Chapter 17 Eukaryotic Gene Regulation

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• Chapter 17 Eukaryotic Gene Regulation

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• In multicellular eukaryotes, cells become
  specialized as a zygote develops into a mature
  organism
• Different types of cells make different kinds of
  proteins
• Different combinations of genes are active in
  each type

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• Most differentiated cells retain a complete set
  of genes
• In general, all somatic cells of a multicellular
  organism have the same genes

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• So a carrot plant can be grown from a single
  carrot cell

Root ofcarrot plant
Plantlet
Cell divisionin culture
Single cell
Adult plant
Root cells cultured in nutrient medium
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• The cloning of tadpoles showed that the nuclei of
  differentiated animal cells retain their full
  genetic potential

Frog egg cell
Tadpole (frog larva)
UV
Nucleus
Intestinal cell
Nucleus
Transplantationof nucleus
Nucleusdestroyed
Tadpole
Eight-cellembryo
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• The first mammalian clone, a sheep named Dolly,
  was produced in 1997

• further evidence for the developmental potential
  of cell nuclei

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Reproductive cloning of nonhuman mammals has
applications in basic research, agriculture, and
medicine
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Differentiation of embryonic stem cells in culture
Liver cells
Culturedembryonicstem cells
Nerve cells
Heart muscle cells
Different cultureconditions
Different types ofdifferentiated cells
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How is gene activity controlled?
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DNAdoublehelix(2-nmdiameter)
Histones
Beads ona string
Differential Packaging of DNA
Nucleosome(10-nm diameter)
Tight helical fiber(30-nm diameter)
Supercoil(200-nm diameter)
700nm
Metaphase chromosome
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Main Point of Gene Regulation

• Initiation of transcription
• Cis-acting elements
• Regions near the gene
• Promoter and enhancer
• Trans-acting elements
• Proteins that interact with promoter or enhancer
• Direct binding
• Bind to a protein that binds

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Complex assemblies of proteins control
eukaryotic transcription

• A variety of regulatory proteins interact with
  DNA and each other
• These interactions turn the transcription of
  eukaryotic genes on or off

Promoter
Enhancers
Gene
DNA
Activatorproteins
Transcriptionfactors
Otherproteins
RNA polymerase
Bendingof DNA
Transcription
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Regulation after Transcription

• Influences RNA Production
• Influences Protein Synthesis
• Influences Protein Stability

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• After transcription, alternative splicing may
  generate two or more types of mRNA from the same
  transcript

Exons
DNA
RNAtranscript
RNA splicing
or
mRNA
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mRNA stability controls the amount of gene product
mRNA turnover varies from 30 minutes to days
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Translation and later stages of gene expression
are also subject to regulation

• The lifetime of an mRNA molecule helps determine
  how much protein is made
• The protein may need to be activated in some way

Folding of polypeptide andformation of SS
linkages
Cleavage
Initial polypeptide(inactive)
Folded polypeptide(inactive)
Active formof insulin
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Control at the Protein Level

• Altering substrate concentration
• Variation in pH
• Enzyme activators and inhibitors
• Phosphorylation and dephosphorylation
• Subunit association and disassociation

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Review Multiple mechanisms regulate gene
expression in eukaryotes

• Each stage of eukaryotic expression offers an
  opportunity for regulation
• The process can be turned on or off, speeded up,
  or slowed down
• The most important control point is usually the
  start of transcription

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Broken-down mRNA
Tail
Broken-down protein
GENE
Splicing
Cap
DNA unpackingOther changes to DNA
Addition of cap and tail
Breakdown of mRNA
TRANSCRIPTION
Flowthroughnuclear envelope
Cleavage/modification/activation
Breakdownof protein
Translation
mRNA in cytoplasm
RNA transcript
ACTIVE PROTEIN
Polypeptide
GENE
Chromosome
mRNA in nucleus
NUCLEUS
CYTOPLASM
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Cascades of gene expression and cell-to-cell
signaling direct development

• A cascade of gene expression involves genes for
  regulatory proteins that affect other genes
• It determines how an animal or plant develops
  from a fertilized egg

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• Mutant fruit flies show the relationship between
  gene expression and development

