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LECTURE PRESENTATIONSFor CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
© 2011 Pearson Education, Inc.
Lectures byErin Barley
Kathleen Fitzpatrick
From Gene to Protein
Chapter 17
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Overview: The Flow of Genetic Information
• The information content of DNA is in the form of specific sequences of nucleotides
• The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins
• Proteins are the links between genotype and phenotype
• Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation
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Concept 17.1: Genes specify proteins via transcription and translation
• How was the fundamental relationship between genes and proteins discovered?
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Evidence from the Study of Metabolic Defects
• In 1902, British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions
• He thought symptoms of an inherited disease reflect an inability to synthesize a certain enzyme
• Linking genes to enzymes required understanding that cells synthesize and degrade molecules in a series of steps, a metabolic pathway
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Nutritional Mutants in Neurospora: Scientific Inquiry
• George Beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal media
• Using crosses, they and their coworkers identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for synthesizing arginine
• 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
Minimal medium
No growth:Mutant cellscannot growand divide
Growth:Wild-typecells growingand dividing
EXPERIMENT RESULTS
CONCLUSION
Classes of Neurospora crassa
Wild type Class I mutants Class II mutants Class III mutants
Minimalmedium(MM)(control)
MM +ornithine
MM +citrulline
Co
nd
itio
n
MM +arginine(control)
Summaryof results
Can grow withor without anysupplements
Can grow onornithine,citrulline, orarginine
Can grow onlyon citrulline orarginine
Require arginineto grow
Wild type
Class I mutants(mutation in
gene A)
Class II mutants(mutation in
gene B)
Class III mutants(mutation in
gene C)
Gene (codes forenzyme)
Gene A
Gene B
Gene C
Precursor Precursor Precursor PrecursorEnzyme A Enzyme A Enzyme A Enzyme A
Enzyme B Enzyme B Enzyme B Enzyme B
Enzyme C Enzyme C Enzyme C Enzyme C
Ornithine Ornithine Ornithine Ornithine
Citrulline Citrulline Citrulline Citrulline
Arginine Arginine Arginine Arginine
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Figure 17.2a
Minimal medium
No growth:Mutant cellscannot growand divide
Growth:Wild-typecells growingand dividing
EXPERIMENT
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Figure 17.2bRESULTS
Classes of Neurospora crassa
Wild type Class I mutants Class II mutants Class III mutants
Minimalmedium(MM)(control)
MM +ornithine
MM +citrullineC
on
dit
ion
MM +arginine(control)
Summaryof results
Can grow withor without anysupplements
Can grow onornithine,citrulline, orarginine
Can grow onlyon citrulline orarginine
Require arginineto grow
Growth Nogrowth
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Figure 17.2c
CONCLUSION
Wild type
Class I mutants(mutation in
gene A)
Class II mutants(mutation in
gene B)
Class III mutants(mutation in
gene C)
Gene (codes forenzyme)
Gene A
Gene B
Gene C
Precursor Precursor Precursor PrecursorEnzyme A Enzyme A Enzyme A Enzyme A
Enzyme B Enzyme B Enzyme B Enzyme B
Ornithine Ornithine Ornithine Ornithine
Enzyme C Enzyme C Enzyme CEnzyme C
Citrulline Citrulline Citrulline Citrulline
Arginine Arginine Arginine Arginine
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The Products of Gene Expression: A Developing Story
• Some proteins aren’t enzymes, so researchers later revised the hypothesis: one gene–one protein
• Many proteins are composed of several polypeptides, each of which has its own gene
• Therefore, Beadle and Tatum’s hypothesis is now restated as the one gene–one polypeptide hypothesis
• Note that it is common to refer to gene products as proteins rather than polypeptides
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Basic Principles of Transcription and Translation
• RNA is the bridge between genes and the proteins for which they code
• Transcription is the synthesis of RNA using information in DNA
• Transcription produces messenger RNA (mRNA)
• Translation is the synthesis of a polypeptide, using information in the mRNA
• Ribosomes are the sites of translation
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• In prokaryotes, translation of mRNA can begin before transcription has finished
• In a eukaryotic cell, the nuclear envelope separates transcription from translation
• Eukaryotic RNA transcripts are modified through RNA processing to yield the finished mRNA
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• A primary transcript is the initial RNA transcript from any gene prior to processing
• 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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Figure 17.3
DNA
mRNARibosome
Polypeptide
TRANSCRIPTION
TRANSLATION
TRANSCRIPTION
TRANSLATION
Polypeptide
Ribosome
DNA
mRNA
Pre-mRNARNA PROCESSING
(a) Bacterial cell (b) Eukaryotic cell
Nuclearenvelope
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Figure 17.3a-1
TRANSCRIPTIONDNA
mRNA
(a) Bacterial cell
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Figure 17.3a-2
TRANSCRIPTIONDNA
mRNA
(a) Bacterial cell
TRANSLATIONRibosome
Polypeptide
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Figure 17.3b-1
Nuclearenvelope
DNA
Pre-mRNA
(b) Eukaryotic cell
TRANSCRIPTION
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Figure 17.3b-2
RNA PROCESSING
Nuclearenvelope
DNA
Pre-mRNA
(b) Eukaryotic cell
mRNA
TRANSCRIPTION
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Figure 17.3b-3
RNA PROCESSING
Nuclearenvelope
DNA
Pre-mRNA
(b) Eukaryotic cell
mRNA
TRANSCRIPTION
TRANSLATION Ribosome
Polypeptide
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The Genetic Code
• How are the instructions for assembling amino acids into proteins encoded into DNA?
