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Operons

Operons. Big picture. Prokaryotic control of genome expression 2 levels of control Change in # of enzyme molecules produced Change in activity of enzymes present Ex: feedback inhibition Genes turned on/off based on metabolic needs: operons Operator + promoter + group of genes

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Operons

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  1. Operons

  2. Big picture • Prokaryotic control of genome expression • 2 levels of control • Change in # of enzyme molecules produced • Change in activity of enzymes present • Ex: feedback inhibition • Genes turned on/off based on metabolic needs: operons • Operator + promoter + group of genes • Often include several enzymes w/ in a pathway

  3. * Regulator gene - a gene that codes for a repressor protein molecule. The repressor molecule binds to the operator and prevents RNA polymerase from binding to the promoter. • * Promoter - a short sequence of DNA where RNA polymerase first attaches when a gene is to be transcribed. • * Operator - a short sequence of DNA where the repressor binds, preventing RNA polymerase from attaching to the promoter. • * In addition, the part of chromosome (in eukaryotic cells) containing the gene must first decompact to form a "chromosome puffs" before the beginning of transcription.

  4. 2 Types of operons • Inducible: • Default = off • Inducer turns transciption on • ex: an enzymatic pathway that allows the cell to break down lactose • Repressible: • Default = on • Corepressor turns transcription off • Ex: an enzymatic pathway that allows the cell to synthesize tryptophan

  5. The Lac Operon • Bacteria can use lactose if lactose is present and if there’s not enough glucose • B-galactosidase breaks down lactose • Lactose present = transcription is on • Allolactose (inducer) inactivates the repressor • Inactive repressor falls off of the operator • RNA polymerase binds

  6. LAC OPERON

  7. http://www.csam.montclair.edu/%7Esmalley/LacOperon.mov

  8. The trp operon • Cell can get tryptophan from environment or by synthesizing it • Will only synthesize tryptophan if there isn’t any in the environment • Tryptohan in the environment = no transcription • Tryptophan activates repressor • Tryptophan is a corepressor • Repressor is from regulatorygene (trpR) • Repressor binds to operator • Blocks RNA polymerase

  9. Positive gene regulation • Ex: lac operon transcription also depends on [glucose] • CRP: activates transcription (positive regulation) •  glucose   cAMP • cAMP activates CRP

  10. Explain the events that would occur if. . . • A cell was in an environment with plenty of glucose but no lactose, and lactose was added • A cell was in an environment with tryptophan, then the tryptophan disappeared • A cell was in an environment with lactose and plenty of glucose, then the glucose disappeared

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