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Gene Expression: From Gene to Protein. 0. 17. Lecture Presentation by Nicole Tunbridge and Kathleen Fitzpatrick. The Flow of Genetic Information. The information content of genes is in the specific sequences of nucleotides - PowerPoint PPT Presentation
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CAMPBELL
BIOLOGYReece • Urry • Cain • Wasserman • Minorsky • Jackson
© 2014 Pearson Education, Inc.
TENTHEDITION
CAMPBELL
BIOLOGYReece • Urry • Cain • Wasserman • Minorsky • Jackson
TENTHEDITION
17Gene Expression: From Gene to Protein
Lecture Presentation by Nicole Tunbridge andKathleen Fitzpatrick
© 2014 Pearson Education, Inc.
The Flow of Genetic Information
The information content of genes is in the specific sequences of nucleotides
The DNA inherited by an organism leads tospecific 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
© 2014 Pearson Education, Inc.
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
Cells synthesize and degrade molecules in a series of steps, a metabolic pathway
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
It is common to refer to gene products as proteins rather than polypeptides
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Figure 17.UN01
DNA RNA Protein
© 2014 Pearson Education, Inc.
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 anamino acid?
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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′
© 2014 Pearson Education, Inc.
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 strandfor a given gene
© 2014 Pearson Education, Inc.
During translation, the mRNA base triplets, called codons, are read in the 5′ → 3′ direction
Each codon specifies the amino acid (one of 20)to be placed at the corresponding position alonga polypeptide
© 2014 Pearson Education, Inc.
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) butnot ambiguous; no codon specifies more thanone amino acid
Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced
© 2014 Pearson Education, Inc.
Figure 17.5Second mRNA base
Th
ird
mR
NA
ba
se (
3′ e
nd
of
cod
on
)
Fir
st
mR
NA
ba
se (
5′ e
nd
of
cod
on
)
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
Ala
Glu
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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Figure 17.6
Pig expressing a jellyfishgene
(b)Tobacco plant expressinga firefly gene
(a)
© 2014 Pearson Education, Inc.
Concept 17.2: Transcription is the DNA-directed synthesis of RNA: A closer look
Transcription is the first stage of gene expression
© 2014 Pearson Education, Inc.
Molecular Components of Transcription
RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and joins together the RNA nucleotides
The RNA is complementary to the DNA template strand
RNA polymerase does not need any primer
RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine
© 2014 Pearson Education, Inc.
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′
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Synthesis of an RNA Transcript
The three stages of transcription
Initiation
Elongation
Termination
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Figure 17.8Promoter 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 AAAA
TA A T T T T
© 2014 Pearson Education, Inc.
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 thegrowing RNA molecule
© 2014 Pearson Education, Inc.
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
CC CU
U
C CT
TC
A
TT
GG
U
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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 certain interior sections of the molecule are cut out, and the remaining parts spliced together
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
…
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
The RNAs of the spliceosome also catalyze the splicing reaction
© 2014 Pearson Education, Inc.
Figure 17.12
Spliceosome Small RNAs
Exon 2
Cut-outintron
Spliceosomecomponents
mRNA
Exon 1 Exon 2
Pre-mRNA
Exon 1
Intron
5′
5′
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Three properties of RNA enable it to function asan enzyme
It can form a three-dimensional structure because of its ability to base-pair with itself
Some bases in RNA contain functional groupsthat may participate in catalysis
RNA may hydrogen-bond with other nucleicacid molecules
© 2014 Pearson Education, Inc.
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 proteinsan organism can produce is much greater thanits number of genes
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Figure 17.13
GeneDNA
Transcription
RNA processing
Translation
Domain 3
Exon 1 Exon 3Exon 2 IntronIntron
Domain 2
Domain 1
Polypeptide
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Because of hydrogen bonds, tRNA actually twists and folds into a three-dimensional molecule
tRNA is roughly L-shaped
© 2014 Pearson Education, Inc.
