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?