• There are 20 amino acids, but there are only four nucleotide bases in DNA
• How many nucleotides correspond to an amino acid?
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Codons: Triplets of Nucleotides
• The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words
• The words of a gene are transcribed into complementary nonoverlapping three-nucleotide words of mRNA
• These words are then translated into a chain of amino acids, forming a polypeptide
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Figure 17.4
DNAtemplatestrand
TRANSCRIPTION
mRNA
TRANSLATION
Protein
Amino acid
Codon
Trp Phe Gly
5′
5′
Ser
U U U U U3′
3′
5′3′
G
G
G G C C
T
C
A
A
AAAAA
T T T T
T
G
G G G
C C C G G
DNAmolecule
Gene 1
Gene 2
Gene 3
C C
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• 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
• The template strand is always the same strand for a given gene
• During translation, the mRNA base triplets, called codons, are read in the 5′ to 3′ direction
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• Codons along an mRNA molecule are read by translation machinery in the 5′ to 3′ direction
• Each codon specifies the amino acid (one of 20) to be placed at the corresponding position along a polypeptide
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Cracking the Code
• All 64 codons were deciphered by the mid-1960s• Of the 64 triplets, 61 code for amino acids; 3
triplets are “stop” signals to end translation• The genetic code is redundant (more than one
codon may specify a particular amino acid) but not ambiguous; no codon specifies more than one amino acid
• 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
Fir
st m
RN
A b
ase
(5′ e
nd
of
cod
on
)
Th
ird
mR
NA
bas
e (3
′ en
d o
f co
do
n)
UUU
UUC
UUA
CUU
CUC
CUA
CUG
Phe
Leu
Leu
Ile
UCU
UCC
UCA
UCG
Ser
CCU
CCC
CCA
CCG
UAU
UACTyr
Pro
Thr
UAA Stop
UAG Stop
UGA Stop
UGU
UGCCys
UGG Trp
GC
U
U
C
A
U
U
C
C
CA
U
A
A
A
G
G
His
Gln
Asn
Lys
Asp
CAU CGU
CAC
CAA
CAG
CGC
CGA
CGG
G
AUU
AUC
AUA
ACU
ACC
ACA
AAU
AAC
AAA
AGU
AGC
AGA
Arg
Ser
Arg
Gly
ACGAUG AAG AGG
GUU
GUC
GUA
GUG
GCU
GCC
GCA
GCG
GAU
GAC
GAA
GAG
Val Ala
GGU
GGC
GGA
GGGGlu
Gly
G
U
C
A
Met orstart
UUG
G
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Evolution of the Genetic Code
• The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals
• Genes can be transcribed and translated after being transplanted from one species to another
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Figure 17.6
(a) Tobacco plant expressing a firefly gene gene
(b) Pig expressing a jellyfish
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Figure 17.6a
(a) Tobacco plant expressinga firefly gene
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Figure 17.6b
(b) Pig expressing a jellyfishgene
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Concept 17.2: Transcription is the DNA-directed synthesis of RNA: a closer look
• Transcription is the first stage of gene expression
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Molecular Components of Transcription
• RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides
• The RNA is complementary to the DNA template strand
• RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine
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• The DNA sequence where RNA polymerase attaches is called the promoter; in bacteria, the sequence signaling the end of transcription is called the terminator
• The stretch of DNA that is transcribed is called a transcription unit
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Animation: Transcription
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Figure 17.7-1 Promoter
RNA polymeraseStart point
DNA
5′3′
Transcription unit
3′5′
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Figure 17.7-2 Promoter
RNA polymeraseStart point
DNA
5′3′
Transcription unit
3′5′
Initiation
5′3′
3′5′
Nontemplate strand of DNA
Template strand of DNARNAtranscriptUnwound
DNA
1
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Figure 17.7-3 Promoter
RNA polymeraseStart point
DNA
5′3′
Transcription unit
3′5′
Elongation
5′3′
3′5′
Nontemplate strand of DNA