Eye
Antenna
Head of a normal fruit fly

• Some mutants have legs where antennae should be

Head of a developmental mutant
Leg
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Development of head-tail polarity in fruit fly
EGG CELL WITHIN OVARIAN FOLLICLE
Egg cell
Egg protein signaling follicle cells
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Follicle cells
Gene expression in follicle cells
Follicle cell protein signaling egg cell
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Localization of head mRNA
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Head mRNA
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FERTILIZATION AND MITOSIS
ZYGOTE
Translation of head mRNA
EMBRYO
Gradient of regulatory protein
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Gene expression
Gradient of certain other proteins
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Gene expression
Body segments
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EMBRYO
Body segments
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LARVA
Gene expression
ADULT FLY
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Head end
Tail end
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Genes involved in floral development ABC model
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Signal-transduction pathways convert messages
received at the cell surface into responses
within the cell

• Cell-to-cell signaling is important in
• development
• coordination of cellular activities

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• A signal-transduction pathway that turns on a gene

SIGNALING CELL
Signal molecule
Plasma membrane
1
Receptor protein
2
(1) The signaling cell secretes the signal
molecule
TARGET CELL
(2) The signal molecule binds to a receptor
protein in the target cells plasma membrane
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SIGNALING CELL
Signal molecule
1
Plasma membrane
Receptor protein
2
(3) Binding activates the first relay protein,
which then activates the next relay protein, etc.
3
TARGET CELL
Relay proteins
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Transcription factor (activated)
(4) The last relay protein activates a
transcription factor
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Key developmental genes are very ancient

• Homeotic genes
• contain nucleotide sequences called homeoboxes
• are similar in many kinds of organisms
• arose early in the history of life

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• Fruit flies and mice have similar homeotic genes
  (colored boxes)

• The order of homeotic genes is the same
• The gene ordercorresponds toanalogous
  bodyregions

Mouse chromosomes
Fly chromosomes
Mouse embryo (12 days)
Fruit fly embryo (10 hours)
Adult mouse
Adult fruit fly
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THE GENETIC BASIS OF CANCER
Mutations in genes that control cell division

• A mutation can change a proto-oncogene into an
  oncogene
• An oncogene causes cells to divide excessively

Proto-oncogene
DNA
Mutation within the gene
Multiple copies of the gene
Gene moved tonew DNA locus,under new controls
Oncogene
New promoter
Hyperactivegrowth-stimulatingprotein in
normalamount
Normal growth-stimulatingproteinin excess
Normal growth-stimulatingproteinin excess
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• Mutations that inactivate tumor-suppressor genes
  have similar effects

Tumor-suppressor gene
Mutated tumor-suppressor gene
Normalgrowth-inhibitingprotein
Defective,nonfunctioningprotein
Cell divisionunder control
Cell division notunder control
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Oncogene proteins and faulty tumor-suppressor
proteins can interfere with normal
signal-transduction pathways

• Mutations of these genes cause malfunction of the
  pathway

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GROWTH-INHIBITINGFACTOR
Receptor
Relayproteins

• Other cancer-causing mutations inhibit the cells
  ability to repair damaged DNA

Nonfunctional transcriptionfactor (product of
faulty p53tumor-suppressor gene) cannot
trigger transcription
Transcription factor(activated)
Normal productof p53 gene
Transcription
Translation
Protein thatINHIBITScell division
Protein absent(cell divisionnot inhibited)
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GROWTHFACTOR
Receptor
TARGET CELL
Hyperactiverelay protein(product ofras
oncogene)issues signalson its own
Normal productof ras gene
Relayproteins
Transcription factor(activated)
DNA
Transcription
NUCLEUS
Protein thatSTIMULATEScell division
Translation
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Multiple genetic changes underlie the development
of cancer

• Cancers result from a series of genetic changes
  in a cell lineage
• As in many cancers, the development of colon
  cancer is gradual

Colon wall
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2
3
CELLULARCHANGES
Increasedcell division
Growth of polyp
Growth of malignanttumor (carcinoma)
DNACHANGES
Oncogeneactivated
Tumor-suppressorgene inactivated
Second tumor-suppressorgene inactivated
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• Mutations that lead to cancer may accumulate in a
  lineage of somatic cells

Chromosomes
1mutation
2mutations
3mutations
4mutations
Normalcell
Malignantcell
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Avoiding carcinogens can reduce the risk of cancer
Lifestyle choices can help reduce cancer risk