Photosynthesis

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Outline

• Prokaryotic Regulation
• trp Operon
• lac Operon
• Eukaryotic Regulation
• Transcriptional Control
• Posttranscriptional Control
• Translational Control
• Posttranslational Control
• Genetic Mutations
• Cancer

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Prokaryotic RegulationThe 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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Repressible OperonsThe 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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The trp Operon
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Inducible OperonsThe 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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The lac Operon
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Action of CAP
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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

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Regulation of Gene ExpressionLevels of Control
in Eukaryotes
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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

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Levels of Chromatin Structure
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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

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X-Inactivation in Mammalian Females
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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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Lampbrush Chromosomes
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Initiation of Transcription
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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

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Processing of mRNA Transcripts
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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

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Effect of Mutations onProtein 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

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Point Mutation
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Androgen Insensitivity
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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

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Carcinogenesis
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Achondroplasia andXeroderma Pigmentosum
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Causes of Mutations

• Replication Errors
• 1 in 1,000,000,000 replications
• DNA polymerase
• Proofreads new strands
• Generally corrects errors
• Environmental Mutagens
• Carcinogens - Mutagens that increase the chances
  of cancer
• Ultraviolet Radiation
• Tobacco Smoke

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Review

• Prokaryotic Regulation
• trp Operon
• lac Operon
• Eukaryotic Regulation
• Transcriptional Control
• Posttranscriptional Control
• Translational Control
• Posttranslational Control
• Genetic Mutations
• Cancer

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