Template strand of DNARNAtranscriptUnwound
DNA2
3′5′3′5′
3′
RewoundDNA
RNAtranscript
5′
Initiation1
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Figure 17.7-4 Promoter
RNA polymeraseStart point
DNA
5′3′
Transcription unit
3′5′
Elongation
5′3′
3′5′
Nontemplate strand of DNA
Template strand of DNARNAtranscriptUnwound
DNA2
3′5′3′5′
3′
RewoundDNA
RNAtranscript
5′
Termination3
3′5′
5′Completed RNA transcript
Direction of transcription (“downstream”)
5′3′
3′
Initiation1
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Synthesis of an RNA Transcript
• The three stages of transcription– Initiation
– Elongation– Termination
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RNA Polymerase Binding and Initiation of Transcription
• Promoters signal the transcriptional start point and usually extend several dozen nucleotide pairs upstream of the start point
• Transcription factors mediate the binding of RNA polymerase and the initiation of transcription
• The completed assembly of transcription factors and RNA polymerase II bound to a promoter is called a transcription initiation complex
• A promoter called a TATA box is crucial in forming the initiation complex in eukaryotes
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Figure 17.8
Transcription initiationcomplex forms
3
DNAPromoter
Nontemplate strand
5′3′
5′3′
5′3′
Transcriptionfactors
RNA polymerase II
Transcription factors
5′3′
5′3′
5′3′
RNA transcript
Transcription initiation complex
5′ 3′
TATA box
T
T T T T T
A A A AA
A A
T
Several transcriptionfactors bind to DNA
2
A eukaryotic promoter1
Start point Template strand
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Elongation of the RNA Strand
• As RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at a time
• Transcription progresses at a rate of 40 nucleotides per second in eukaryotes
• A gene can be transcribed simultaneously by several RNA polymerases
• Nucleotides are added to the 3′ end of the growing RNA molecule
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Nontemplatestrand of DNA
RNA nucleotides
RNApolymerase
Templatestrand of DNA
3′
3′5′
5′
5′
3′
Newly madeRNA
Direction of transcription
A
A A A
AA
A
T
TT
T
TTT G
GG
C
C C
CC
G
C CC A AA
U
U
U
end
Figure 17.9
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Termination of Transcription
• The mechanisms of termination are different in bacteria and eukaryotes
• In bacteria, the polymerase stops transcription at the end of the terminator and the mRNA can be translated without further modification
• In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequence; the RNA transcript is released 10–35 nucleotides past this polyadenylation sequence
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Concept 17.3: Eukaryotic cells modify RNA after transcription
• Enzymes in the eukaryotic nucleus modify pre-mRNA (RNA processing) before the genetic messages are dispatched to the cytoplasm
• During RNA processing, both ends of the primary transcript are usually altered
• Also, usually some interior parts of the molecule are cut out, and the other parts spliced together
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Alteration of mRNA Ends
• Each end of a pre-mRNA molecule is modified in a particular way
– The 5′ end receives a modified nucleotide 5′ cap
– The 3′ end gets a poly-A tail• These modifications share several functions
– They seem to facilitate the export of mRNA to the cytoplasm
– They protect mRNA from hydrolytic enzymes– They help ribosomes attach to the 5′ end
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Figure 17.10
Protein-codingsegment
Polyadenylationsignal
5′ 3′
3′5′ 5′Cap UTRStartcodon
G P P P
Stopcodon
UTR
AAUAAA
Poly-A tail
AAA AAA…
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Split Genes and RNA Splicing
• Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions
• These noncoding regions are called intervening sequences, or introns
• The other regions are called exons because they are eventually expressed, usually translated into amino acid sequences
• RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence
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Figure 17.11
