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Gene Expression
Chapter 13
Learning Objective 1
•
What early evidence indicated that most
genes specify the structure of proteins?
Garrod’s Work
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Inborn errors of metabolism
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Alkaptonuria
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evidence that genes specify proteins
rare genetic disease
lacks enzyme to oxidize homogentisic acid
Gene mutation
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associated with absence of specific enzyme
Alkaptonuria
Tyrosine
Functional
enzyme absent
Homogentisic acid
Functional
enzyme present
Disease condition
Normal metabolism
ALKAPTONURIA
Maleylacetoacetate
Homogentisic acid
excreted in urine;
turns black when
exposed to air
CO2
H2O
Fig. 13-1, p. 280
Learning Objective 2
•
Describe Beadle and Tatum’s experiments
with Neurospora
Beadle and Tatum
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Exposed Neurospora spores
•
•
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to X-rays or ultraviolet radiation
induced mutations prevented metabolic
production of essential molecules
Each mutant strain
•
•
had mutation in only one gene
each gene affected only one enzyme
Beadle-Tatum Experiments
Expose Neurospora
spores to UV light or X-rays
1
Each irradiated spore is used to
establish culture on complete
growth medium (minimal medium
plus amino acids, vitamins, etc.)
Fungal growth
(mycelium)
2
Transfer cells to
minimal medium
plus vitamins
Transfer cells to Transfer cells to
minimal medium minimal medium
plus amino acids
(control)
3
Minimal
Minimal Minimal Minimal
Minimal
medium
medium medium medium medium plus
plus
plus
plus
plus
other amino
arginine tryptophan lysine
leucine
acids
Fig. 13-2, p. 281
KEY CONCEPTS
•
Beadle and Tatum demonstrated the
relationship between genes and proteins
in the 1940s
Learning Objective 3
•
How does genetic information in cells flow
from DNA to RNA to polypeptide?
DNA to Protein
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Information encoded in DNA
•
•
codes sequences of amino acids in proteins
2-step process:
1. Transcription
2. Translation
Transcription
•
Synthesizes messenger RNA (mRNA)
•
•
complementary to template DNA strand
specifies amino acid sequences of
polypeptide chains
Translation
•
Synthesizes polypeptide chain
•
•
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specified by mRNA
also requires tRNA and ribosomes
Codon
•
•
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sequence of 3 mRNA nucleotide bases
specifies one amino acid
or a start or stop signal
DNA to Protein
Nontemplate strand
‘
‘
‘
DNA
Transcription
‘
‘
mRNA
(complementary
copy of
template
‘
DNA strand)
Template strand
Codon 1 Codon 2 Codon 3 Codon 4 Codon 5 Codon 6
Polypeptide
Met
Thr
Cys
Glu
Cys
Phe
Translation
Fig. 13-4, p. 283
KEY CONCEPTS
•
Transmission of information in cells is
typically from DNA to RNA to polypeptide
Learning Objective 4
•
What is the difference between the
structures of DNA and RNA?
RNA
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RNA nucleotides
•
•
•
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ribose (sugar)
bases (uracil, adenine, guanine, or cytosine)
3 phosphates
RNA subunits
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covalently joined by 5′ – 3′ linkages
form alternating sugar-phosphate backbone
RNA Structure
Uracil
Adenine
Cytosine
Guanine
Fig. 13-3, p. 282
Learning Objective 5
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Why is genetic code said to be redundant
and virtually universal?
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How may these features reflect its
evolutionary history?
Genetic Code
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mRNA codons
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specify a sequence of amino acids
64 codons
•
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61 code for amino acids
3 codons are stop signals
Codons
Genetic Code
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Is redundant
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•
some amino acids have more than one codon
Is virtually universal
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suggesting all organisms have a common
ancestor
few minor exceptions to standard code found
in all organisms
KEY CONCEPTS
•
A sequence of DNA base triplets is
transcribed into RNA codons
Learning Objective 6
•
What are the similarities and differences
between the processes of transcription
and DNA replication?
Enzymes
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Similar enzymes
•
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RNA polymerases (RNA synthesis)
DNA polymerases (DNA replication)
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Carry out synthesis in 5′ → 3′ direction
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Use nucleotides with 3 phosphate groups
Antiparallel Synthesis
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Strands of DNA are antiparallel
•
Template DNA strand and complementary
RNA strand are antiparallel
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DNA template read in 3′ → 5′ direction
RNA synthesized in 5′ → 3′ direction
Antiparallel Synthesis
5’
Promoter
region
mRNA transcript
5’
RNA polymerase
3’
3’
Gene 1
Gene 2
Promoter Promoter
region
region
mRNA transcript
5’
5’
3’
5’
mRNA transcript
3’
Gene 3
3’
Fig. 13-9, p. 287
Base-Pairing Rules
•
In RNA synthesis and DNA replication
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•
are the same
except uracil is substituted for thymine
Transcription
Growing RNA strand
5’ end
Nucleotide
added to
growing
chain by RNA
polymerase
3’end
Template DNA strand
3’ direction
5’ direction
Fig. 13-7, p. 286
Learning Objective 7
•
What features of tRNA are important in
decoding genetic information and
converting it into “protein language”?
