Download - Gene Expression: From Gene to Protein
CAMPBELL BIOLOGY IN FOCUSUrry • Cain • Wasserman • Minorsky • Jackson • Reece
14Gene Expression: From Gene to Protein
鄭先祐 (Ayo) 教授國立臺南大學 生態科學與技術學系
Ayo website: http://myweb.nutn.edu.tw/~hycheng/
Overview: The Flow of Genetic Information
The information content of genes is in the form of specific sequences of nucleotides in DNA.
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
Concept 14.1: Genes specify proteins via transcription and translation
How was the fundamental relationship between genes and proteins discovered?
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.
Nutritional Mutants in Neurospora: Scientific Inquiry
George Beadle and Edward Tatum disabled genes in bread mold one by one and looked for phenotypic changes.
They studied the haploid bread mold because it would be easier to detect recessive mutations.
They studied mutations that altered the ability of the fungus to grow on minimal medium.
Figure 14.2a
Neurosporacells
Each survivingcell forms a colony ofgeneticallyidentical cells.
Individual Neurosporacells placed on completegrowth medium.
Cells subjectedto X-rays.
2
1 3
Figure 14.2b
No growth
Growth
Mutant cells placedin a series of vials,each containingminimal mediumplus one additionalnutrient.
Surviving cellstested for inabilityto grow onminimal medium.
Growth
Control: Wild-typecells in minimalmedium
4
5
The researchers amassed a valuable collection of Neurospora mutant strains, catalogued by their defects.
For example, one set of mutants all required arginine for growth
It was determined that different classes of these mutants were blocked at a different step in the biochemical pathway for arginine biosynthesis
The Products of Gene Expression: A Developing Story
Some proteins are not enzymes, so researchers later revised the one gene–one enzyme 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.
Basic Principles of Transcription and Translation
RNA is the bridge between DNA and protein synthesis.
RNA is chemically similar to DNA, but RNA has a ribose sugar and the base uracil (U) rather than thymine (T).
RNA is usually single-stranded.
Getting from DNA to protein requires two stages: transcription and translation.
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
In prokaryotes, translation of mRNA can begin before transcription has finished.
In eukaryotes, the nuclear envelope separates transcription from translation.
Eukaryotic RNA transcripts are modified through RNA processing to yield the finished mRNA.
Eukaryotic mRNA must be transported out of the nucleus to be translated.
Figure 14.4
Nuclearenvelope
Pre-mRNA
mRNA
DNA
RNA PROCESSING
TRANSCRIPTION
TRANSLATION
Polypeptide
Ribosome
mRNA
DNATRANSCRIPTION
TRANSLATION
Polypeptide
Ribosome
(a) Bacterial cell (b) Eukaryotic cell
Figure 14.4a-2
mRNA
DNA
TRANSCRIPTION
TRANSLATION
Polypeptide
Ribosome
(a) Bacterial cell
Figure 14.4b-1
Nuclearenvelope
Pre-mRNA
DNATRANSCRIPTION
(b) Eukaryotic cell
Figure 14.4b-2
Nuclearenvelope
Pre-mRNA
mRNA
DNA
RNA PROCESSING
TRANSCRIPTION
(b) Eukaryotic cell
Figure 14.4b-3
Nuclearenvelope
Pre-mRNA
mRNA
DNA
RNA PROCESSING
TRANSCRIPTION
TRANSLATION
Polypeptide
Ribosome
(b) Eukaryotic cell
Figure 14.UN01
DNA RNA Protein
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?
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
Figure 14.5
DNAtemplatestrand
Protein
mRNA
3
Trp
TRANSCRIPTION
TRANSLATION
Amino acid
Codon
5
35
3
5
Phe Gly Ser
GU G U UU G G UC C A
CA C A AA C C AG G T
GT G T TT G G TC C 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.
The template strand is always the same strand for any given gene.
During translation, the mRNA base triplets, called codons, are read 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.
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.
Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced.
Codons are read one at a time in a nonoverlapping fashion.
