN

A b

ase

(5

end

)

Th

ird

mR

NA

bas

e (3

en

d)

Glu


Figure 17.6 A tobacco plant expressing a firefly gene


Figure 17.7 The stages of transcription: initiation, elongation, and termination (layer 1)

PromoterTranscription unit

RNA polymerase

Start point

53

35

DNA




RNA polymerase

Start point

53

35

35

53

Unwound

DNA

RNA

transcript

Template strand of DNA

1 Initiation. After RNA polymerase binds to

the promoter, the DNA strands unwind, and

the polymerase initiates RNA synthesis at the

start point on the template strand.

DNA




RNA polymerase

Start point

53

35

35

53

53

35

5

Rewound

RNA

RNA

transcript

3

Unwound

DNA

RNA

transcript






2 Elongation. The polymerase moves downstream, unwinding the

DNA and elongating the RNA transcript 5 3 . In the wake of

transcription, the DNA strands re-form a double helix.

DNA




RNA polymerase

Start point

53

35

35

53

53

35

53

35

5

5

Rewound

RNA

RNA

transcript

3

3

Completed RNA transcript

Unwound

DNA

RNA

transcript






3 Termination. Eventually, the RNA

transcript is released, and the

polymerase detaches from the DNA.

2 Elongation. The polymerase moves downstream, unwinding the

DNA and elongating the RNA transcript 5 3 . In the wake of

transcription, the DNA strands re-form a double helix.

DNA


Elongation

RNApolymerase

Non-templatestrand of DNA

RNA nucleotides

3 end

C A E G C AA

U

T A G G T TA

AC

G

U

AT

CA

T C C A AT

T

GG

3

5

5

Newly madeRNA

Direction of transcription(“downstream) Template

strand of DNA


Figure 17.8 The initiation of transcription at a eukaryotic promoter

TRANSCRIPTION

RNA PROCESSING

TRANSLATION

DNA

Pre-mRNA

mRNA

Ribosome

Polypeptide

Eukaryotic promoters

T A T AAA A

ATAT T T T

TATA box Start point TemplateDNA strand

53

35

Several transcriptionfactors

Additional transcriptionfactors

Transcriptionfactors

53

35

1

2

3

Promoter

53

355

RNA polymerase IITranscription factors

RNA transcript

Transcription initiation complex


Figure 17.9 RNA processing: addition of the 5 cap and poly-A tail

A modified guanine nucleotideadded to the 5 end

50 to 250 adenine nucleotidesadded to the 3 end

Protein-coding segment Polyadenylation signal

Poly-A tail3 UTRStop codonStart codon

5 Cap 5 UTR

AAUAAA AAA…AAA

TRANSCRIPTION

RNA PROCESSING

DNA

Pre-mRNA

mRNA

TRANSLATIONRibosome

Polypeptide

G P P P

5 3


Figure 17.10 RNA processing: RNA splicing

TRANSCRIPTION

RNA PROCESSING

DNA

Pre-mRNA

mRNA

TRANSLATION

Ribosome

Polypeptide

5 CapExon Intron

1

5

30 31

Exon Intron

104 105 146

Exon 3Poly-A tail

Poly-A tail

Introns cut out andexons spliced together

Codingsegment

5 Cap1 146

3 UTR3 UTR

Pre-mRNA

mRNA


Figure 17.11 The roles of snRNPs and spliceosomes in pre-mRNA splicing

RNA transcript (pre-mRNA)

Exon 1 Intron Exon 2

Other proteinsProtein

snRNA

snRNPs

Spliceosome

Spliceosomecomponents

Cut-outintron

mRNA

Exon 1 Exon 2

5

5

5

1

2

3


Figure 17.12 Correspondence between exons and protein domains

GeneDNA

Exon 1 Intron Exon 2 Intron Exon 3

Transcription

RNA processing

Translation

Domain 3

Domain 1

Domain 2

Polypeptide


Figure 17.13 Translation: the basic concept

TRANSCRIPTION

TRANSLATION

DNA

mRNARibosome

Polypeptide

Polypeptide

Aminoacids

tRNA withamino acidattached

Ribosome

tRNA

Anticodon

mRNA

Trp

Phe Gly

AG C

A A A

CC

G

U G G U U U G G C

Codons5 3


Figure 17.14 The structure of transfer RNA (tRNA)3ACCACGCUUAA

GACACCU

GC

*G U G U

CUGAG

GU

A

AA G

UC

AGACC

C G A GA G G

G

GACUCA

UUUAGGCG5

Amino acidattachment site

Hydrogenbonds

AnticodonTwo-dimensional structure. The four base-paired regions and three loops are characteristic of all tRNAs, as is the base sequence of the amino acid attachment site at the 3 end. The anticodon triplet is unique to each tRNA type. (The asterisks mark bases that have been chemically modified, a characteristic of tRNA.)

