BIOLOGYA Global Approach
Campbell • Reece • Urry • Cain • Wasserman • Minorsky • Jackson
© 2015 Pearson Education Ltd
TENTH EDITION
Global Edition
Lecture Presentation by Nicole Tunbridge andKathleen Fitzpatrick
17Expression of
Genes
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The Flow of Genetic Information
a) The information content of genes is in the specific sequences of nucleotides
b) The DNA inherited by an organism leads tospecific traits by dictating the synthesis of proteins
c) Proteins are the links between genotype and phenotype
d)Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation
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Figure 17.1
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Figure 17.1a
An albino racoon
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Concept 17.1: Genes specify proteins via transcription and translation
a) How was the fundamental relationship between genes and proteins discovered?
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Evidence from the Study of Metabolic Defects
a) In 1902, British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions
b) He thought symptoms of an inherited disease reflect an inability to synthesize a certain enzyme
c) Cells synthesize and degrade molecules in a series of steps, a metabolic pathway
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Nutritional Mutants in Neurospora: Scientific Inquiry
a) George Beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal media
b) Using crosses, they and their coworkers identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for synthesizing arginine
c) They developed a one gene–one enzyme hypothesis, which states that each gene dictates production of a specific enzyme
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Figure 17.2
Precursor
Enzyme A
Enzyme B
Enzyme C
Ornithine
Citrulline
Arginine
No growth:Mutant cellscannot growand divide
Growth:Wild-typecells growingand dividing
Control: Minimal medium
Results Table
Wild type
Minimalmedium(MM)(control)
MM +ornithine
MM +citrulline
MM +arginine(control)
Summaryof results
Can grow withor without anysupplements
Gene(codes forenzyme) Wild type
Precursor
Ornithine
Gene A
Gene B
Gene C
Enzyme A
Enzyme B
Enzyme C
Enzyme A
Enzyme B
Enzyme C
Citrulline
Arginine
Precursor
Ornithine
Citrulline
Arginine
Precursor
Ornithine
Citrulline
Arginine
Precursor
Ornithine
Citrulline
Arginine
Enzyme A
Enzyme B
Enzyme C
Enzyme A
Enzyme B
Enzyme C
Class I mutants(mutation in
gene A)
Class II mutants(mutation in
gene B)
Class III mutants(mutation in
gene C)
Can grow onornithine,citrulline,or arginine
Can grow onlyon citrulline orarginine
Require arginineto grow
Class I mutants Class II mutants Class III mutants
Classes of Neurospora crassa
Con
ditio
n
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Figure 17.2a
Precursor
Enzyme A
Enzyme B
Enzyme C
Ornithine
Citrulline
Arginine
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Figure 17.2b
No growth:Mutant cellscannot growand divide
Growth:Wild-typecells growingand dividing
Control: Minimal medium
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Figure 17.2c
Results Table
Wild type
Minimalmedium(MM)(control)
MM +ornithine
MM +citrulline
MM +arginine(control)
Summaryof results
Can grow withor without anysupplements
Can grow onornithine,citrulline,or arginine
Can grow onlyon citrulline orarginine
Require arginineto grow
Class I mutants Class II mutants Class III mutants
Classes of Neurospora crassaC
ondi
tion
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Figure 17.2d
Gene(codes forenzyme) Wild type
Precursor
Ornithine
Gene A
Gene B
Gene C
Enzyme A
Enzyme B
Enzyme C
Citrulline
Arginine
Class I mutants(mutation in
gene A)
Class II mutants(mutation in
gene B)
Class III mutants(mutation in
gene C)Precursor
Ornithine
Enzyme A
Enzyme B
Enzyme C
Citrulline
Arginine
Precursor
Ornithine
Enzyme A
Enzyme B
Enzyme C
Citrulline
Arginine
Precursor
Ornithine
Enzyme A
Enzyme B
Enzyme C
Citrulline
Arginine
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The Products of Gene Expression:A Developing Story
a) Some proteins aren’t enzymes, so researchers later revised the hypothesis: one gene–one protein
b) Many proteins are composed of several polypeptides, each of which has its own gene
c) Therefore, Beadle and Tatum’s hypothesis is now restated as the one gene–one polypeptide hypothesis
d) It is common to refer to gene products as proteins rather than polypeptides
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Basic Principles of Transcription and Translation
a) RNA is the bridge between genes and the proteins for which they code
b)Transcription is the synthesis of RNA using information in DNA
c) Transcription produces messenger RNA (mRNA)
d)Translation is the synthesis of a polypeptide, using information in the mRNA
e) Ribosomes are the sites of translation
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a) In prokaryotes, translation of mRNA can begin before transcription has finished