Figure 17.15
Amino acidattachmentsite
Amino acidattachment site
5′
3′ACCACGCUUA
G
GC
GAUUU
A
AGAA CC
CU*
**C
GG U UG
C*
**
*C CUA G
GG
GAGAGC
CC
*U* G A
GGU
**
*A
A
A
G
C
UG
AA
Hydrogenbonds
Hydrogenbonds
Anticodon Anticodon
Symbol usedin this book
Three-dimensionalstructure
(a) Two-dimensional structure (b) (c)
Anticodon
5′3′A A G
3′
5′
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Figure 17.16-3
Aminoacyl-tRNAsynthetase
1 Amino acidand tRNAenter activesite.
Tyr-tRNA
Tyrosine (Tyr)(amino acid)
Tyrosyl-tRNAsynthetase
ComplementarytRNA anticodon
A AU
tRNA
Aminoacid
Using ATP,synthetasecatalyzescovalentbonding.
23
ATP
AMP + 2P i
AminoacyltRNAreleased.
Computer model
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Figure 17.17a
Exit tunnel
Largesubunit
Smallsubunit
mRNA 3′5′
E P A
tRNAmolecules
Growingpolypeptide
(a) Computer model of functioning ribosome
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Figure 17.17c
Growingpolypeptide
Next aminoacid to beadded topolypeptidechain
tRNA
3′
5′
mRNA
Amino end
Codons
E
(c) Schematic model with mRNA and tRNA
© 2014 Pearson Education, Inc.
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
Energy is required for some steps also
© 2014 Pearson Education, Inc.
Ribosome Association and Initiation of Translation
Initiation 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
© 2014 Pearson Education, Inc.
Figure 17.18
1 2
P site3′
Largeribosomalsubunit
Translation initiation complex
Large ribosomal subunitcompletes the initiationcomplex.
Small ribosomal subunit binds to mRNA.
mRNA binding site
Smallribosomalsubunit
InitiatortRNA
Start codon3′5′
E A
3′5′
GTP GDP
P i+
3′ 5′5′
U A C
GA UMetMet
mRNA
© 2014 Pearson Education, Inc.
Elongation of the Polypeptide Chain
During elongation, amino acids are added oneby one to 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
Energy expenditure occurs in the first andthird steps
Translation proceeds along the mRNA in a5′ → 3′ direction
© 2014 Pearson Education, Inc.
Figure 17.19-1Amino endof polypeptide
Codonrecognition
1
3′
5′
E
P Asitesite
E
P A
mRNA
GTP
P iGDP +
© 2014 Pearson Education, Inc.
Figure 17.19-2Amino endof polypeptide
Codonrecognition
1
3′
5′
E
P Asitesite
E
P A
mRNA
GTP
P i
2
GDP +
E
P A
Peptide bondformation
© 2014 Pearson Education, Inc.
Figure 17.19-3Amino endof polypeptide
Codonrecognition
1
3′
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
© 2014 Pearson Education, Inc.
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 comes apart
© 2014 Pearson Education, Inc.
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
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 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
A gene determines primary structure, and primary structure in turn determines shape
Post-translational modifications may be required before the protein can begin doing its particular job in the cell
© 2014 Pearson Education, Inc.
Targeting Polypeptides to Specific Locations
Two populations of ribosomes are evident in cells: free ribosomes (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 tothe 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 17.21
Protein
ERmembrane
Signalpeptideremoved
SRP receptorprotein
Translocation complex
ER LUMEN
CYTOSOL
SRP
Signalpeptide
mRNA
Ribosome
Polypeptidesynthesisbegins.
SRPbinds tosignalpeptide.
SRPbinds toreceptorprotein.
SRPdetachesandpolypeptidesynthesisresumes.
Signal-cleavingenzyme cutsoff signalpeptide.
Completedpolypeptidefolds intofinalconformation.
1 2 3 4 5 6
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Making Multiple Polypeptides in Bacteria and Eukaryotes
Multiple 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 17.22
0.1 µm
mRNA
Ribosomes
A large polyribosome in a bacterialcell (TEM)
(b)
Several ribosomes simultaneously translatingone mRNA molecule
(a)
Incomingribosomalsubunits
Completedpolypeptide
Growingpolypeptides
End ofmRNA(3′ end)
PolyribosomeStart ofmRNA(5′ end)
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A bacterial cell ensures a streamlined process by coupling transcription and translation
In this case the newly made protein can quickly diffuse to its site of function
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Figure 17.23
RNA polymerase
Polyribosome
DNAmRNA
0.25 µm
Ribosome
DNA
mRNA (5′ end)
Polyribosome
Polypeptide(amino end)
Direction oftranscriptionRNA
polymerase
© 2014 Pearson Education, Inc.
