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Chapter 15.4

Chapter 15.4. Prokaryotic Gene Control. AP Biology Fall 2010. Lac Operon. http://www.youtube.com/watch?v=49siJ7XowhU&NR=1 Watch and take notes!. Negative Control of the Lactose Operon.

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Chapter 15.4

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  1. Chapter 15.4 Prokaryotic Gene Control AP Biology Fall 2010

  2. Lac Operon • http://www.youtube.com/watch?v=49siJ7XowhU&NR=1 • Watch and take notes!

  3. Negative Control of the Lactose Operon • E. coli bacteria (common in the human digestive tract) can metabolize lactose because of a series of three genes that code for lactose-digesting enzymes

  4. Negative Control of the Lactose Operon • The three genes are preceded by a promoter and an operator • Altogether called an operon • http://www.youtube.com/watch?v=iPQZXMKZEfw

  5. Negative Control of the Lactose Operon • A regulator gene nearby codes for a repressor protein • Which binds to the operator when lactose concentrations are low and effectively blocks RNA polymerase’s access to the promoter  Not able to bind when repressor is bound

  6. Negative Control of the Lactose Operon • When milk is consumed the lactose binds to the repressor • Changing its shape and effectively removing its blockage of the promotor • Thus, RNA polymerase can now initiate transcription of the genes

  7. Positive Control of the Lactose Operon • The lactose operon is also subject to positive control by an activator protein called CAP • CAP: catabolite activator protein • cAMP: cyclic adenosine monophosphate • RNA polymerase will bind to the promoter if CAP is already there • In turn, CAP must first be activated by cAMP

  8. Positive Control of the Lactose Operon • When glucose is scarce, the CAP – cAMP complex forms and turns on the lactose metabolism genes • Converts lactose to glucose • Makes a promoter far more inviting to RNA polymerase • Complex called transcription factor • Why is this used? • E.coli cells pay far more attention to glucose than lactose • Lactose operon is not used much, unless there is no glucose

  9. Positive Control of the Lactose Operon • Glucose plentiful  ATP forms by glycolysis • Synthesis of enzyme necessary to synthesize cAMP is blocked • Glucose scarce  blocking ends, lactose available • CAP-cAMP complex forms • Lactose operon genes transcribed • Gene products allow lactose to be converted to glucose

  10. Positive Control of the Lactose Operon • Humans may develop lactose intolerance as we age due to declining concentrations of the lactose – digesting enzyme (lactase) • Too much lactose results and bacterial populations in the gut explode • Produces gas and diarrhea as by-products of their metabolism

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