3. How Does A Cell Know? Which Gene To Express Which Gene To Express& Which Gene Should Stay Silent? Which Gene Should Stay Silent?

4. When a Gene is Expressed: When a Gene is Expressed: – It Is Transcribed Into mRNA When a Gene is Silent: When a Gene is Silent: – It Is Not Transcribed

5. What is an ? What is an Operon ? Group of Genes That Operate Together Group of Genes That Operate Together For Example: For Example: – E. coli ferments lactose To Do That It Needs Three Enzymes (Proteins), It Makes Them All At Once! To Do That It Needs Three Enzymes (Proteins), It Makes Them All At Once! – 3 Genes Turned On & Off Together. This is known as the – 3 Genes Turned On & Off Together. This is known as the lac Operon ( lac tose Operon)

6. The lac Operon – Regulates Lactose Metabolism – It Turns On Only When Lactose Is Present & Glucose is Absent – It Turns On Only When Lactose Is Present & Glucose is Absent. Lactose is a Disaccharide – A Combination of Galactose & Glucose To Ferment Lactose E. coli Must: 1. Transport Lactose Across Cell Membrane 2. Separate The Two Sugars

7. Each Task Requires A Specific Protein but Protein’s Not Needed If Glucose Present (why waste energy if you already have food?) so Genes Coding For Proteins Expressed Only When There Is Present But Lactose Is Present Genes Coding For Proteins Expressed Only When There Is No Glucose Present But Lactose Is Present

9. = Lactose ADDLACTOSE ADD LACTOSE

11. Key Concept Key Concept : The lac Genes Are: Turned Off By Repressors And And Turned On By The Presence Of Lactose

12. Operon Has 2 Regulatory Regions Operon Has 2 Regulatory Regions – Promoter (RNA Polymerase Binding) – Operator (O region) Bound To A lac Repressor

13. lac Repressor lac Repressor – When Bound To O Region : Prevents Binding of RNA Polymerase To Promoter – Turns The Operon “OFF”

14. lac Repressor Also Binds To Lactose lac Repressor Also Binds To Lactose – Higher Affinity For Lactose When Lactose Present lac Repressor Is Released From O Region When Lactose Present lac Repressor Is Released From O Region – Allows Transcription of All Three Genes

15. 1. Based On Repressors 2. Based On Enhancers 3. Regulated At Protein Synthesis