Figure 14.6
UUU
Second mRNA base
UUC
UUA
UUG
UCU
UCC
UCA
UCG
UAU
UAC
UAA
UAG
UGU
UGC
UGA
UGG
CUU
CUC
CUA
CUG
CCU
CCC
CCA
CCG
CAU
CAC
CAA
CAG
CGU
CGC
CGA
CGG
AUU
AUC
AUA
AUG
ACU
ACC
ACA
ACG
AAU
AAC
AAA
AAG
AGU
AGC
AGA
AGG
GUU
GUC
GUA
GUG
GCU
GCC
GCA
GCG
GAU
GAC
GAA
GAG
GGU
GGC
GGA
GGG
Fir
st m
RN
A b
ase
(5
en
d o
f co
do
n)
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
U C A G
Phe
Leu
Ser
Tyr Cys
Trp
Met orstart
Stop
Stop Stop
Arg
Gln
His
ProLeu
Val Ala
Asp
Glu
Gly
IIeThr
Lys
Asn
Arg
Ser
Th
ird
mR
NA
bas
e (
3 e
nd
of
cod
on
)
Evolution of the Genetic Code
The genetic code is nearly universal, shared by the simplest bacteria and the most complex animals.
Genes can be transcribed and translated after being transplanted from one species to another.
Figure 14.7
(a) Tobacco plant expressinga firefly gene
(b) Pig expressing a jellyfishgene
Concept 14.2: Transcription is the DNA-directed synthesis of RNA: a closer look
Transcription is the first stage of gene expression.
RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and joins together the RNA nucleotides.
RNA polymerases assemble polynucleotides in the 5 to 3 direction.
However, RNA polymerases can start a chain without a primer.
Figure 14.8-1Transcription unit
RNA polymerase
Promoter
Template strand of DNA
Start point
RNA transcript
UnwoundDNA
Initiation
35
35 3
5
35
1
Figure 14.8-2Transcription unit
RNA polymerase
Promoter
Template strand of DNA
Start point
RNA transcript
UnwoundDNA
RewoundDNA
RNA transcript
Direction oftranscription(“downstream”)
Initiation
Elongation
35
35
35 3
5
35
35
35
2
1
Figure 14.8-3Transcription unit
RNA polymerase
Promoter
Template strand of DNA
Start point
Termination
Completed RNA transcript
RNA transcript
UnwoundDNA
RewoundDNA
RNA transcript
Direction oftranscription(“downstream”)
Initiation
Elongation
35
35
35 3
5
35
35
35
35
35
35
3
2
1
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.
Synthesis of an RNA Transcript
The three stages of transcription
Initiation
Elongation
Termination
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.
Figure 14.UN02
DNA
Pre-mRNA
mRNA
Ribosome
Polypeptide
TRANSLATION
TRANSCRIPTION
RNA PROCESSING
Figure 14.9
Transcription factors
TATA box
Promoter Nontemplate strand
Start point
Transcriptioninitiationcomplex forms.
Transcription initiation complex
DNA
RNA transcript
A eukaryoticpromoter
Several transcriptionfactors bind to DNA.
35
5 3 35
35
35
3
2
1
Templatestrand
Transcription factors
RNA polymerase II
35
35 TA T A A A A
A T A T T T T
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.
Figure 14.10
Nontemplate strand of DNA
Direction of transcription
RNA polymerase
3
53
5
RNA nucleotides
Template strand of DNA
Newly made RNA
3 end
5
UC
U
G
A
A
A
A
AA
A
AA
A
T T T
TT
T
T
CC
C
CCC C
G
GG
U
Concept 14.3: 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 altered.
Also, usually some interior parts of the molecule are cut out and the other parts spliced together.
Alteration of mRNA Ends
Each end of a pre-mRNA molecule is modified in a particular way
The 5 end receives a modified G nucleotide 5 cap.
The 3 end gets a poly-A tail.
These modifications share several functions.
Facilitating the export of mRNA to the cytoplasm.
Protecting mRNA from hydrolytic enzymes.
Helping ribosomes attach to the 5 end.
Figure 14.UN03
DNA
Pre-mRNA
mRNA
Ribosome
Polypeptide
TRANSLATION
TRANSCRIPTION
RNA PROCESSING
Figure 14.11
Protein-coding segmentPolyadenylation
signal
G P
A modified guaninenucleotide added tothe 5 end
50–250 adeninenucleotides added to the 3 end
35
5 Cap 5 UTR 3 UTR Poly-A tailStartcodon
Stopcodon
P P AAUAAA …AAA AAA
Split Genes and RNA Splicing
Most eukaryotic mRNAs have long noncoding stretches of nucleotides that lie between coding regions.