Transfer RNA (tRNA)
•
“Decoding” molecule in translation
•
Anticodon
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complementary to mRNA codon
specific for 1 amino acid
tRNA
’
’
Loop 3
Hydrogen bonds
Loop 1
Loop 2
Anticodon
Fig. 13-6a, p. 285
OH 3’ end
Amino acid
accepting end
P 5’ end
Hydrogen bonds
Loop 3
Loop 1
Modified nucleotides
Loop 2
Anticodon
Fig. 13-6b, p. 285
Amino acid
(phenylalanine)
‘
Anticodon
‘
Fig. 13-6c, p. 285
Transfer RNA (tRNA)
•
tRNA
•
•
attaches to specific amino acid
covalently bound by aminoacyl-tRNA
synthetase enzymes
Aminoacyl-tRNA
AMP+
Phenylalanine
+
Aminoacyl-tRNA
synthetase
Anticodon
Amino acid
tRNA
Aminoacyl-tRNA
Fig. 13-11, p. 289
AMP+
Phenylalanine
+
Aminoacyl-tRNA
synthetase
Anticodon
Amino acid
tRNA
Aminoacyl-tRNA
Stepped Art
Fig. 13-11, p. 289
Learning Objective 8
•
How do ribosomes function in polypeptide
synthesis?
Ribosomes
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Bring together all machinery for translation
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Couple tRNAs to mRNA codons
Catalyze peptide bonds between amino acids
Translocate mRNA to read next codon
Ribosomal Subunits
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Each ribosome is made of
•
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1 large ribosomal subunit
1 small ribosomal subunit
Each subunit contains
•
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ribosomal RNA (rRNA)
many proteins
Ribosome
Structure
Front view
Large
subunit
E P A
Ribosome
Small
subunit
Fig. 13-12a, p. 290
Large ribosomal
subunit
E
site
mRNA
binding site
P
site
A
site
Small
ribosomal
subunit
Fig. 13-12b, p. 290
KEY CONCEPTS
•
A sequence of RNA codons is translated
into a sequence of amino acids in a
polypeptide
Animation: Structure of a
Ribosome
CLICK
TO PLAY
Learning Objective 9
•
Describe the processes of initiation,
elongation, and termination in polypeptide
synthesis
Initiation
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1st stage of translation
Initiation factors
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bind to small ribosomal subunit
which binds to mRNA at start codon (AUG)
Initiator tRNA
•
•
binds to start codon
then binds large ribosomal subunit
Elongation
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A cyclic process
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adds amino acids to polypeptide chain
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Proceeds in 5′ → 3′ direction along mRNA
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Polypeptide chain grows
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from amino end to carboxyl end
Termination
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Final stage of translation
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when ribosome reaches stop codon
A site binds to release factor
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triggers release of polypeptide chain
dissociation of translation complex
Stages of Transcription
RNA polymerase binds to
promoter region in DNA
DNA
Promoter
region
Direction of
transcription
Termination
sequence
DNA template
strand
RNA transcript
Rewinding Unwinding
of DNA
of DNA
DNA
RNA transcript
RNA
polymerase
Fig. 13-8, p. 287
Learning Objective 10
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What is the functional significance of the
structural differences between bacterial
and eukaryotic mRNAs?
Eukaryotes
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Genes and mRNA molecules
•
are more complicated than those of bacteria
Eukaryotic mRNA
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After transcription
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5′ cap (modified guanosine triphosphate) is
added to 5′ end of mRNA molecule
Poly-A tail (adenine-containing nucleotides)
•
may be added at 3′ end of mRNA molecule
Posttranscriptional Modification
Promoter
1st
1st 2nd 2nd
exon intron exon intron
Template
DNA strand
7-methylguanosine cap
3rd
exon
mRNA termination
sequence
Transcription, capping of 5’ end
5’ end
Start codon
Formation of pre-mRNA
Small nuclear
ribonucleoprotein complex
Stop codon
1st intron
2nd intron
–AAA...
Poly-A tail
3’ end
5’ end
Processing of pre-mRNA (addition of
poly-A tail and removal of introns)
1st 2nd
exon exon
5’ end
Mature mRNA in nucleus
3rd
exon
Protein-coding
region
Nuclear pore
–AAA...
