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?