Regulation of Gene Expression
• Important for cellular control and differentiation.
• Understanding “expression” is an important area in Biology where much research is taking place.
How does the cell determine which genes will be expressed?
• A small fraction of the genes in a cell are expressed.
• Important to understand the structure of a gene. – Recall: the promotor is a certain sequence of DNA
that serves as the binding site for RNA polymerase.
Operon Model
• Jacob and Monod (1961) - Prokaryotic model of gene control.– Gene regulation in bacteria.
• Always on the National AP Biology exam !
An example of an “Operon”
• E. coli – common bacterium in digestive tract • Contains 3 genes that operate together
• = Operon
• Operon genes must be expressed in order for the organisms to be able to use the sugar lactose.
• = lac operon
Operon Structure
1. Regulatory Gene – makes repressor protein- Repressor protein – turns gene “off”
2. Operon Area a. Promoter – where RNA polymerase binds to begin
transcription b. Operator – where repressor binds c. Structural Genes – make the enzymes
Lac Operon
• For digesting Lactose.– Recall: Lactose is a disaccharide.
• 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 present
• Repressor binds to Lactose instead of operator.
• Operon is "on”, transcription occurs, enzymes are made.
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.
• Bozeman: Lac Operon
If no Tryptophan
• Repressor protein is inactive, Operon "on” Tryptophan made.
• “Normal” state for the cell.
If Tryptophan present
• Repressor protein is active, Operon "off”, no transcription, no enzymes.
• Result - no Tryptophan made.
Repressible Operons
• Are examples of Feedback Inhibition.• Result - keeps the substrate at a constant
level.
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