Gene Control

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Gene Control

• Prokaryotic Operons and Eukaryotic Gene Expression

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Prokaryotic Gene ControlThe Operon Model

• Proposed by Francois Jacob and Jacques Monod
• Operon
• Promoter
• 1 Structural Genes
• Operator -DNA sequence between promoter and
  structural gene
• Transcription of structural genes depends on
  another gene
• Regulatory Gene
• Produces the Repressor
• When Repressor binds to Operator
• RNA Polymerase cannot bind
• no Transcription of Structural Genes

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Ability of Repressor to bind to Operator Depends
on 2 factors

• Co-Repressor Activates a Repressor
• Seen in the TRP Operon
• Co-Repressor is tryptophan
• Turns normally on Operon off
• Inducer Inactivates a Repressor, Induces the
  Gene to be Transcribed
• Seen in the LAC Operon
• Inducer is allolactose
• Turns normally off Operon on

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Trp Operon Repressor with Co-Repressor blocks
transcription

• E. Coli adjust to Environmental (metabolic)
  Changes
• Amino Acid Tryptophan
• Needed by E. Coli
• E. Coli produces it by a single Operon consisting
  of 5 genes
• Operon usually produces trp
• If in the environment, it does not need to make
  trp
• turns off genes, save energy
• 2 Ways to turn off Operon
• 1. Feedback Inhibition trp in medium acts as
  co-repressor
• 2. trp in medium acts as enzyme inhibitor in
  pathway

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Figure 18.19 Regulation of a metabolic pathway
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Trp present, Operon is Off

• Repressor gene is always transcribed
• When trp present, trp acts as co-repressor
• Binds to Repressor activating it
• Repressor is then able to bind to Operator
• Turns Operon off
• Genes to synthesis trp not Transcribed

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Lac Operon Remove repressor by binding with
Inducer
Lac Operon contains genes for lactose sugar
metabolism Operon is usually off Repressor is
always transcribed If lactose enters the medium,
an isomer of lactose, Allolactose, binds to the
Repressor inactivating it Allolactose is an
Inducer, as it induces the genes to be
transcribed by inactivating the Repressor Elegant
control if lactose is present the genes are
transcribed, if not, then the energy is saved
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Positive Regulation cAMP Receptor Protein (CRP)

• Positive control of Operon
• Binds to DNA next to Promoter of Operon
• Stimulates Transcription of the genes
• Glucose is preferred over Lactose
• If Glucose is low, Lactose will be metabolized
• Lactose binds to Repressor, deactivates
• Low glucose means high cAMP in cell
• cAMP binds to CRP, activates it
• CRP cAMP binds to DNA near promoter
• Stimulates Transcription

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Switching Gears to Eukaryotic Gene Control
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Fun Fact

• Fun Fact one human cell has about 3 meters of
  DNA in it
• How does the cell go about condensing that amount
  and fitting it into the nucleus?
• How can the cell control that many genes on that
  much DNA?

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Eukaryotic Chromosome Structure

• Chromatin term for DNA Proteins in the
  Nucleus of a Eukaryotic cell
• DNA is wound around Histones (positively charged
  proteins which attract negatively charged DNA)
• Histones (8) with wound DNA make up a Nucleosome
  - the fundamental packing unit of Chromosome
  (10nm fiber)
• Nucleosomes further condense and form 30nm
  Chromatin Fibers
• Nucleosomes scaffold, loop, fold to make Looped
  Domains (300nm 700nm fibers)
• Metaphase Chromosome most condensed, largest
  form of DNA (1400nm)

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Figure 19.7 Opportunities for the control of
gene expression in eukaryotic cells

• Gene Control in Eukaryotes Overview
• Chromatin Packing, modification
• Assembling of Transcription Factors
• RNA Processing
• Regulation of mRNA degradation and Control of
  Translation
• Protein Processing and Degradation

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1. Chromatin Availability

• Euchromatin (true chromatin) less condensed and
  available for Transcription
• Heterochromatin (highly condensed) is present
  during interphase is not Transcribed
• DNA Methylation
• Enzymes Add CH3 to DNA
• Inactivates genes
• Histone Acetylation
• Attaches COCH3 to histone proteins
• Causes histones to change shape
• Allows easier access of Transcription proteins to
  DNA
• Coupled with initiation of Transcription

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How does DNA methylation prevent gene expression?