In eukaryotes, the nuclear envelop separates the processes of transcription and translation
RNA undergoes processes before leavingthe nucleus
© 2014 Pearson Education, Inc.
Figure 17.24DNA
RNApolymerase
RNA transcript(pre-mRNA)
Intron
Exon
Aminoacyl-tRNAsynthetase
Aminoacid
tRNA
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 GGG
A A AAnticodon
Codon
Ribosome
CA
U
3′
A
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
If a mutation has an adverse effect on the phenotype of the organism the condition is referred to as a genetic disorder or hereditary disease
© 2014 Pearson Education, Inc.
Figure 17.25
Wild-type β-globin Sickle-cell β-globin
Mutant β-globin DNAWild-type β-globin DNA
mRNA mRNA
Normal hemoglobin Sickle-cell hemoglobin
Val
U GG
A CCG GT
Glu
G GA
G GAC CT3′
5′
5′ 5′
5′ 5′ 3′
3′
3′ 3′
3′ 5′
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Figure 17.26a
Wild type
DNA template strand T T TA C C A A A C C G A T T
AAAAA T G G T T T G G C T3′ 5′ 3′
5′
AAUCGGUUUGAAGA U5′ 3′ mRNAProtein
Amino endMet Lys Phe Gly Stop
Carboxyl end
Nucleotide-pair substitution: silent
A instead of G
U instead of C3′
3′ 5′
StopMet Lys Phe Gly
GGUUUGAAGA U U A AU
T T TA C C A A A C C T TAA
AAA T G G T T T G G T AAT
5′
5′ 3′
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Figure 17.26b
Wild type
DNA template strand T T TA C C A A A C C G A T T
AAAAA T G G T T T G G C T3′ 5′ 3′
5′
AAUCGGUUUGAAGA U5′ 3′ mRNAProtein
Amino endMet Lys Phe Gly Stop
Carboxyl end
Nucleotide-pair substitution: missense
T instead of C
StopMet Lys Phe Ser
A instead of G5′ 3′
5′
5′ 3′
3′ A U G A A G U U U U A ACGA
A T G A A AATCGTTTG A
T A C C CA A A AT T T TT G
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New Mutations and Mutagens
Spontaneous mutations can occur during DNA replication, recombination, or repair
Mutagens are physical or chemical agents that can cause mutations
© 2014 Pearson Education, Inc.
Figure 17.26Wild type
Stop
5′ 3′
3′
Stop
3′ 5′
5′ Met
(b) Nucleotide-pair insertion or deletion
Carboxyl endAmino end
mRNAProtein
3′ 5′
5′
DNA template strand
(a) Nucleotide-pair substitution
A instead of G
U instead of C
5′ 3′
3′
Stop
Extra A
Extra U
Frameshift (1 nucleotide-pair insertion)
missing
missing
Frameshift (1 nucleotide-pair deletion)
missing
3′ 5′
5′
missing
3′ 5′
3′ Met Lys Leu Ala
3′ 5′
3′
T A C T T C A A A C C G A T TATA G T A A A AC TTTTG G G
G G C U A AAAA U U UUUGG
A U G A A G G G C UUU
U
A A
AATCGGTTT GG A AA
T A C T T C A A C C G A T T
A
Met Lys Phe Gly
T A C C C C GA A A AA AAAA
A U G
G G G GT T T T
TTTTT C
G G G C UUUUAA A A
3′ 5′
5′
T
A A A A A
T T T TA A A A AACCCC
T T T T TTGGGG
G G G GUUU U A AA A AU
Met Lys Phe Gly Stop
U
Silent
3′ 5′
5′
5′ 3′
3′
Stop
Stop Stop3 nucleotide-pair deletionNonsense
A instead of T
U instead of A5′
5′
5′ 3′
3′
3′
Met Lys Phe Ser
Met Met Phe Gly
A
T
A
A
T
A
A
T
A
T instead of C
A instead of G
Missense
U G G G G A AU U U U UU A
A A A
T T TA A A A AGCCCC A
T T T T T TCGGGG3′ 5′
5′
5′ 3′
3′
T T
A A
AA
A A G
U U U U UCGGG
T T T T T
AGCC
CGGG
C
CTT
AAAA
T T T TT
A
AA AA C C C GA
A A A ATTTT GGGT C
A A AAUCGGGU U U U A
© 2014 Pearson Education, Inc.