Poly-A tail
3’ end
Nuclear envelope
Transport through nuclear
envelope to cytosol
5’ end
Mature mRNA in cytosol
Start codon
Stop codon
Cytosol
–AAA...
Poly-A tail
3’ end
Fig. 13-17, p. 295
Introns and Exons
•
Introns
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noncoding regions (interrupt exons)
removed from original pre-mRNA
Exons
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coding regions in eukaryotic genes
spliced to produce continuous polypeptide
coding sequence
Learning Objective 11
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What is the difference between translation
in bacterial and eukaryotic cells?
Bacterial Cells
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Transcription and translation are coupled
•
Bacterial ribosomes
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•
bind to 5′ end of growing mRNA
initiate translation before message is fully
synthesized
Bacterial mRNA
Promoter
region
Transcribed region
mRNA termination
sequence
DNA
Upstream
leader
sequences
Protein-coding
sequences
Translated region
Downstream
trailing
sequences
Start codon Stop codon
mRNA
–OH
3 ′ end
5 ′ end
Polypeptide
Fig. 13-10, p. 288
Initiation
Leader
sequence
mRNA
Small
ribosomal
subunit
Initiation factor
Start codon
Fig. 13-13a, p. 291
fMet
Initiator
tRNA
Fig. 13-13b, p. 291
fMet
P site
E site
Large
ribosomal
subunit
A site
Initiation complex
Fig. 13-13c, p. 291
Elongation
tRNA with an amino
acid
Amino acids
Amino acids
GDP
GTP
E
P
E
A
P
A
AminoacyltRNA binds to
codon in A site
mRNA
Ribosome ready to
accept another
aminoacyl-tRNA
Amino end of
polypeptide
E
P
A
Peptide bond
formation
Translocation
toward 3 ′ end
of mRNA
New
peptide
bond
E
P
A
GTP
GDP
Fig. 13-14, p. 292
Termination
Release
factor
E
P
A
mRNA
Stop codon
(UAA, UAG,
or UGA)
Fig. 13-15a, p. 293
Polypeptide
chain is
released
Stop codon
(UAA, UAG,
or UGA)
Fig. 13-15b, p. 293
Large ribosomal
subunit
Release
factor
mRNA
Small
ribosomal
subunit
tRNA
Fig. 13-15c, p. 293
Polyribosome
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Many ribosomes bound to a single mRNA
KEY CONCEPTS
•
Prokaryotic and eukaryotic cells differ in
the details of transcription and translation
Learning Objective 12
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Describe retroviruses and the enzyme
reverse transcriptase
Retroviruses
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Synthesize DNA from an RNA template
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HIV-1 (virus that causes AIDS)
Enzyme reverse transcriptase
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reverses flow of genetic information
Reverse Transcription
Chromosome
DNA in nucleus
of host cell
Provirus inserted
into chromosome
DNA
DNA provirus
DNA replication
RNA /DNA hybrid
Viral RNA
Digestion of
RNA strand
Reverse
transcription
RNA virus
Fig. 13-19a, p. 297
Provirus DNA
transcribed
Viral mRNA
Viral RNA
Viral
proteins
RNA virus
2
Fig. 13-19b, p. 297
Learning Objective 13
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Give examples of the different classes of
mutations that affect the base sequence of
DNA
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What effects does each have on the
polypeptide produced?
Base Substitution
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May alter or destroy protein function
•
missense mutation
• codon change specifies a different amino acid
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nonsense mutation
• codon becomes a stop codon
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May have minimal effects
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if amino acid is not altered
if codon change specifies a similar amino acid
Normal DNA sequence
Normal mRNA sequence
Normal
protein
sequence
(Stop)
BASE-SUBSTITUTION
MUTATIONS
Missense
mutation
(Stop)
Nonsense
mutation
(Stop)
Fig. 13-20a, p. 299
Animation: Base-Pair
Substitution
CLICK
TO PLAY
Frameshift Mutations
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Insertion or deletion of one or two base
pairs in a gene
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destroys protein function
changes codon sequences downstream from
the mutation
Normal DNA
sequence
Normal mRNA
sequence
(Stop)
Normal protein
sequence
FRAMESHIFT
MUTATIONS
Deletion causing
nonsense
(Stop)
Deletion causing
altered amino acid
sequence
Fig. 13-20b, p. 299
Animation: Frameshift
Mutation
CLICK
TO PLAY
Transposons
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Movable DNA sequences
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“jump” into the middle of a gene
Retrotransposons
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replicate by forming RNA intermediate
reverse transcriptase converts to original DNA
sequence before jumping into gene
KEY CONCEPTS
•
Mutations can cause changes in
phenotype
Animation: Protein Synthesis
Summary
CLICK
TO PLAY