5′ Exon Intron Exon
5′ CapPre-mRNACodonnumbers
1−30 31−104
mRNA 5′ Cap
5′
Intron Exon
3′ UTR
Introns cut out andexons spliced together
3′
105− 146
Poly-A tail
Codingsegment
Poly-A tail
UTR1−146
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• In some cases, RNA splicing is carried out by spliceosomes
• Spliceosomes consist of a variety of proteins and several small nuclear ribonucleoproteins (snRNPs) that recognize the splice sites
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Figure 17.12-1RNA transcript (pre-mRNA)
5′Exon 1
Protein
snRNA
snRNPs
Intron Exon 2
Other proteins
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Figure 17.12-2RNA transcript (pre-mRNA)
5′Exon 1
Protein
snRNA
snRNPs
Intron Exon 2
Other proteins
Spliceosome
5′
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Figure 17.12-3RNA transcript (pre-mRNA)
5′Exon 1
Protein
snRNA
snRNPs
Intron Exon 2
Other proteins
Spliceosome
5′
Spliceosomecomponents
Cut-outintronmRNA
5′Exon 1 Exon 2
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Ribozymes
• Ribozymes are catalytic RNA molecules that function as enzymes and can splice RNA
• The discovery of ribozymes rendered obsolete the belief that all biological catalysts were proteins
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• Three properties of RNA enable it to function as an enzyme
– It can form a three-dimensional structure because of its ability to base-pair with itself
– Some bases in RNA contain functional groups that may participate in catalysis
– RNA may hydrogen-bond with other nucleic acid molecules
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The Functional and Evolutionary Importance of Introns
• Some introns contain sequences that may regulate gene expression
• Some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during splicing
• This is called alternative RNA splicing• Consequently, the number of different proteins an
organism can produce is much greater than its number of genes
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• Proteins often have a modular architecture consisting of discrete regions called domains
• In many cases, different exons code for the different domains in a protein
• Exon shuffling may result in the evolution of new proteins
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GeneDNA
Exon 1 Exon 2 Exon 3Intron Intron
Transcription
RNA processing
Translation
Domain 3
Domain 2
Domain 1
Polypeptide
Figure 17.13
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Concept 17.4: Translation is the RNA-directed synthesis of a polypeptide: a closer look
• Genetic information flows from mRNA to protein through the process of translation
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Molecular Components of Translation
• A cell translates an mRNA message into protein with the help of transfer RNA (tRNA)
• tRNAs transfer amino acids to the growing polypeptide in a ribosome
• Translation is a complex process in terms of its biochemistry and mechanics
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Figure 17.14
Polypeptide
Ribosome
Trp
Phe Gly
tRNA withamino acidattached
Aminoacids
tRNA
Anticodon
Codons
U U U UG G G G C
AC C
C
CG
A A A
CGC
G
5′ 3′mRNA
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The Structure and Function of Transfer RNA
• Molecules of tRNA are not identical– Each carries a specific amino acid on one end
– Each has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA
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BioFlix: Protein Synthesis
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• A tRNA molecule consists of a single RNA strand that is only about 80 nucleotides long
• Flattened into one plane to reveal its base pairing, a tRNA molecule looks like a cloverleaf
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Figure 17.15
Amino acidattachmentsite
3′
5′
Hydrogenbonds
Anticodon
(a) Two-dimensional structure (b) Three-dimensional structure(c) Symbol used
in this book
Anticodon Anticodon3′ 5′
Hydrogenbonds
Amino acidattachmentsite5′
3′
A A G
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Figure 17.15a
Amino acidattachmentsite