Figure 17.26c
Wild type
DNA template strand T T TA C C A A A C C G A T T
AAAAA T G G T T T G G C T3′ 5′ 3′
5′
AAUCGGUUUGAAGA U5′ 3′ mRNAProtein
Amino endMet Lys Phe Gly Stop
Carboxyl end
Nucleotide-pair substitution: nonsense
A instead of T
U instead of A5′
5′
3′ 3′
3′
5′
Met Stop
A U G G U U U G G U U A A
AATCGGTTTGAGA T
T A C T C A A A C C G A T TA
T
AU
© 2014 Pearson Education, Inc.
Figure 17.26d
Wild type
DNA template strand T T TA C C A A A C C G A T T
AAAAA T G G T T T G G C T3′ 5′ 3′
5′
AAUCGGUUUGAAGA U5′ 3′ mRNAProtein
Amino endMet Lys Phe Gly Stop
Carboxyl end
Nucleotide-pair insertion: frameshift causing immediate nonsense
3′
3′
5′ 5′ 3′
5′
StopMet
A U G A A G U U U UG G C A A
Extra A
U
A
T
T G A A G T T T G G C T A A
A C T T C A A A C C G A T TA
T
© 2014 Pearson Education, Inc.
Figure 17.26e
Wild type
DNA template strand T T TA C C A A A C C G A T T
AAAAA T G G T T T G G C T3′ 5′ 3′
5′
AAUCGGUUUGAAGA U5′ 3′ mRNAProtein
Amino endMet Lys Phe Gly Stop
Carboxyl end
Nucleotide-pair deletion: frameshift causing extensive missense
5′
5′ 3′ 5′
3′
3′
A missingA
AACGGUUGAAGUA
A
U
A A A A
AAA C C C C G T TTT
T G G T T G G TC
Met Lys Leu Ala
missingU
T
© 2014 Pearson Education, Inc.
Figure 17.26f
Wild type
DNA template strand
3 nucleotide-pair deletion: no frameshift, but one amino acid missing
mRNAProtein
Amino endMet Lys Phe Gly Stop
Carboxyl end
T T TA C C A A A C C G A T T
AAAAA T G G T T T G G C T
AAUCGGUUUGAAGA U
T
T T C
A C A A A C C G A T T
T T T T TA A ACGGG
missing
missingA A G
G G G C UUUUUA A A
GlyPheMet Stop
3′ 5′
5′
3′ 5′
3′
5′ 3′
3′
3′ 5′
5′
© 2014 Pearson Education, Inc.
Figure 17.UN02
TRANSCRIPTION
RNA PROCESSING
TRANSLATION
DNA
Pre-mRNA
mRNA
Ribosome
Polypeptide
© 2014 Pearson Education, Inc.
Figure 17.UN03
TRANSCRIPTION
RNA PROCESSING
TRANSLATION
DNA
Pre-mRNA
mRNA
Ribosome
Polypeptide
© 2014 Pearson Education, Inc.
Figure 17.UN04
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
© 2014 Pearson Education, Inc.
Figure 17.UN06
Transcription unit
Template strandof DNARNA
polymerase
Promoter
RNA transcript
3′ 5′
5′ 3′ 3′
5′
© 2014 Pearson Education, Inc.
Figure 17.UN07
Pre-mRNA
mRNA
IntronPoly-A tail
Exon 3′Intron ExonExon5′ Cap
5′
5′ UTR 3′ UTRCodingsegment
© 2014 Pearson Education, Inc.
Figure 17.UN08
Polypeptide
Aminoacid
Anti-codon
mRNA
Codon
Ribosome
E A
tRNA