AP Biology
Chapter 18: Gene Regulation

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

Gene Structures

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 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.
• Bozeman: Lac Operon

trp Operon
• Makes Tryptophan.
• Repressible Operon .

If no Tryptophan
• Repressor protein is inactive, Operon "on”
Tryptophan made.
• “Normal” state for the cell.

Tryptophan absent

If Tryptophan present
• Repressor protein is active, Operon "off”, no transcription, no enzymes.
• Result - no Tryptophan made.

If Tryptophan present

Repressible Operons
• Are examples of Feedback Inhibition.
• Result - keeps the substrate at a constant level.

Questions for Review
Are these operons examples of positive or negative feedback? Why?