biology chapter 13: 10th edition regulation of gene … · 2 outline prokaryotic regulation trp...
TRANSCRIPT
Sylv
ia S
. Ma
der
Copyright © The McGraw Hill Companies Inc. Permission required for reproduction or display
PowerPoint® Lecture Slides are prepared by Dr. Isaac Barjis, Biology Instructor
BIOLOGY 10th Edition
Regulation of Gene Activity
Chapter 13: pp. 233 - 248
1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
regulator gene promoter operator structural genes
DNA
RNA polymerase
RNA polymerase cannot bind to promoter.
mRNA
enzymes inactive repressor
a. Tryptophan absent. Enzymes needed to synthesize tryptophan are produced.
DNA
inactive repressor
b. Tryptophan present. Presence of tryptophan prevents production of enzymes used to synthesize tryptophan.
tryptophan
active repressor
5 3
2
Outline
Prokaryotic Regulation
trp Operon
lac Operon
Eukaryotic Regulation
Chromatin Structure
Transcriptional Control
Posttranscriptional Control
Translational Control
Posttranslational Control
Genetic Mutations
Cancer
3
Prokaryotic Regulation
Bacteria do not require the same enzymes all the time
Enzymes are produced as needed
Francois Jacob and Jacques Monod (1961) proposed the operon model to explain regulation of gene expression in prokaryotes
Operon is a group of structural and regulatory genes that function as a single unit
4
Prokaryotic Regulation: The Operon Model
Operon consist of three components
Promoter
DNA sequence where RNA polymerase first attaches
Short segment of DNA
Operator
DNA sequence where active repressor binds
Short segment of DNA
Structural Genes
One to several genes coding for enzymes of a metabolic
pathway
Translated simultaneously as a block
Long segment of DNA
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6
Repressible Operons: The trp Operon
The regulator codes for a repressor
If tryptophan (an amino acid) is absent:
Repressor is unable to attach to the operator (expression is normally “on”)
RNA polymerase binds to the promoter
Enzymes for synthesis of tryptophan are produced
If tryptophan is present:
Combines with repressor as corepressor
Repressor becomes functional
Blocks synthesis of enzymes and tryptophan
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8
The trp Operon
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When the repressor
binds to the operator,
transcription is prevented.
active
repressor
structural genes
regulator gene
promoter operator
9
The trp Operon
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regulator gene promoter operator structural genes
DNA
RNA polymerase
RNA polymerase cannot bind to promoter.
mRNA
enzymes inactive repressor
a. Tryptophan absent. Enzymes needed to synthesize tryptophan are produced.
DNA
inactive repressor
b. Tryptophan present. Presence of tryptophan prevents production of enzymes used to synthesize tryptophan.
tryptophan
active repressor
5 3
10
Inducible Operons: The lac Operon
The regulator codes for a repressor
If lactose (a sugar that can be used for food) is absent:
Repressor attaches to the operator
Expression is normally “off”
If lactose is present:
It combines with repressor and renders it unable to bind to operator
RNA polymerase binds to the promoter
The three enzymes necessary for lactose catabolism are produced
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12
The lac Operon
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regulator gene promoter operator structural genes
DNA
RNA polymerase cannot bind to promoter.
RNA polymerase can bind to promoter.
active repressor
active repressor
mRNA
enzymes
active repressor
inactive repressor
b. Lactose present. Enzymes needed to take up and use lactose are produced only when lactose is present.
a. Lactose absent. Enzymes needed to take up and use lactose are not produced.
lactose
DNA
5 3
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14
Action of CAP
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DNA
inactive CAP
inactive CAP
active CAP
a. Lactose present, glucose absent (cAMP level high)
b. Lactose present, glucose present (cAMP level low)
DNA
cAMP
promoter CAP binding site
RNA polymerase binds
fully with promoter.
