1. Control Mechanisms

2. Four Levels of Control of Gene Expression Type of ControlDescription Transcriptional Regulates which genes are transcribed. Controls rate of transcription. Posttranscriptional Modification of mRNA. Introns removed & exons spliced together.

3. Four Levels of Control of Gene Expression Type of ControlDescription Translational Controls rate of translation. Controls rate of mRNA activation and destruction. Posttranslational Controls rate at which a protein becomes active and the time it remains functional.

4. The lac Operon E. coli uses β-galactosidase to break down lactose in order to grow. The 3 genes for β-galactosidase are part of the lac operon.

5. The lac Operon When lactose is absent: – LacI repressor protein binds to the operator and partially blocks the promoter. – Prevents transcription of the lac operon genes.

6. The lac Operon With sufficient lactose: – Binds to the LacI repressor protein. – LacI changes its shape, allowing transcription of the lac operon genes. WATCH THIS!!!

7. The trp Operon E. coli uses tryptophan for the production of protein. The trp operon consists of 5 genes that code for 3 enzymes needed to synthesize tryptophan.

8. The trp Operon When tryptophan is absent: – The shape of the trp repressor protein changes. – No longer binds to the trp operator. – RNA polymerase transcribes trp operon genes.

9. The trp Operon With sufficient tryptophan: – Binds to the trp repressor protein, causing a change in shape. – trp repressor-tryptophan complex binds to the operator. – Prevents transcription of the trp operon genes. WATCH THIS!!!

10. Summary lac Operon Transcription induced when high levels of lactose present. Effector = level of lactose trp Operon Transcription repressed when high levels of tryptophan present. Effector = level of tryptophan