b) In a eukaryotic cell, the nuclear envelope separates transcription from translation
c) Eukaryotic RNA transcripts are modified through RNA processing to yield the finished mRNA
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Figure 17.3
Nuclear envelope
CYTOPLASM
DNA
Pre-mRNA
mRNA
RibosomeTRANSLATION
(b) Eukaryotic cell
NUCLEUS
RNA PROCESSING
TRANSCRIPTION
(a) Bacterial cell
Polypeptide
DNA
mRNARibosome
CYTOPLASM
TRANSCRIPTION
TRANSLATION
Polypeptide
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Figure 17.3a-1
(a) Bacterial cell
DNA
mRNACYTOPLASM
TRANSCRIPTION
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Figure 17.3a-2
(a) Bacterial cell
Polypeptide
DNA
mRNARibosome
CYTOPLASM
TRANSCRIPTION
TRANSLATION
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Figure 17.3b-1
Nuclear envelope
CYTOPLASM
DNA
Pre-mRNA
(b) Eukaryotic cell
NUCLEUS
TRANSCRIPTION
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Figure 17.3b-2
Nuclear envelope
CYTOPLASM
DNA
Pre-mRNA
mRNA
(b) Eukaryotic cell
NUCLEUS
RNA PROCESSING
TRANSCRIPTION
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Figure 17.3b-3
Nuclear envelope
CYTOPLASM
DNA
Pre-mRNA
mRNA
RibosomeTRANSLATION
(b) Eukaryotic cell
NUCLEUS
RNA PROCESSING
TRANSCRIPTION
Polypeptide
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a) A primary transcript is the initial RNA transcript from any gene prior to processing
b) The central dogma is the concept that cells are governed by a cellular chain of command: DNA → RNA → protein
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Figure 17.UN01
DNA RNA Protein
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The Genetic Code
a) How are the instructions for assembling amino acids into proteins encoded into DNA?
b) There are 20 amino acids, but there are only four nucleotide bases in DNA
c) How many nucleotides correspond to anamino acid?
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Codons: Triplets of Nucleotides
a) The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words
b) The words of a gene are transcribed into complementary nonoverlapping three-nucleotide words of mRNA
c) These words are then translated into a chain of amino acids, forming a polypeptide
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Figure 17.4
A C C A A A C C G A G T
ACTTTT CGGGGT
U G G U U U G G C CU A
SerGlyPheTrp
CodonTRANSLATION
TRANSCRIPTION
Protein
mRNA 5′
5′
3′
Amino acid
DNAtemplatestrand
5′
3′
3′
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a) During transcription, one of the two DNA strands, called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript
b) The template strand is always the same strandfor a given gene
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a) During translation, the mRNA base triplets, called codons, are read in the 5′ → 3′ direction
b) Each codon specifies the amino acid (one of 20)to be placed at the corresponding position alonga polypeptide
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Cracking the Code
a) All 64 codons were deciphered by the mid-1960s
b) Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation
c) The genetic code is redundant (more than one codon may specify a particular amino acid) butnot ambiguous; no codon specifies more thanone amino acid
d) Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced
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Figure 17.5Second mRNA base
Third
mR
NA
base
(3′
end
of c
odon
)
Firs
t mR
NA
base
(5′
end
of c
odon
)
UUU
UUC
UUA
UUG
Phe
Leu
Leu
Ile
Val
CUU
CUC
CUA
CUG
AUU
AUC
AUA
AUG
GUU
GUC
GUA
GUG
UCU
UCC
UCA
UCG
CCU
CCC
CCA
CCG
ACU
ACC
ACA
ACG
GCU
GCC
GCA
GCG GAG
GAA
GAC
GAU
AAG
AAA
AAC
AAU
CAG
CAA
CAC
CAU
Ser
Pro
Thr
AlaGlu
Asp
Lys
Asn
Gln
His
Tyr Cys
Trp
Arg
Ser
Arg
Gly
GGG
GGA
GGC
GGU
AGG
AGA
AGC
AGU
CGG
CGA
CGC
CGU
UGG
UGA
UGC
UGUUAU
UAC
UAA
UAG Stop
Stop Stop
Met orstart
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
U C A G
G
A
C
U
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Evolution of the Genetic Code
a) The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals
b) Genes can be transcribed and translated after being transplanted from one species to another
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Figure 17.6
Pig expressing a jellyfishgene
(b)Tobacco plant expressinga firefly gene
(a)
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Figure 17.6a
Tobacco plant expressinga firefly gene
(a)
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Figure 17.6b
Pig expressing a jellyfishgene
(b)
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Concept 17.2: Transcription is the DNA-directed synthesis of RNA: A closer look
a) Transcription is the first stage of gene expression
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Molecular Components of Transcription
a) RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and joins together the RNA nucleotides
b) The RNA is complementary to the DNA template strand