(a)

*

*

**

*

**

*

***


Amino acidattachment site

Hydrogen bonds

AnticodonAnticodon

(b) Three-dimensional structure

A A G

53

3 5

Symbol used in this book

(c)


Figure 17.15 An aminoacyl-tRNA synthetase joins a specific amino acid to a tRNA

Amino acid

ATP

Adenosine

Pyrophosphate

Adenosine

Adenosine

Phosphates

tRNA

P P P

P

P Pi

Pi

Pi

P

AMP

Aminoacyl tRNA(an “activatedamino acid”)

AppropriatetRNA covalentlyBonds to aminoAcid, displacing

AMP.

Active site binds theamino acid and ATP. 1

3

Activated amino acidis released by the enzyme.

4

Aminoacyl-tRNAsynthetase (enzyme)

ATP loses two P groupsand joins amino acid as AMP.2


Figure 17.16 The anatomy of a functioning ribosome

TRANSCRIPTION

TRANSLATION

DNA

mRNA

Ribosome

PolypeptideExit tunnel

Growingpolypeptide

tRNAmolecules

EP A

Largesubunit

Smallsubunit

mRNA

Computer model of functioning ribosome. This is a model of a bacterial ribosome, showing its overall shape. The eukaryotic ribosome is roughly similar. A ribosomal subunit is an aggregate of ribosomal RNA molecules and proteins.

(a)

53


P site (Peptidyl-tRNAbinding site)

E site (Exit site)

mRNAbinding site

A site (Aminoacyl-tRNA binding site)

Largesubunit

Smallsubunit

E P A

Schematic model showing binding sites. A ribosome has an mRNA binding site and three tRNA binding sites, known as the A, P, and E sites. This schematic ribosome will appear in later diagrams.

(b)


Amino end Growing polypeptide

Next amino acidto be added topolypeptide chain

tRNA

mRNA

Codons

3

5

Schematic model with mRNA and tRNA. A tRNA fits into a binding site when its anticodon base-pairs with an mRNA codon. The P site holds the tRNA attached to the growing polypeptide. The A site holds the tRNA carrying the next amino acid to be added to the polypeptide chain. Discharged tRNA leaves via the E site.

(c)


Figure 17.17 The initiation of translation

The arrival of a large ribosomal subunit completes the initiation complex. Proteins called initiationfactors (not shown) are required to bring all the translation components together. GTP provides the energy for the assembly. The initiator tRNA is in the P site; the A site is available to the tRNA bearing the next amino acid.

2

Initiator tRNA

mRNA

mRNA binding site Smallribosomalsubunit

Largeribosomalsubunit

Translation initiation complex

P site

GDPGTP

Start codon

A small ribosomal subunit binds to a molecule of mRNA. In a prokaryotic cell, the mRNA binding site on this subunit recognizes a specific nucleotide sequence on the mRNA just upstream of the start codon. An initiator tRNA, with the anticodon UAC, base-pairs with the start codon, AUG. This tRNA carries the amino acid methionine (Met).

1

MetMet

U A C

A U G

E A

3

5

5

3

35 35


Figure 17.18 The elongation cycle of translation

Amino endof polypeptide

mRNA

Codon recognition. The anticodon of an incoming aminoacyl tRNA base-pairs with the complementary mRNA codon in the A site. Hydrolysisof GTP increases the accuracy andefficiency of this step.

Ribosome ready fornext aminoacyl tRNA

Translocation. The ribosome translocates the tRNA in the A site to the P site. The empty tRNA in the P site is moved to the E site, where it is released. The mRNA moves along with its bound tRNAs,bringing the next codon to be translated into the A site.

Peptide bond formation. An rRNA molecule of the large Subunit catalyzes the formation of a peptide bond between the new amino acid in the A site and the carboxyl end of the growing polypeptide in the P site. This step attaches the polypeptide to the tRNA in the A site.

E

P A

E

P A

E

P A

E

P A

GDPGTP

GTP

GDP

2

2

site site5

3

3

2

1TRANSCRIPTION

TRANSLATION

DNA

mRNARibosome

Polypeptide


Figure 17.19 The termination of translation

Release factor

Freepolypeptide

Stop codon(UAG, UAA, or UGA)

When a ribosome reaches a stop codon on mRNA, the A site of the ribosome accepts a protein called a release factor instead of tRNA.

The release factor hydrolyzes the bond between the tRNA in the P site and the last amino acid of the polypeptide chain. The polypeptide is thus freed from the ribosome.

