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Chapter 11
GENE EXPRESSION
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.
General Mechanisms
1. Regulate Gene Expression
2. Regulate Enzyme Activity
Initiating Transcription
Operon Model
Jacob and Monod (1961) - Prokaryotic model of gene control.
Operon Structure
1. Regulatory Gene
2. Operon Area a. Promoter
b. Operator
c. Structural Genes
Regulatory Gene
Makes Repressor Protein which may bind to the operator.
Promoter
Attachment sequence on the DNA for RNA polymerase.
Operator
The "Switch”, binding site for Repressor Protein.
If blocked, will not permit RNA polymerase to pass, preventing transcription.
Structural Genes
Make the enzymes for the metabolic pathway.
Lac Operon
For digesting Lactose. Inducible Operon - only
works (on) when the substrate (lactose) is present.
If no Lactose
Repressor binds to operator. Operon is "off”,
no transcription, no enzymes made
If Lactose is absent
If Lactose is present
Repressor binds to Lactose instead of operator.
Operon is "on”, transcription occurs, enzymes are made.
If Lactose is present
Enzymes
Digest Lactose. When enough Lactose is
digested, the Repressor can bind to the operator and switch the Operon "off”.
Net Result
The cell only makes the Lactose digestive enzymes when the substrate is present, saving time and energy.
Repressible Operons
Are examples of Feedback Inhibition.
Result - keeps the substrate at a constant level.
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
Gene Expression and Cancer
Cancer - loss of the genetic control of cell division.
Balance between growth-stimulating pathway (accelerator) and growth-inhibiting pathway (brakes).
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
Proto-oncogenes
Tumor-Suppressor Genes
Genes that inhibit cell division.
Ex - p53, p21
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.