3′
5′
Hydrogenbonds
Anticodon
(a) Two-dimensional structure
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(b) Three-dimensional structure(c) Symbol used
Anticodon Anticodon3′ 5′
Hydrogenbonds
Amino acidattachmentsite5′
3′
in this book
A A G
Figure 17.15b
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• Because of hydrogen bonds, tRNA actually twists and folds into a three-dimensional molecule
• tRNA is roughly L-shaped
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• Accurate translation requires two steps– First: a correct match between a tRNA and an
amino acid, done by the enzyme aminoacyl-tRNA synthetase
– Second: a correct match between the tRNA anticodon and an mRNA codon
• Flexible pairing at the third base of a codon is called wobble and allows some tRNAs to bind to more than one codon
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Aminoacyl-tRNAsynthetase (enzyme)
Amino acid
P P P Adenosine
ATP
Figure 17.16-1
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Aminoacyl-tRNAsynthetase (enzyme)
Amino acid
P P P Adenosine
ATP
P
P
P
PPi
i
i
Adenosine
Figure 17.16-2
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Aminoacyl-tRNAsynthetase (enzyme)
Amino acid
P P P Adenosine
ATP
P
P
P
PPi
i
i
Adenosine
tRNA
AdenosineP
tRNA
AMP
Computer model
Aminoacid
Aminoacyl-tRNAsynthetase
Figure 17.16-3
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Aminoacyl-tRNAsynthetase (enzyme)
Amino acid
P P P Adenosine
ATP
P
P
P
PPi
i
i
Adenosine
tRNA
AdenosineP
tRNA
AMP
Computer model
Aminoacid
Aminoacyl-tRNAsynthetase
Aminoacyl tRNA(“charged tRNA”)
Figure 17.16-4
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Ribosomes
• Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis
• The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA)
• 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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tRNAmolecules
Growingpolypeptide Exit tunnel
E PA
Largesubunit
Smallsubunit
mRNA5′
3′
(a) Computer model of functioning ribosome
Exit tunnel Amino end
A site (Aminoacyl-tRNA binding site)
Smallsubunit
Largesubunit
E P AmRNA
E
P site (Peptidyl-tRNAbinding site)
mRNAbinding site
(b) Schematic model showing binding sites
E site (Exit site)
(c) Schematic model with mRNA and tRNA
5′ Codons
3′tRNA
Growing polypeptide
Next aminoacid to beadded topolypeptidechain
Figure 17.17
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Figure 17.17a
tRNAmolecules
Growingpolypeptide Exit tunnel
E P A
Largesubunit
Smallsubunit
mRNA5′
3′
(a) Computer model of functioning ribosome
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Figure 17.17b
Exit tunnel
A site (Aminoacyl-tRNA binding site)
Smallsubunit
Largesubunit
P A
P site (Peptidyl-tRNAbinding site)
mRNAbinding site
(b) Schematic model showing binding sites
E site (Exit site)
E
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Figure 17.17c
Amino end
mRNAE
(c) Schematic model with mRNA and tRNA
5′ Codons
3′tRNA
Growing polypeptide
Next aminoacid to beadded topolypeptidechain
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• A ribosome has three binding sites for tRNA– The P site holds the tRNA that carries the
growing polypeptide chain
– The A site holds the tRNA that carries the next amino acid to be added to the chain
– The E site is the exit site, where discharged tRNAs leave the ribosome
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Building a Polypeptide
• The three stages of translation– Initiation– Elongation
– Termination
• All three stages require protein “factors” that aid in the translation process
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Ribosome Association and Initiation of Translation
• The initiation stage of translation brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits
• First, a small ribosomal subunit binds with mRNA and a special initiator tRNA
• Then the small subunit moves along the mRNA until it reaches the start codon (AUG)
• Proteins called initiation factors bring in the large subunit that completes the translation initiation complex
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Figure 17.18