c) RNA polymerase does not need any primer
d) RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine
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Figure 17.7-1Promoter Transcription unit
RNA polymeraseStart point
1
Template strand of DNARNAtranscript
UnwoundDNA
Initiation
5′3′
5′3′
5′3′
5′3′
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Figure 17.7-2Promoter Transcription unit
RNA polymeraseStart point
1
Template strand of DNARNAtranscript
UnwoundDNA
RewoundDNA
RNAtranscript
Direction oftranscription(“downstream”)
Initiation
Elongation2
5′3′
5′
5′3′
5′3′
5′3′ 3′
5′3′
5′3′
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Figure 17.7-3Promoter Transcription unit
RNA polymeraseStart point
1
Template strand of DNARNAtranscript
UnwoundDNA
RewoundDNA
RNAtranscript
Direction oftranscription(“downstream”)
Completed RNA transcript
Initiation
Elongation
Termination
2
3
5′3′
5′
5′3′
5′3′
5′3′
5′
5′3′ 3′
5′3′
3′
5′3′
5′3′
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Animation: Transcription
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a) The DNA sequence where RNA polymerase attaches is called the promoter; in bacteria, the sequence signaling the end of transcription is called the terminator
b) The stretch of DNA that is transcribed is called a transcription unit
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Synthesis of an RNA Transcript
a) The three stages of transcription
a)Initiation
b)Elongation
c)Termination
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Figure 17.8 Promoter Nontemplate strand
15′3′
5′3′
Start point
RNA polymerase II
Templatestrand
TATA box
Transcriptionfactors
DNA3′5′
3′5′
3′5′
2
3
Transcription factors
RNA transcript
Transcription initiation complex
3′5′5′3′
A eukaryoticpromoter
Severaltranscriptionfactors bindto DNA.
Transcriptioninitiationcomplexforms.
T A T AAAATA A T T T T
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Elongation of the RNA Strand
a) As RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at a time
b) Transcription progresses at a rate of 40 nucleotides per second in eukaryotes
c) A gene can be transcribed simultaneously by several RNA polymerases
d) Nucleotides are added to the 3′ end of thegrowing RNA molecule
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Figure 17.9
Nontemplatestrand of DNA
5′
3′3′ end
5′
3′
5′
Direction of transcription
RNApolymerase
Templatestrand of DNA
Newly madeRNA
RNA nucleotides
A
A
AA
A AA
AA
CC
G G T TT
C C CU
U
C CT
TC
A
TT
GG
U
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Concept 17.3: Eukaryotic cells modify RNA after transcription
a) Enzymes in the eukaryotic nucleus modify pre-mRNA (RNA processing) before the genetic messages are dispatched to the cytoplasm
b) During RNA processing, both ends of the primary transcript are usually altered
c) Also, usually certain interior sections of the molecule are cut out, and the remaining parts spliced together
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Alteration of mRNA Ends
a) Each end of a pre-mRNA molecule is modified in a particular way
a)The 5′ end receives a modified nucleotide 5′ cap
b)The 3′ end gets a poly-A tail
b) These modifications share several functions
a)They seem to facilitate the export of mRNA to the cytoplasm
b)They protect mRNA from hydrolytic enzymes
c)They help ribosomes attach to the 5′ end
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Figure 17.10
A modified guaninenucleotide added tothe 5′ end
Region that includesprotein-coding segments
5′
5′ Cap
5′ UTR Startcodon
Stopcodon
G P P P
3′ UTR
3′AAUAAA AAA AAA
Poly-A tail
Polyadenylationsignal
50–250 adeninenucleotides addedto the 3′ end
…
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Split Genes and RNA Splicing
a) Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions
b) These noncoding regions are called intervening sequences, or introns
c) The other regions are called exons because they are eventually expressed, usually translated into amino acid sequences
d)RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence
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Figure 17.11
Pre-mRNA Intron Intron
Introns cut outand exonsspliced together
Poly-A tail5′ Cap
5′ Cap Poly-A tail
1–30 31–104 105–146
1–1463′ UTR5′ UTR
Codingsegment
mRNA
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Figure 17.12
Spliceosome Small RNAs
Exon 2
Cut-outintron
Spliceosomecomponents
mRNA
Exon 1 Exon 2
Pre-mRNA
Exon 1
Intron
5′
5′
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Concept 17.4: Translation is the RNA-directed synthesis of a polypeptide: A closer look
a) Genetic information flows from mRNA to protein through the process of translation
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Molecular Components of Translation
a) A cell translates an mRNA message into protein with the help of transfer RNA (tRNA)
b) tRNAs transfer amino acids to the growing polypeptide in a ribosome
c) Translation is a complex process in terms of its biochemistry and mechanics
