chapter 18 - biolympiads...enzymes •digest lactose. •when enough lactose is digested, the...
TRANSCRIPT
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Chapter 18
Regulation of
Gene Expression
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Regulation of Gene Expression
• Important for cellular control and differentiation.
• Understanding “expression” is a “hot” area in Biology.
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General Mechanisms
1. Regulate Gene Expression
2. Regulate Protein Activity
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Operon Model
• Jacob and Monod (1961) - Prokaryotic model of gene control.
• Always on the National AP Biology exam!
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Operon Structure
1. Regulatory Gene
2. Operon Area
a. Promoter
b. Operator
c. Structural Genes
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Gene Structures
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Regulatory Gene
• Makes Repressor Protein which may bind to the operator.
• Repressor protein blocks transcription.
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Promoter
• Attachment sequence on the DNA for RNA polymerase to start transcription.
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Operator
• The "Switch”, binding site for Repressor Protein.
• If blocked, will not permit RNA polymerase to pass, preventing transcription.
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Structural Genes
• Make the enzymes for the metabolic pathway.
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Lac Operon
• For digesting Lactose.
• Inducible Operon - only works (on) when the substrate (lactose) is present.
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If no Lactose
• Repressor binds to operator.
• Operon is "off”, no transcription, no enzymes made
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If Lactose is absent
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If Lactose is present
• Repressor binds to Lactose instead of operator.
• Operon is "on”, transcription occurs, enzymes are made.
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If Lactose is present
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Enzymes
• Digest Lactose.
• When enough Lactose is digested, the Repressor can bind to the operator and switch the Operon "off”.
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Net Result
• The cell only makes the Lactose digestive enzymes when the substrate is present, saving time and energy.
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Animation
• http://www.biostudio.com/d_%20Lac%20Operon.htm
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trp Operon
• Makes/synthesizes Tryptophan.
• Repressible Operon.
– Predict how it is different from the inducible operon…
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If no Tryptophan
• Repressor protein is inactive, Operon "on” Tryptophan made.
• “Normal” state for the cell.
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Tryptophan absent
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If Tryptophan present
• Repressor protein is active, Operon "off”, no transcription, no enzymes
• Result - no Tryptophan made
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If Tryptophan present
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Repressible Operons
• Are examples of Feedback Inhibition.
• Result - keeps the substrate at a constant level.
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Positive Gene Regulation
• Positive increase of the level of transcription.
• Uses CAP - Catabolite Activator Protein
• Uses cAMP as a secondary cell signal.
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CAP - Mechanism
• Binds to cAMP.
• Complex binds to the Promoter, helping RNA polymerase with transcription.
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Result
• If the amount of glucose is low (as shown by cAMP) and lactose is present, the lac operon can kick into high gear.
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Eukaryotic Gene Regulation
• Can occur at any stage between DNA and Protein.
• Be prepared to talk about several mechanisms in some detail.
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Chromatin Structure
• Histone Modifications
• DNA Methylation
• Epigenetic Inheritance
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Histone Acetylation
• Attachment of acetyl groups (-COCH3) to AAs in histones.
• Result - DNA held less tightly to the nucleosomes, more accessible for transcription.
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DNA Methylation
• Addition of methyl groups(-C H3) to DNA bases.
• Result - long-term shut-down of DNA transcription.
• Ex: Barr bodies, genomic imprinting
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Epigenetics
• Another example of DNA methylation effecting the control of gene expression.
• Long term control from generation to generation.
• Tends to turn genes “off”.
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Do Identical Twins have Identical DNA?
• Yes – at the early stages of their lives.
• Later – methylation patterns change their DNA and they become less alike with age.
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Transcriptional Control
• Enhancers and Repressors
• Specific Transcription Factors
• Result – affect the transcription of DNA into mRNA
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Enhancers
• Areas of DNA that increase transcription.
• May be widely separated from the gene (usually upstream).
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Posttranscriptional Control
• Alternative RNA Processing/Splicing – Ex. - introns and exons
• Can have choices on which exons to keep and which to discard.
• Result – different mRNA and different proteins.
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Another Example
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Results
• Bcl-XL – inhibits apoptosis
• Bcl-XS – induces apoptosis
• Two different and opposite effects!!
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DSCAM Gene
• Found in fruit flies
• Has 100 potential splicing sites.
• Could produce 38,000 different polypeptides
• Many of these polypeptides have been found
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Commentary
• Alternative Splicing is going to be a BIG topic in Biology.
