Prokaryotes

• Prokaryotes need control of gene expression to make quick adjustments in response to the changing environment.
• Prokaryotes primarily use transcriptional control to regulate gene expression.
• Operon: multiple prokaryotic genes that share a promoter and are transcribed together
• Operon transcription is controlled by proteins: Positive control: an activator protein stimulates transcription by enhancing the binding of RNA polymerase to the promoter
• Operon transcription is controlled by proteins: Negative control: a repressor protein decreases transcription by blocking the binding of RNA polymerase to the promoter
• When glucose is present, cAMP levels are low, causing cAMP not to bind to the activator protein.
• When glucose is absent, cAMP levels are high, causing cAMP to bind to and activate the activator protein.
• Lac operon codes for proteins that let the cell digest lactose.
• Lactose is digested only when glucose is not available.
• Lac repressor binds to the operator region, blocking transcription initiation by RNA polymerase.
• A repressor blocks transcription through the binding of a protein to the promoter.
• An activator stimulates transcription by binding upstream of a promoter and enhances the binding of RNA polymerase.
• Negative control is when a repressor protein decreases transcription by blocking the binding of RNA polymerase to the promoter.
• When lactose is present for a lac operon, it is a negative control. When glucose is absent for a lac operon, it is a positive control..
• When lactose is absent, the repressor protein is active, the active repressor binds to DNA near the promoter and blocks RNA polymerase. The operon is not transcribed (off).
• When lactose is present, lactose binds to and deactivates the repressor protein. RNA polymerase is not blocked and the operon is transcribed (on).