The noncoding regions are called intervening ( 介於其間的 ) sequences, or introns.
The other regions are called exons and are usually translated into amino acid sequences.
RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence.
Figure 14.12
Introns cut out andexons spliced together
31–104
5 Cap
5 UTR 3 UTR
Poly-A tail
Codingsegment
1–146
AAUAAA
105– 146
5 Cap Poly-A tail
1–30
mRNA
Pre-mRNA
Intron Intron
Many genes can give rise to two or more different polypeptides, depending on which segments are used as exons.
This process is called alternative RNA splicing.
RNA splicing is carried out by spliceosomes.
Spliceosomes consist of proteins and small RNAs
Figure 14.13
Spliceosomecomponents
5
Pre-mRNA
5
mRNA
Intron
Spliceosome
Exon 1
Small RNAs
Exon 2
Exon 2Exon 1
Cut-outintron
Ribozymes
Ribozymes are RNA molecules that function as enzymes
RNA splicing can occur without proteins, or even additional RNA molecules
The introns can catalyze their own splicing
Concept 14.4: Translation is the RNA-directed synthesis of a polypeptide: a closer look
Genetic information flows from mRNA to protein through the process of translation.
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.
Figure 14.UN04
DNA
mRNA
Ribosome
Polypeptide
TRANSLATION
TRANSCRIPTION
Figure 14.14
5
tRNA
Polypeptide
Ribosome
Anticodon
mRNA
Codons 3
tRNA withamino acidattached
Amino acids
Gly
Trp
Phe
A A A
A C C
CC
G
U U U G G CU G G
The Structure and Function of Transfer RNA
Each tRNA can translate a particular mRNA codon into a given amino acid.
A tRNA molecule consists of a single RNA strand that is only about 80 nucleotides long.
Flattened into one plane, a tRNA molecule looks like a cloverleaf.
In three dimensions, tRNA is roughly L-shaped, where one end of the L contains the anticodon that base-pairs with an mRNA codon.
Figure 14.15a
5
3
Anticodon
Amino acidattachment site
Hydrogenbonds
A
A
G
A
CG
CC
C
C
UA
C G
UU
A
A A
A
C
G U
A
C G U
A
C
GU
AC
G
U
*
CG
*
G
GU
A
AA
A
C C
C
CC
GG
GG
G
UUU
U
GG
G
G
A
A
**
*
*
*
*
**
*
(a) Two-dimensional structure
*
Figure 14.15b
Anticodon
Amino acidattachment site
Anticodon
A A G53
Hydrogenbonds
53
(b) Three-dimensional structure
(c) Symbol used in this book
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(擺動 ) and allows some tRNAs to bind to more than one codon.
Figure 14.16-1Tyrosyl-tRNAsynthetase
Tyrosine (Tyr)(amino acid)
Amino acidand tRNAenter activesite.
Tyr-tRNA
ComplementarytRNA anticodon
1
UA A
Figure 14.16-2Tyrosyl-tRNAsynthetase
Tyrosine (Tyr)(amino acid)
Amino acidand tRNAenter activesite.
Tyr-tRNA
ComplementarytRNA anticodon
Using ATP,synthetasecatalyzescovalentbonding.
AMP 2
ATP
2
1
UA A
P i
Figure 14.16-3Tyrosyl-tRNAsynthetase
Tyrosine (Tyr)(amino acid)
Amino acidand tRNAenter activesite.
Tyr-tRNA
ComplementarytRNA anticodon
AminoacyltRNAreleased.
Using ATP,synthetasecatalyzescovalentbonding.
AMP 2
ATP
2
3
1
UA A
P i
Ribosomes
Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons during protein synthesis.
The large and small ribosomal are made of proteins and ribosomal RNAs (rRNAs).
In bacterial and eukaryotic ribosomes the large and small subunits join to form a ribosome only when attached to an mRNA molecule.