InitiatortRNA
mRNA
5′
5′3′Start codon
Smallribosomalsubunit
mRNA binding site
3′
Translation initiation complex
5′ 3′3′ U
UA
A GC
P
P site
i+
GTP GDP
Met Met
Largeribosomalsubunit
E A
5′
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Elongation of the Polypeptide Chain
• During the elongation stage, amino acids are added one by one to the preceding amino acid at the C-terminus of the growing chain
• Each addition involves proteins called elongation factors and occurs in three steps: codon recognition, peptide bond formation, and translocation
• Translation proceeds along the mRNA in a 5′ to 3′ direction
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Amino end ofpolypeptide
mRNA
5′
E
Psite
Asite
3′
Figure 17.19-1
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Amino end ofpolypeptide
mRNA
5′
E
Psite
Asite
3′
E
GTP
GDP + P i
P A
Figure 17.19-2
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Amino end ofpolypeptide
mRNA
5′
E
Psite
Asite
3′
E
GTP
GDP + P i
P A
E
P A
Figure 17.19-3
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Amino end ofpolypeptide
mRNA
5′
E
Asite
3′
E
GTP
GDP + P i
P A
E
P A
GTP
GDP + P i
P A
E
Ribosome ready fornext aminoacyl tRNA
Psite
Figure 17.19-4
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Termination of Translation
• Termination occurs when a stop codon in the mRNA reaches the A site of the ribosome
• The A site accepts a protein called a release factor
• The release factor causes the addition of a water molecule instead of an amino acid
• This reaction releases the polypeptide, and the translation assembly then comes apart
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Animation: Translation
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Figure 17.20-1
Releasefactor
Stop codon(UAG, UAA, or UGA)
3′
5′
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Figure 17.20-2
Releasefactor
Stop codon(UAG, UAA, or UGA)
3′
5′
3′
5′
Freepolypeptide
2 GTP
2 GDP + 2 iP
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Figure 17.20-3
Releasefactor
Stop codon(UAG, UAA, or UGA)
3′
5′
3′
5′
Freepolypeptide
2 GTP
5′
3′
2 GDP + 2 iP
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Polyribosomes
• A number of ribosomes can translate a single mRNA simultaneously, forming a polyribosome (or polysome)
• Polyribosomes enable a cell to make many copies of a polypeptide very quickly
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Figure 17.21Completedpolypeptide
Incomingribosomalsubunits
Start ofmRNA(5′ end)
End ofmRNA(3′ end)(a)
Polyribosome
Ribosomes
mRNA
(b)0.1 µm
Growingpolypeptides
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Figure 17.21a
Ribosomes
mRNA
0.1 µm
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Completing and Targeting the Functional Protein
• Often translation is not sufficient to make a functional protein
• Polypeptide chains are modified after translation or targeted to specific sites in the cell
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Protein Folding and Post-Translational Modifications
• During and after synthesis, a polypeptide chain spontaneously coils and folds into its three-dimensional shape
• Proteins may also require post-translational modifications before doing their job
• Some polypeptides are activated by enzymes that cleave them
• Other polypeptides come together to form the subunits of a protein
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Targeting Polypeptides to Specific Locations
• Two populations of ribosomes are evident in cells: free ribsomes (in the cytosol) and bound ribosomes (attached to the ER)
• Free ribosomes mostly synthesize proteins that function in the cytosol
• Bound ribosomes make proteins of the endomembrane system and proteins that are secreted from the cell
• Ribosomes are identical and can switch from free to bound
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• Polypeptide synthesis always begins in the cytosol
• Synthesis finishes in the cytosol unless the polypeptide signals the ribosome to attach to the ER
• Polypeptides destined for the ER or for secretion are marked by a signal peptide
