ch 11 gene expression lecture presentation
TRANSCRIPT
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Chapter 11
GENE EXPRESSION
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Gene Expression in Eukaryotes
Gene expression – the activation of a gene that results in the formation of a gene
Transcription causes expression Important for cellular control and
differentiation.
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General Mechanisms
1. Regulate Gene Expression
2. Regulate Enzyme Activity
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Initiating Transcription
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Operon Model
Jacob and Monod (1961) - Prokaryotic model of gene control.
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Operon Structure
1. Regulatory Gene
2. Operon Area a. Promoter
b. Operator
c. Structural Genes
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Regulatory Gene
Makes Repressor Protein which may bind to the operator.
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Promoter
Attachment sequence on the DNA for RNA polymerase.
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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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Repressible Operons
Are examples of Feedback Inhibition.
Result - keeps the substrate at a constant level.
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CELL DIFFERENTIATION Development of specialized cells Homeotic Genes – determine
where certain anatomical structures such as appendages will occur Ex HOX genes –control position
front/back/top/bottom
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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
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.