Figure 14.17a
tRNAmolecules
A
Largesubunit
Smallsubunit
Growing polypeptide Exit tunnel
E P
mRNA5
3
(a) Computer model of functioning ribosome
Figure 14.17b
PE A Largesubunit
Smallsubunit
P site (Peptidyl-tRNA binding site)
Exit tunnel
E site (Exit site)
mRNA binding site
(b) Schematic model showing binding sites
A site (Aminoacyl-tRNA binding site)
Figure 14.17c
Growing polypeptide
tRNA
5
3
EmRNA
(c) Schematic model with mRNA and tRNA
Codons
Amino end Next amino acidto be added to
polypeptidechain
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
Building a Polypeptide
The three stages of translation
Initiation
Elongation
Termination
All three stages require protein “factors” that aid in the translation process
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
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)
Figure 14.18
A
Small ribosomal subunit bindsto mRNA.
GTP
P siteU A
mRNA
53
Met
P i
mRNA binding site
Start codonSmallribosomalsubunit
InitiatortRNA
53
53
Large ribosomal subunit completes the initiation complex.
U G
C53
Translation initiation complex
Largeribosomalsubunit
AE
Met
GDP
1 2
The start codon is important because it establishes the reading frame for the mRNA
The addition of the large ribosomal subunit is last and completes the formation of the translation initiation complex
Proteins called initiation factors bring all these components together
Elongation of the Polypeptide Chain
During elongation, amino acids are added one by one to the previous 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
Figure 14.19-1Amino endof polypeptide
mRNAP
site
P i
5
3E
GTP
Asite
GDP
Codon recognition
E
P A
1
Figure 14.19-2Amino endof polypeptide
mRNAP
site
P i
5
3E
GTP
Asite
GDP
Peptide bondformation
Codon recognition
E
P A
E
P A
2
1
Figure 14.19-3Amino endof polypeptide
mRNARibosome ready fornext aminoacyl tRNA P
site
P i
5
3E
GTP
Asite
GDP
Peptide bondformation
Codon recognition
Translocation
E
P A
E
P A
P i
GTP
GDP
E
P A
3
2
1
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
Figure 14.20-1
Ribosome reaches astop codon on mRNA.
5
3
Releasefactor
Stop codon(UAG, UAA, or UGA)
1
Figure 14.20-2
Freepolypeptide
Ribosome reaches astop codon on mRNA.
5
3
Releasefactor
Stop codon(UAG, UAA, or UGA)
5
3
Release factorpromoteshydrolysis.
21
Figure 14.20-3
Freepolypeptide
Ribosome reaches astop codon on mRNA.
5
3
2 GTP
2 GDP Pi
Releasefactor
Stop codon(UAG, UAA, or UGA)
5
35
3
Ribosomalsubunits and othercomponentsdissociate.
Release factorpromoteshydrolysis.
2 31
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.
During synthesis, a polypeptide chain spontaneously coils and folds into its three-dimensional shape.
Proteins may also require post-translational modifications before doing their jobs.
Targeting Polypeptides to Specific Locations
Two populations of ribosomes are evident in cells: free ribosomes (in the cytosol ( 細胞質液 )) 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.
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 .
A signal-recognition particle (SRP) binds to the signal peptide.
The SRP brings the signal peptide and its ribosome to the ER.
Figure 14.21
Polypeptidesynthesisbegins.
SRPbinds tosignalpeptide.
SRPbinds toreceptorprotein.
SRPdetachesandpolypeptidesynthesisresumes.
Signal-cleavingenzyme cutsoff signalpeptide.
Completedpolypeptidefolds intofinalconformation.