1 2 3

5

3

5

3

5

3

The two ribosomal subunits and the other components of the assembly dissociate.


Figure 17.20 Polyribosomes

Growingpolypeptides

Completedpolypeptide

Incomingribosomalsubunits

Start of mRNA(5 end)

End of mRNA(3 end)

Polyribosome

An mRNA molecule is generally translated simultaneously by several ribosomes in clusters called polyribosomes.

(a)

Ribosomes

mRNA

This micrograph shows a large polyribosome in a prokaryotic cell (TEM).

0.1 µm(b)


Figure 17.21 The signal mechanism for targeting proteins to the ER

The rest ofthe completedpolypeptide leaves the ribosome andfolds into its finalconformation.

Polypeptidesynthesis beginson a freeribosome inthe cytosol.

An SRP binds to the signal peptide, halting synthesismomentarily.

The SRP binds to areceptor protein in the ERmembrane. This receptoris part of a protein complex(a translocation complex)that has a membrane poreand a signal-cleaving enzyme.

The SRP leaves, andthe polypeptide resumesgrowing, meanwhiletranslocating across themembrane. (The signalpeptide stays attachedto the membrane.)

The signal-cleaving enzymecuts off thesignal peptide.

1 2 3 4 5 6

Ribosome

mRNASignalpeptide

Signal-recognitionparticle(SRP) SRP

receptorprotein

Translocationcomplex

CYTOSOL

Signalpeptideremoved

ERmembrane

Protein


Figure 17.22 Coupled transcription and translation in bacteria

DNA

Polyribosome

mRNA

Direction oftranscription

0.25 mRNApolymerase

Polyribosome

Ribosome

DNA

mRNA (5 end)

RNA polymerase

Polypeptide(amino end)


Figure 17.23 The molecular basis of sickle-cell disease: a point mutation

In the DNA, themutant templatestrand has an A where the wild-type template has a T.

The mutant mRNA has a U instead of an A in one codon.

The mutant (sickle-cell) hemoglobin has a valine (Val) instead of a glutamic acid (Glu).

Mutant hemoglobin DNAWild-type hemoglobin DNA

mRNA mRNA

Normal hemoglobin Sickle-cell hemoglobin

Glu Val

C T T C A T

G A A G U A

3 5 3 5

5 35 3


Figure 17.24 Base-pair substitutionWild type

A U G A A G U U U G G C U A AmRNA5

Protein Met Lys Phe Gly Stop

Carboxyl endAmino end

3

A U G A A G U U U G G U U A A

Met Lys Phe Gly

Base-pair substitution

No effect on amino acid sequenceU instead of C

Stop

A U G A A G U U U A G U U A A

Met Lys Phe Ser Stop

A U G U A G U U U G G C U A A

Met Stop

Missense A instead of G

NonsenseU instead of A


Figure 17.25 Base-pair insertion or deletion

mRNA

Protein

Wild type

A U G A A G U U U G G C U A A5

Met Lys Phe Gly

Amino end Carboxyl end

Stop

Base-pair insertion or deletion

Frameshift causing immediate nonsense

A U G U A A G U U U G G C U A

A U G A A G U U G G C U A A

A U G U U U G G C U A A

Met Stop

U

Met Lys Leu Ala

Met Phe GlyStop

MissingA A G

Missing

Extra U

Frameshift causing extensive missense

Insertion or deletion of 3 nucleotides:no frameshift but extra or missing amino acid

3


Figure 17.26 A summary of transcription and translation in a eukaryotic cell

TRANSCRIPTION RNA is transcribedfrom a DNA template.

DNA

RNApolymerase

RNAtranscript

RNA PROCESSING

In eukaryotes, theRNA transcript (pre-mRNA) is spliced andmodified to producemRNA, which movesfrom the nucleus to thecytoplasm.

Exon

Poly-A

RNA transcript(pre-mRNA)

Intron

NUCLEUSCap

FORMATION OFINITIATION COMPLEX

After leaving thenucleus, mRNA attachesto the ribosome.

CYTOPLASM

mRNA

Poly-A

Growingpolypeptide

Ribosomalsubunits

Cap

Aminoacyl-tRNAsynthetase

AminoacidtRNA

AMINO ACID ACTIVATION

Each amino acidattaches to its proper tRNAwith the help of a specificenzyme and ATP.

Activatedamino acid

TRANSLATION

A succession of tRNAsadd their amino acids tothe polypeptide chainas the mRNA is movedthrough the ribosomeone codon at a time.(When completed, thepolypeptide is releasedfrom the ribosome.)

Anticodon

A CC

A A AUG GUU UA U G

UACE A

Ribosome

1

Poly-A

5

5

3

Codon

2

3 4

5

chapter 17

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