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• A signal-recognition particle (SRP) binds to the signal peptide
• The SRP brings the signal peptide and its ribosome to the ER
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Figure 17.22
Ribosome
mRNA
Signalpeptide
SRP
1
SRPreceptorprotein
Translocationcomplex
ERLUMEN
2
3
45
6
Signalpeptideremoved
CYTOSOL
Protein
ERmembrane
Page 101
Concept 17.5: Mutations of one or a few nucleotides can affect protein structure and function
• Mutations are changes in the genetic material of a cell or virus
• Point mutations are chemical changes in just one base pair of a gene
• The change of a single nucleotide in a DNA template strand can lead to the production of an abnormal protein
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Figure 17.23
Wild-type hemoglobin
Wild-type hemoglobin DNA3′
3′
3′5′
5′ 3′
3′5′
5′5′5′3′
mRNA
A AGC T T
A AGmRNA
Normal hemoglobin
Glu
Sickle-cell hemoglobin
Val
AA
AUG
GT
T
Sickle-cell hemoglobin
Mutant hemoglobin DNAC
Page 103
Types of Small-Scale Mutations
• Point mutations within a gene can be divided into two general categories
– Nucleotide-pair substitutions– One or more nucleotide-pair insertions or
deletions
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Substitutions
• A nucleotide-pair substitution replaces one nucleotide and its partner with another pair of nucleotides
• Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the genetic code
• Missense mutations still code for an amino acid, but not the correct amino acid
• Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein
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Wild type
DNA template strand
mRNA5′
5′
3′
Protein
Amino end
A instead of G
(a) Nucleotide-pair substitution
3′
3′
5′
Met Lys Phe Gly StopCarboxyl end
T T T T T
TTTTTA A A A A
AAAACC
C
C
A
A A A A A
G G G G
GC C
G GGU U U U UG
(b) Nucleotide-pair insertion or deletion
Extra A
3′5′
5′3′
Extra U
5′ 3′
T T T T
T T T T
A
A A A
A
AT G G G G
GAAA
AC
CCCC A
T3′5′
5′ 3′
5′T T T T TAAAACCA AC C
TTTTTA A A A ATG G G G
U instead of C
Stop
UA A A A AG GGU U U U UG
MetLys Phe Gly
Silent (no effect on amino acid sequence)
T instead of C
T T T T TAAAACCA GT C
T A T T TAAAACCA GC C
A instead of G
CA A A A AG AGU U U U UG UA A A AG GGU U U G AC
AA U U A AU UGU G G C UA
GA U A U AA UGU G U U CG
Met Lys Phe Ser
Stop
Stop Met Lys
missing
missing
Frameshift causing immediate nonsense(1 nucleotide-pair insertion)
Frameshift causing extensive missense (1 nucleotide-pair deletion)
missing
T T T T TTCAACCA AC G
AGTTTA A A A ATG G G C
Leu Ala
Missense
A instead of T
TTTTTA A A A ACG G A G
A
CA U A A AG GGU U U U UG
TTTTTA T A A ACG G G G
Met
Nonsense
Stop
U instead of A
3′5′
3′5′
5′3′
3′5′
5′3′
3′5′ 3′Met Phe Gly
No frameshift, but one amino acid missing(3 nucleotide-pair deletion)
missing
3′5′
5′3′
5′ 3′U
T CA AA CA TTAC G
TA G T T T G G A ATC
T T C
A A G
Met
3′
T
A
Stop
3′5′
5′3′
5′ 3′
Figure 17.24
Page 106
Figure 17.24a
Wild type
DNA template strand
mRNA5′
5′
Protein
Amino endStopCarboxyl end
3′3′
3′
5′
Met Lys Phe Gly
A instead of G
(a) Nucleotide-pair substitution: silent
StopMet Lys Phe Gly
U instead of C
A
A
A A
A A A A
A AT
T T T T T
T T TT
C C C C
C
C
G G G G
G
G
A
A A A AG GGU U U U U
5′3′
3′5′A
A A
A A A A
A AT
T T T T T
T T TT
C C C C
G G G G
A
A
A G A A A AG GGU U U U U
T
U 3′5′
Page 107
Figure 17.24b
Wild type
DNA template strand
mRNA5′
5′
Protein
Amino endStopCarboxyl end
3′3′
3′
5′
Met Lys Phe Gly
T instead of C
(a) Nucleotide-pair substitution: missense
StopMet Lys Phe Ser
A instead of G
A
A
A A
A A A A
A AT
T T T T T
T T TT
C C C C
C
C
G G G G
G
G
A
A A A AG GGU U U U U
5′3′
3′5′A
A A
A A A A
A AT
T T T T T
T T TT
C C T C
G
G
GA
A G A A A AA GGU U U U U 3′5′
A C
C
G
Page 108
Figure 17.24c
Wild type
DNA template strand
mRNA5′
5′
Protein
Amino endStopCarboxyl end
3′3′
3′
5′
Met Lys Phe Gly
A instead of T
(a) Nucleotide-pair substitution: nonsense
Met
A
A
A A
A A A A
A AT
T T T T T
T T TT
C C C C
C
C
G G G G
G
G
A
A A A AG GGU U U U U
5′3′
3′5′A
A
A A A A
A AT
T A T T T
T T TT
C C C
G
G
GA
A G U A A AGGU U U U U 3′5′
C
C
G
T instead of C
C
GT
U instead of A
G
Stop
Page 109
Insertions and Deletions