Signalpeptideremoved
ERmembrane
Signalpeptide
Protein
SRP receptorprotein
Ribosome
mRNA
CYTOSOL
ER LUMEM
SRP
Translocation complex
1 2 3 4 5 6
Making Multiple Polypeptides in Bacteria and Eukaryotes
In bacteria and eukaryotes multiple ribosomes translate an mRNA at the same time
Once a ribosome is far enough past the start codon, another ribosome can attach to the mRNA
Strings of ribosomes called polyribosomes (or polysomes) can be seen with an electron microscope
Figure 14.22a
Incomingribosomalsubunits
(a) Several ribosomes simultaneously translating onemRNA molecule
Start of mRNA(5 end)
End of mRNA(3 end)
Growingpolypeptides
Completedpolypeptide
Polyribosome
Figure 14.22b
(b) A large polyribosome in a bacterialcell (TEM)
Ribosomes
mRNA
0.1 m
Bacteria and eukaryotes can also transcribe multiple mRNAs form the same gene
In bacteria, the transcription and translation can take place simultaneously
In eukaryotes, the nuclear envelope separates transcription and translation
Figure 14.23
RNA polymerase
mRNA
0.25 m
DNA
Polyribosome
RNA polymerase DNA
Polyribosome
Direction oftranscription
Ribosome
mRNA (5 end)
Polypeptide(amino end)
Figure 14.24
A
U A
tRNA
5 Cap
3
mRNA
Ribosomalsubunits
Aminoacyl(charged)tRNA
PE
G
5
3
Ribosome
Codon
Anticodon
TRANSLATION
Aminoacid
Aminoacyl-tRNAsynthetase
CYTOPLASM
NUCLEUS
AMINO ACIDACTIVATION
Intron
5 C
ap
Poly-A
TRANSCRIPTION
RNAPROCESSING
RNAtranscript
RNA transcript(pre-mRNA)
RNApolymerase
Exon
DNA
Poly-A
Poly-A
U GUG U U
A A A
A C C UA
CAE
Figure 14.24a
tRNA
mRNA
5
3
Aminoacid
Aminoacyl-tRNAsynthetase
CYTOPLASM
NUCLEUS
AMINO ACIDACTIVATION
Intron
5 C
ap
TRANSCRIPTION
RNAPROCESSING
RNAtranscript
RNA transcript(pre-mRNA)
RNApolymerase
Exon
DNA
Poly-A
Poly-A
Aminoacyl(charged)tRNA
Figure 14.24b
A
U A
5 Cap
3
mRNA
Ribosomalsubunits
Aminoacyl(charged)tRNA
PE
G
Ribosome
Codon
Anticodon
TRANSLATION
5 C
ap
Poly-A
U GUG U U
A A A
A C C UA
CAE
Growing polypeptide
Concept 14.5: 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 or a few nucleotide pairs of a gene
The change of a single nucleotide in a DNA template strand can lead to the production of an abnormal protein
Figure 14.25
AG G
Wild-type hemoglobin
mRNA
53
mRNA
Wild-type hemoglobin DNA
535
3TC C
TG GAC C 5
3
AG G5 53 UG G 3
Normal hemoglobin
Sickle-cell hemoglobin
Mutant hemoglobin DNA
Sickle-cell hemoglobin
ValGlu
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
Figure 14.26
mRNA
DNA template strand
StopCarboxyl end
ProteinAmino end
Phe GlyMet Lys
A G3 T CC 5T T T TA A A AC C
5 3TA A A A AT T T TG G G G C
U5 3G CA U U G GGA A A AU U
Wild type
Phe GlyMet Lys
A A3 T CC 5T T T TA A A AC C
5 3TA A A A AT T T TG G G G T
U5 3G UA U U G GGA A A AU U
Phe SerMet Lys
A G3 T CC 5T T T TA A A AC T
5 3TA A A A AT T T TG G A G C
U5 3G CA U U A GGA A A AU U
Met
A C3 T CT 5A T A TC A A GC A
5 3TA T A T AG T T CG A T G G
U5 3G GA G U U GAU A U AU U
Leu AlaMet Lys
A A3 T GC 5T T T
A
A A C TC C
5 3TA A A AGT T AG G G C T
U5 3G UA U G G GGA A A
U
U A
GlyMet Phe
A T3 T TA 5A A
T T
C C G
C
C A
5 3TA T T
A
A
G G C
T
G T T A A
U5 3G AA G C U AUU U
A G
G
A
Met
A G3 T CC 5A T T TA A A AC C
5 3TA T A A AT T T TG G G G C
U5 3G UA U U G GGU A A AU U
Stop Stop
Stop
Stop
Stop
A instead of G
Silent (no effect on amino acid sequence)
(a) Nucleotide-pair substitution
U instead of C
T instead of C