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Figure 17.14
PolypeptideAminoacids
tRNA withamino acidattached
Ribosome
tRNA
Anticodon
Codons
mRNA
5′
U U U UG G G G C
A C C
A A A
CC
G
Phe
Trp
3′
Gly
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The Structure and Function of Transfer RNA
a) Molecules of tRNA are not identical
a)Each carries a specific amino acid on one end
b)Each has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA
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a) A tRNA molecule consists of a single RNA strand that is only about 80 nucleotides long
b) Flattened into one plane to reveal its base pairing, a tRNA molecule looks like a cloverleaf
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a) Because of hydrogen bonds, tRNA actually twists and folds into a three-dimensional molecule
b) tRNA is roughly L-shaped
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Figure 17.15
Amino acidattachmentsite Amino acid
attachment site5′
3′ACCACGCUUA
G
GC
GAUUUA
AGAA CC
CU*
**CG
G U UGC*
**
*C CUA G
GGGA
GAGC
CC
*U* G A
GGU**
*A
A
AG
CU
GAA
Hydrogenbonds
Hydrogenbonds
Anticodon Anticodon
Symbol usedin this book
Three-dimensionalstructure
(a) Two-dimensional structure (b) (c)Anticodon
5′3′A A G
3′5′
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Figure 17.15a
Amino acidattachmentsite 5′
3′
Hydrogenbonds
(a) Two-dimensional structureAnticodon
ACCAC
CG
GCUUAA
GGAUUUAA GCC
CA * C CU
A G **G
GGAGAGC
***
*
*
U
GC
CCAGA
CU
GAA
A*
**
U U
U
G GC
* G AGGU
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Figure 17.15b
Amino acidattachment site
Hydrogenbonds
Anticodon Anticodon
Symbol usedin this book
Three-dimensionalstructure
(b) (c)
5′3′
3′5′
A A G
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Video: Stick and Ribbon Rendering of a tRNA
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Ribosomes
a) Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis
b) The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA)
c) Bacterial and eukaryotic ribosomes are somewhat similar but have significant differences: some antibiotic drugs specifically target bacterial ribosomes without harming eukaryotic ribosomes
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Figure 17.17
Exit tunnel
Largesubunit
Smallsubunit
mRNA 3′5′
E P A
tRNAmolecules
Growingpolypeptide
(a) Computer model of functioning ribosome
Growingpolypeptide Next amino
acid to beadded topolypeptidechain
tRNA3′
5′
mRNA
Amino end
Codons
E
(c) Schematic model with mRNA and tRNA(b) Schematic model showing binding sites
Smallsubunit
Largesubunit
Exit tunnel
A site (Aminoacyl-tRNA binding site)
P site (Peptidyl-tRNA binding site)
E site(Exit site)
mRNAbinding site
E P A
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Figure 17.17a
Exit tunnel
Largesubunit
Smallsubunit
mRNA 3′5′
E P A
tRNAmolecules
Growingpolypeptide
(a) Computer model of functioning ribosome
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Figure 17.17b
(b) Schematic model showing binding sites
Smallsubunit
Largesubunit
Exit tunnel
A site (Aminoacyl-tRNA binding site)
P site (Peptidyl-tRNA binding site)
E site(Exit site)
mRNAbinding site
E P A
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Figure 17.17c
Growingpolypeptide
Next aminoacid to beadded topolypeptidechain
tRNA
3′
5′
mRNA
Amino end
Codons
E
(c) Schematic model with mRNA and tRNA
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Elongation of the Polypeptide Chain
a) During elongation, amino acids are added oneby one to the C-terminus of the growing chain
b) Each addition involves proteins called elongation factors and occurs in three steps: codon recognition, peptide bond formation, and translocation
c) Energy expenditure occurs in the first andthird steps
d) Translation proceeds along the mRNA in a5′ → 3′ direction
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Figure 17.19-1Amino endof polypeptide
Codonrecognition
13′
5′
E
P Asitesite
E
P A
mRNA
GTP
P iGDP +
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Figure 17.19-2Amino endof polypeptide
Codonrecognition
13′
5′
E
P Asitesite
E
P A
mRNA
GTP
P i
2
GDP +
E
P A
Peptide bondformation
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Figure 17.19-3Amino endof polypeptide
Codonrecognition
13′
5′
E
P Asitesite
E
P A
mRNA
GTP
P i
23 GTP
P i
GDP +
GDP +
Translocation
E
P A
Peptide bondformation
E
P A
Ribosome ready fornext aminoacyl tRNA
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Termination of Translation
a) Termination occurs when a stop codon in the mRNA reaches the A site of the ribosome
b) The A site accepts a protein called a release factor
c) The release factor causes the addition of a water molecule instead of an amino acid
d) This reaction releases the polypeptide, and the translation assembly comes apart
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Figure 17.20-1
1
Releasefactor
3′5′
Stop codon(UAG, UAA, or UGA)
Ribosome reaches astop codon on mRNA.