• Insertions and deletions are additions or losses of nucleotide pairs in a gene
• These mutations have a disastrous effect on the resulting protein more often than substitutions do
• Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation
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Figure 17.24d
Wild type
DNA template strand
mRNA5′
5′
Protein
Amino endStopCarboxyl end
3′3′
3′
5′
Met Lys Phe Gly
A
A
A A
A A A A
A AT
T T T T T
T T TT
C C C C
C
C
G G G G
G
G
A
A A A AG GGU U U U U
(b) Nucleotide-pair insertion or deletion: frameshift causingimmediate nonsense
Extra A
Extra U
5′3′
5′
3′
3′
5′
Met
1 nucleotide-pair insertion
Stop
A C A A GT T A TC T A C G
T A T AT G T CT GG A T GA
A G U A U AU GAU G U U C
A T
A
AG
Page 111
Figure 17.24e
DNA template strand
mRNA5′
5′
Protein
Amino endStopCarboxyl end
3′3′
3′
5′
Met Lys Phe Gly
A
A
A A
A A A A
A AT
T T T T T
T T TT
C C C C
C
C
G G G G
G
G
A
A A A AG GGU U U U U
(b) Nucleotide-pair insertion or deletion: frameshift causingextensive missense
Wild type
missing
missing
A
U
A A AT T TC C A T TC C G
A AT T TG GA A ATCG G
A G A A GU U U C A AG G U 3′
5′3′
3′5′
Met Lys Leu Ala
1 nucleotide-pair deletion
5′
Page 112
Figure 17.24f
DNA template strand
mRNA5′
5′
Protein
Amino endStopCarboxyl end
3′3′
3′
5′
Met Lys Phe Gly
A
A
A A
A A A A
A AT
T T T T T
T T TT
C C C C
C
C
G G G G
G
G
A
A A A AG GGU U U U U
(b) Nucleotide-pair insertion or deletion: no frameshift, but oneamino acid missing
Wild type
AT C A A A A T TC C G
T T C missing
missing
Stop
5′3′
3′5′
3′5′
Met Phe Gly
3 nucleotide-pair deletion
A GU C A AG GU U U U
T GA A AT T TT CG G
A A G
Page 113
Mutagens
• Spontaneous mutations can occur during DNA replication, recombination, or repair
• Mutagens are physical or chemical agents that can cause mutations
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Concept 17.6: While gene expression differs among the domains of life, the concept of a gene is universal
• Archaea are prokaryotes, but share many features of gene expression with eukaryotes
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Comparing Gene Expression in Bacteria, Archaea, and Eukarya
• Bacteria and eukarya differ in their RNA polymerases, termination of transcription, and ribosomes; archaea tend to resemble eukarya in these respects
• Bacteria can simultaneously transcribe and translate the same gene
• In eukarya, transcription and translation are separated by the nuclear envelope
• In archaea, transcription and translation are likely coupled
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Figure 17.25
RNA polymerase
DNAmRNA
Polyribosome
RNA polymerase DNA
Polyribosome
Polypeptide(amino end)
mRNA (5′ end)
Ribosome
0.25 µmDirection oftranscription
Page 117
Figure 17.25a
RNA polymerase
DNAmRNA
Polyribosome
0.25 µm
Page 118
What Is a Gene? Revisiting the Question
• The idea of the gene has evolved through the history of genetics
• We have considered a gene as– A discrete unit of inheritance
– A region of specific nucleotide sequence in a chromosome
– A DNA sequence that codes for a specific polypeptide chain
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Figure 17.26TRANSCRIPTION
DNA
RNApolymerase
ExonRNAtranscript
RNAPROCESSING
NUCLEUS
Intron
RNA transcript(pre-mRNA)
Poly-A
Poly-
A
Aminoacyl-tRNA synthetase
AMINO ACIDACTIVATION
Aminoacid
tRNA
5′ Cap
Poly-A
3′
GrowingpolypeptidemRNA
Aminoacyl(charged)tRNA
Anticodon
Ribosomalsubunits
A
AETRANSLATION
5′ Cap
CYTOPLASM
P
E
Codon
Ribosome
5′
3′
Page 120
• In summary, a gene can be defined as a region of DNA that can be expressed to produce a final functional product, either a polypeptide or an RNA molecule
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Figure 17.UN02
Transcription unit
RNA polymerase
Promoter
RNA transcript
5′
5′3′
3′ 3′5′
Template strandof DNA
Page 122
Figure 17.UN03
Pre-mRNA
mRNA
Poly-A tail5′ Cap
Page 123
Figure 17.UN04
tRNA
Polypeptide
Aminoacid
E A Anti-codon
Ribosome mRNACodon
Page 124
Figure 17.UN05
Type of RNA Functions
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Primary transcript
Small nuclear RNA (snRNA)
Plays catalytic (ribozyme) roles andstructural roles in ribosomes