Missense
A instead of G
A instead of T
Nonsense
U instead of A
Extra A
Frameshift causing immediate nonsense(1 nucleotide-pair insertion)
(b) Nucleotide-pair insertion or deletion
Extra U
Frameshift causing extensive missense(1 nucleotide-pair deletion)
missing
missing
missing
missing
No frameshift, but one amino acid missing(3 nucleotide-pair deletion)
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
Figure 14.26a
DNA template strand
mRNA
StopCarboxyl end
Protein
Amino endPhe GlyMet Lys
3 55 35 3
Wild type
Phe GlyMet Lys
3 55 3
5 3
Stop
A instead of G
Nucleotide-pair substitution: silent
U instead of C
T A T T A A A A T TC C C C GTA T TA A AAT TG G G CG
UA U UA A AAU UG G G CG
T A T T A A A A T TC C C C ATA T TA A AAT TG G G TG
UA U UA A AAU UG G G UG
Missense mutations still code for an amino acid, but not the correct amino acid
Substitution mutations are usually missense mutations
Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein
Figure 14.26b
DNA template strand
mRNA
StopCarboxyl end
Protein
Amino endPhe GlyMet Lys
3 55 35 3
Wild type
Phe SerMet Lys
3 55 3
5 3
Stop
T instead of C
A instead of G
Nucleotide-pair substitution: missense
T A T T A A A A T TC C C C G
TA T TA A AAT TG G G CG
UA U UA A AAU UG G G CG
T A T T A A A A T TC C T C G
TA T TA A AAT TG A G CG
UA U UA A AAU UG A G CG
Figure 14.26c
DNA template strand
mRNA
StopCarboxyl end
Protein
Amino endPhe GlyMet Lys
3 55 35 3
Wild type
Nucleotide-pair substitution: nonsense
3 55 3
5 3Met
Stop
A instead of T
U instead of A
T A T T A A A A T TC C C C G
TA T TA A AAT TG G G CG
UA U UA A AAU UG G G CG
T A A T A A A A T TC C C C G
TA T TT A AAT TG G G CG
UA U UU A AAU UG G G CG
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 of the genetic message, producing a frameshift mutation
Figure 14.26d
DNA template strand
mRNA
StopCarboxyl end
Protein
Amino endPhe GlyMet Lys
3 55 35 3
Wild type
Nucleotide-pair insertion: frameshift causing immediate nonsense
Met
3 55 3
5 3
Stop
Extra A
Extra U
T A T T A A A A T TC C C C G
TA T TA A AAT TG G G CG
UA U UA A AAU UG G G CG
T A T T A A A A T TC C C C G
TA T TA A AAT TG G G CG
UA U UA A AAU UG G G CG
A
T
U
Figure 14.26e
DNA template strand
mRNA
StopCarboxyl end
Protein
Amino endPhe GlyMet Lys
3 55 35 3
Wild type
Nucleotide-pair deletion: frameshift causing extensive missense
Leu AlaMet Lys
3 5A
5 3
5 3U
missing
missing
T A T T A A A A T TC C C C G
TA T TA A AAT TG G G CG
UA U UA A AAU UG G G CG
T A T T A AG
A T TC C C C G
TA T TA A AATG G CG
UA U UA A AAG UG G CG
Figure 14.26f
DNA template strand
mRNA
StopCarboxyl end
Protein
Amino endPhe GlyMet Lys
3 55 35 3
Wild type
3 nucleotide-pair deletion: no frameshift, but one amino acidmissing
GlyMet Phe
3 5T T C
5 3
5 3A GA
Stop
missing
missing
T A T T A A A A T TC C C C G
TA T TA A AAT TG G G CG
UA U UA A AAU UG G G CG
T A A A C C GC A A T T
TA G GT T CT T A AG
UA U U UG G G C U A A
Mutagens
Spontaneous mutations can occur during DNA replication, recombination, or repair
Mutagens are physical or chemical agents that can cause mutations
Researchers have developed methods to test the mutagenic activity of chemicals
Most cancer-causing chemicals (carcinogens) are mutagenic, and the converse is also true
What Is a Gene? Revisiting the Question
The definition of a 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