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Figure 17.20-2
1 2
Releasefactor
3′5′5′
3′
Stop codon(UAG, UAA, or UGA)
Ribosome reaches astop codon on mRNA.
Release factorpromotes hydrolysis.
Freepolypeptide
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Figure 17.20-3
31 2
Releasefactor
3′5′5′
3′
Stop codon(UAG, UAA, or UGA)
Ribosome reaches astop codon on mRNA.
Release factorpromotes hydrolysis.
Ribosomal subunitsand other componentsdissociate.
Freepolypeptide
3′
5′
2 GTP
2 GDP +2 P i
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Completing and Targeting the Functional Protein
a) Often translation is not sufficient to make a functional protein
b) Polypeptide chains are modified after translation or targeted to specific sites in the cell
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Protein Folding and Post-Translational Modifications
a) During its synthesis, a polypeptide chain begins to coil and fold spontaneously to form a protein with a specific shape—a three-dimensional molecule with secondary and tertiary structure
b) A gene determines primary structure, and primary structure in turn determines shape
c) Post-translational modifications may be required before the protein can begin doing its particular job in the cell
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Targeting Polypeptides to Specific Locations
a) Two populations of ribosomes are evident in cells: free ribosomes (in the cytosol) and bound ribosomes (attached to the ER)
b) Free ribosomes mostly synthesize proteins that function in the cytosol
c) Bound ribosomes make proteins of the endomembrane system and proteins that are secreted from the cell
d) Ribosomes are identical and can switch from free to bound
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a) In eukaryotes, the nuclear envelop separates the processes of transcription and translation
b) RNA undergoes processes before leavingthe nucleus
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Figure 17.24DNA
RNApolymerase
RNA transcript(pre-mRNA)Intron
Exon
Aminoacyl-tRNAsynthetase
AminoacidtRNA
Aminoacyl(charged)tRNA
mRNA
CYTOPLASM
NUCLEUS
RNAtranscript
3′
5′ Poly-
A
5′ Cap
5′ Cap
TRANSCRIPTION
RNAPROCESSING
Poly-A
Poly-A
AMINO ACIDACTIVATION
TRANSLATION
Ribosomalsubunits
E PA
E AA C C
U U U U U GGGA A A Anticodon
CodonRibosome
CAU
3′
A
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Figure 17.24a
TRANSCRIPTION DNA
Poly-A
Poly-A
5′ Cap
RNApolymerase
RNA transcript(pre-mRNA)Intron
Exon
Aminoacyl-tRNAsynthetase
AminoacidtRNA
AMINO ACIDACTIVATION
Aminoacyl(charged)tRNA
mRNA
CYTOPLASM
NUCLEUS
RNAPROCESSING
RNAtranscript
3′
5′
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Figure 17.24b
mRNA Growingpolypeptide
Ribosomalsubunits
Aminoacyl(charged)tRNA
Anticodon
TRANSLATION
Poly-A
5′ Cap
AE
U U U U U GGG AAAA
A C CC
UA
CodonRibosome
5′ Cap
E PA 3′
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BioFlix: Protein Synthesis
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Animation: Translation
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Figure 17.UN10