RNA polymerase does
not bind fully with promoter.
promoter operator
operator
CAP binding site
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16
Eukaryotic Regulation
A variety of mechanisms
Five primary levels of control:
Nuclear levels
Chromatin Packing
Transcriptional Control
Posttranscriptional Control
Cytoplasmic levels
Translational Control
Posttranslational Control
Animation
17
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18
Regulation of Gene Expression:
Levels of Control in Eukaryotes
functional protein
plasma
membrane
polypeptide chain
Posttranslational
control
Posttranscriptional control
Transcriptional control
Translational
control
nuclear pore
mRNA
pre-
mRNA intron exon
histones
nuclear envelope
Chromatin
structure
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3
5
3
5
19
Chromatin Structure
Eukaryotic DNA associated with histone proteins
Together make up chromatin
As seen in the interphase nucleus
Nucleosomes:
DNA wound around balls of eight molecules of histone
proteins
Looks like beads on a string
Each bead a nucleosome
The levels of chromatin packing determined by
degree of nucleosome coiling
20
Chromatin Structure Regulates
Gene Expression
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DNA
histone protein
a. Darkly stained heterochromatin and lightly stained euchromatin
b. A nucleosome c. DNA unpacking
H2A
H2B
H3
H1
H4 histone
tail
nucleosome
accessible
promoter
DNA to be transcribed
inaccessible
promoter
a: Courtesy Stephen Wolfe
euchromatin heterochromatin nucleolus
1 mm
chromatin remodeling complex
21
Chromatin Packing
Euchromatin
Loosely coiled DNA
Transcriptionally active
Heterochromatin
Tightly packed DNA
Transcriptionally inactive
Barr Bodies
Females have two X chromosomes, but only one is active
Other is tightly packed along its entire length
Inactive X chromosome is Barr body
22
X-Inactivation in Mammalian Females
Coats of tortoiseshell
cats have patches
of orange and black.
One X chromosome is inactivated in
each cell. Which one is by chance. Females have two
X chromosomes.
active X chromosome
inactive X
inactive X
active X chromosome
allele for
orange color
allele for
black color
cell division Barr bodies
© Chanan Photo 2004
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23
Transcriptional Control
Transcription controlled by proteins called
transcription factors
Bind to enhancer DNA
Regions of DNA where factors that regulate
transcription can also bind
Always present in cell, but most likely have to
be activated before they will bind to DNA
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27
Eukaryotic Transcription Factors
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promoter
DNA
enhancer
transcription
activator
mediator proteins
mRNA transcription
RNA polymerase
transcription
factor complex
gene
28
Posttranscriptional Control
Posttranscriptional control operates on primary mRNA transcript
Given a specific primary transcript: Excision of introns can vary
Splicing of exons can vary
Determines the type of mature transcript that leaves the nucleus
May also control speed of mRNA transport from nucleus to cytoplasm Will affect the number of transcripts arriving at rough
ER
And therefore the amount of gene product realized per unit time
29
Processing of mRNA Transcripts
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intron intron
intron
cap
protein product 1
mRNA
RNA splicing
poly-A
tail
exon intron
protein product 2
RNA splicing
exon
a. b.
cap
A B C D E
A B C D E
A B C
C
D E
A D E B
pre-mRNA
mRNA
pre-mRNA poly-A
tail
5 3 5 3
30
Function of microRNAs
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pre-mRNA
MicroRNA is cut from
a pre-mRNA and binds with
proteins to form RISC.
Complementary base pairing
between RNAs allows RISC
to bind to mRNA.
Translation
is inhibited.
The mRNA
is degraded.
mRNA
RISC
(RNA-induced
silencing complex)
microRNA
(miRNA)
proteins
or
RISC
5
3
3 5
5
3
31
Translational Control
Translational Control - Determines degree to
which mRNA is translated into a protein product
Presence of 5′ cap
Length of poly-A tail on 3′ end
Posttranslational Control - Affects the activity of a
protein product
Activation
Degradation rate
32
Regulation Through Gene Mutation
Mutation is a permanent change in the
sequence of bases in DNA.