•     DNA methylation is often recognized by
  proteins that contain  a conserved methyl-CpG
  binding domain (MBD). MBDs can bind to
  5-methylated cytosine residues because these
  project into the major groove of DNA where
  DNA-protein interactions can occur without
  disrupting the double helical structure. When an
  MBD-containing protein binds to m5Cs in the
  promoter region of a gene, inhibition of
  transcription can occur.

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2. Assembling of Transcription Factors

• For Transcription to occur
• Initiation Complex must be assembled
• RNA polymerase can then move along the DNA, make
  mRNA
• Control elements segments of non coding DNA that
  bind Transcription Factors
• Activators proteins bind to Enhancer sequences on
  DNA
• DNA bending brings activators to promoter
• Protein binding domains on Activators attach to
  Transcription Factors and help form Transcription
  Initiation Complex

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Figure 19.10 Three of the major types of
DNA-binding domains in transcription factors
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3. Post Transcriptional MechanismsmRNA Processing

• rRNA Processing in the Nucleus
• Splicing removing Introns from primary mRNA
  transcript leaves functional Exons
• Alternative Splicing of mRNA
• Different mRNA produced from primary transcript
• Response to environmental changes
• Depends on which mRNA segments are treated as
  Exons

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Alternative Splicing offers new combinations of
Exons New Proteins
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4. Translational Control of GenesRegulation of
mRNA Degradation

• Control of Translation - Cytoplasm
• Proteins bind to mRNA or Phosphorylation of mRNA
• Block ability to bind to Ribosome for Translation
• Ex Egg cells produce large amounts of mRNA but
  wait to Translate them until just after
  fertilization

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5. Protein Processing and Degradation

• Post Translational Modifications of Polypeptides
• Degradation of Proteins prior to reaching its
  destination lifespan of proteins
• Proteins tagged with small ubiquitin protein
  signal proteasomes to degrade them
• CF channel protein never reaches plasma
  membrane, degrades and causes disease symptoms
• Mutations that make cell cycle proteins immune to
  proteasomes can lead to cancer
• Many proteins need chemical modification
• Ex Activated or Inactivated by addition of
  phosphate group
• Many proteins need cleavage to function
• Ex Post Translational cleavage of insulin

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CancerOncogenes vs. Proto-Oncogences

• Proto-Oncogenes are normal genes that code for
  proteins which stimulate normal cell growth and
  division
• Oncogenes cancer causing genes lead to
  abnormal stimulation of cell cycle
• Arises from a genetic change in proto-oncogene
• Amplification of proto-oncogenes
• Point mutation in proto-oncogene
• Movement of DNA within genome

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Genetic Changes that can turn Proto-oncogenes
into Oncogenes
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Movement of DNA in Genome

• Chromosomes in cancer cells often broken and
  translocated incorrectly
• Proto-oncogene may move to active promoter
• Increase transcription may make it an oncogene
• Transposition of gene or promoter may increase
  the translation of proto-oncogene making an
  oncogene

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Gene Amplification and Point Mutation

• Point Mutation
• Cause more reactive protein product
• Cause protein more resistant to degradation
• Gene Amplification
• Increases number of genes in the cell
• Increased number of genes in the cell can lead to
  uncontrolled cell growth

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P53 Tumor Suppressor and RAS Proto-Oncogenes

• Mutations in these genes are found in many
  cancers
• Part of signal transduction pathways that send
  extracellular signals to DNA in the nucleus
• Product of RAS gene is G Protein
• Relays a growth signal
• Stimulates cell cycle
• Point mutation -gt oncogene protein that is
  hyperactive, causes cell stimulation even in
  absence of signal
• P53 protein guardian angel of the genome
• DNA damage (UV, toxins) signals expression of
  p53
• p53 protein acts as transcription factor for
  gene p21
• p21 halts cell cycle, allowing DNA repair
• P53 also can cause cell suicide if damage is
  too great
• many cancer patients p53 gene product does not
  function properly

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Figure 19.14 Signaling pathways that regulate
cell growth (Layer 2)
RAS and P53 contribute to uninhibited cell
stimulation and growth- Tumor Formation
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Figure 19.15 A multi-step model for the
development of colorectal cancer