• About 60% of genes are estimated to have alternative splicing sites. (way to increase the number of our genes)
• One “gene” does not equal one polypeptide (or RNA).
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Other post transcriptional control points
• RNA Transport - moving the mRNA into the cytoplasm.
• RNA Degradation - breaking down old mRNA.
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Translation Control
• Regulated by the availability of initiation factors.
• Availability of tRNAs, AAs and other protein synthesis factors. (review Chapter 17).
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Protein Processing and Degradation
• Changes to the protein structure after translation.
• Ex: Cleavage– Modifications
– Activation
– Transport
– Degradation
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Protein Degradation
• By Proteosomes using Ubiquitin to mark the protein.
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Noncoding RNA
• Small RNA molecules that are not translated into protein.
• Whole new area in gene regulation.
• Ex - RNAi
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Types of RNA
• MicroRNAs or miRNAs.
• RNA Interference or RNAi using small interfering RNAs or siRNAs.
• Both made from RNA molecule that is “diced” into double stranded (ds) segments.
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RNAi
• siRNAs or miRNAs can interact with mRNA and destroy the mRNA or block transcription.
• A high percentage of our DNA produces regulatory RNA.
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Morphogenesis
• The generation of body form is a prime example of gene expression control.
• How do cells differentiate from a single celled zygote into a multi-cellular organism?
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Clues?
• Some of the clues are already in the egg.
• Cytoplasmic determinants – chemicals in the egg that signal embryo development.
• Made by Maternal genes, not the embryo’s.
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Induction
• Cell to cell signaling of neighboring cells gives position and clues to development of the embryo.
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Fruit Fly Studies
• Have contributed a great deal of information on how an egg develops into an embryo and the embryo into the adult.
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Homeotic (Hox) Genes
• Any of the “master” regulatory genes that control placement of the body parts.
• Usually contain “homeobox” sequences of DNA (180 bases) that are highly conserved between organisms.
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Comment
• Evolution is strongly tied to gene regulation. Why?
• What happens if you mutate the homeoticgenes?
• Stay tuned for more “evo-devo” links in the future.
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When things go wrong
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Example case
• Bicoid (two tailed) – gene that controls the development of a head area in fruit flies.
• Gene produces a protein gradient across the embryo.
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Result
• Head area develops where Bicoid protein levels are highest.
• If no bicoid gradient – get two tails.
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Other Genes
• Control the development of segments and the other axis of the body.
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Gene Expression and Cancer
• Cancer - loss of the genetic control of cell division.
• Balance between growth-stimulating pathway (accelerator) and growth-inhibiting pathway (brakes).
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Proto-oncogenes
• Normal genes for cell growth and cell division factors.
• Genetic changes may turn them into oncogenes (cancer genes).
• Ex: Gene Amplification, Translocations, Transpositions, Point Mutations
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Proto-oncogenes
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Tumor-Suppressor Genes
• Genes that inhibit cell division.
• Ex - p53, p21
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Cancer Examples
• RAS - a G protein.
• When mutated, causes an increase in cell division by over-stimulating protein kinases.
• Several mutations known.
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Cancer Examples
• p53 - involved with several DNA repair genes and “checking” genes.
• When damaged (e.g. cigarette smoke), can’t inhibit cell division or cause damaged cells to apoptose.
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Carcinogens
• Agents that cause cancer.
• Ex: radiation, chemicals
• Most work by altering the DNA, or interfering with control or repair mechanisms.
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Multistep Hypothesis
• Cancer is the result of several control mechanisms breaking down (usually).
• Ex: Colorectal Cancer requires 4 to 5 mutations before cancer starts.
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Colorectal Cancer
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News Flash
• Severe damage to a chromosome that causes it to “shatter” can lead to immediate cancer.
• Doesn’t always take a long time and multiple steps.
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Can Cancer be Inherited?
• Cancer is caused by genetic changes but is not inherited.
• However, oncogenes can be inherited.
• Multistep model suggests that this puts a person “closer” to developing cancer.
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Example – BRAC1
• BRAC1 is a tumor suppressor gene linked with breast cancer.
• Normal BRAC1 – 2% risk.
• Abnormal BRAC1 – 60% risk.
• Runs in families. Some will have breasts removed to avoid cancer risk.
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Summary
• Know Operons
• Be able to discuss several control mechanisms of gene expression.
• Be familiar with gene expression and development of organisms.
• How control of DNA can lead to cancer.