No effect on protein activity
Protein is completely inactivated
Germ-line mutations occur in sex cells
Somatic mutations occur in body cells
33
Causes of Mutations
Spontaneous mutation DNA can undergo a chemical change
Movement of transposons from one chromosomal location to another
Replication Errors 1 in 1,000,000,000 replications DNA polymerase
Proofreads new strands Generally corrects errors
Induced mutation: Mutagens such as radiation, organic chemicals
Many mutagens are also carcinogens (cancer causing) Environmental Mutagens
Ultraviolet Radiation Tobacco Smoke
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35
The Ames Test For Mutagenicity
bacterial
strain
(requires
histidine)
Control
Mutation did not occur Mutation occurred
Suspected
chemical
mutagen
bacterial
strain
(requires
histidine) Plate onto petri plates
that lack histidine.
Incubate overnight bacterial
growth
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36
Causes of Mutations
Ultraviolet (UV) radiation is easily absorbed
by the pyrimidines in DNA.
Cause neighboring thymine molecules next
to one another to bond together
Thymine dimers.
C G
C
A
A
G
kink
thymine
dimer
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T
T
37
Causes of Mutations
Usually, these dimers are removed by DNA
repair enzymes
Deficient DNA repair enzymes leave the skin
cells vulnerable to the mutagenic effects of
ultraviolet light
Accumulation of mutation
High incidence of cancer
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39
XerodermaPigmentosome
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© Ken Greer/Visuals Unlimited
40
Effect of Mutations on Protein Activity
Point Mutations Involve change in a single DNA nucleotide Changes one codon to a different codon Affects on protein vary:
Nonfunctional Reduced functionality Unaffected
Frameshift Mutations One or two nucleotides are either inserted or deleted
from DNA Protein always rendered nonfunctional
Normal : THE CAT ATE THE RAT After deletion: THE ATA TET HER AT After insertion: THE CCA TAT ETH ERA T
41
Point Mutations in Hemoglobin
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b. Normal red blood cell
a.
c. Sickled red blood cell
No mutation
Val His Leu Thr Pro Glu Glu
(normal protein)
His His
(abnormal protein)
Glu Val
(incomplete protein)
Glu Stop
C T C C T C T G G A G T C A C G T G G A G
C T C C T C T G G A G T C A C G T G A G
Val His Leu Thr Pro Glu Glu
C T C C A C T G G A G T C A C G T G G A G
Val His Leu Thr Pro Glu
C T C C A T G G A G T C A C G T G G A G T
Val His Leu Thr Pro Stop
A
b, c: © Stan Flegler/Visuals Unlimited.
Val
3 5
42
Carcinogenesis
Development of cancer involves a series of
mutations
Proto-oncogenes – Stimulate cell cycle
Tumor suppressor genes – inhibit cell cycle
Mutation in oncogene and tumor suppressor
gene:
Stimulates cell cycle uncontrollably
Leads to tumor formation
43
Cell Signaling Pathway
Cell signaling pathway that stimulates a mutated tumor suppressor gene
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receptor
inhibiting growth factor
cytoplasm
plasma
membrane
signal
transducers
transcription factor
nucleus
protein that is
unable to inhibit
the cell cycle
or promote
apoptosis
mutated tumor suppressor gene
44
Cell Signaling Pathway
Cell signaling pathway that stimulates a proto-oncogene
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receptor
stimulating growth factor
cytoplasm
plasma
membrane
signal
transducers
transcription factor
nucleus
protein that
overstimulates
the cell cycle
oncogene
45
Review
Prokaryotic Regulation
trp Operon
lac Operon
Eukaryotic Regulation
Transcriptional Control
Posttranscriptional Control
Translational Control
Posttranslational Control
Genetic Mutations
Cancer
Sylv
ia S
. Ma
der
Copyright © The McGraw Hill Companies Inc. Permission required for reproduction or display
PowerPoint® Lecture Slides are prepared by Dr. Isaac Barjis, Biology Instructor
BIOLOGY 10th Edition
Regulation of Gene Activity
Chapter 13: pp. 233 - 248
46
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
regulator gene promoter operator structural genes
DNA
RNA polymerase
RNA polymerase cannot bind to promoter.
mRNA
enzymes inactive repressor
a. Tryptophan absent. Enzymes needed to synthesize tryptophan are produced.
DNA
inactive repressor
b. Tryptophan present. Presence of tryptophan prevents production of enzymes used to synthesize tryptophan.
tryptophan
active repressor
5 3