Control Mechanisms. Four Levels of Control of Gene Expression Type of ControlDescription Transcriptional Regulates which genes are transcribed. Controls.

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Control Mechanisms

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.

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.

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.

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.

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!!!

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.

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.

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!!!

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