Chapt 07


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Chapt 07

  2. 2. GENE REGULATIONLearning Outcomes After completing this lecture, students will be able to: (a) Define gene regulation & operon (b) Describe lac operon – operator, promoter & terminator (c) Explain tryptophan operon Topic Slide 2 of 10 © 2010 Cosmopoint
  3. 3. GENE REGULATIONTopic Outlines 1.1. Operon 1.2. Lac Operon 1.3 Tryptophan Operon Slide 3 of 10 © 2010 Cosmopoint
  4. 4. GENE REGULATION 1.1. OperonDefinition Gene regulation: process that cells use to turn the information on genes into gene products Operon: a functioning unit of key nucleotide sequences of DNA including an operator, a common promoter, & one or more structural genes, which is controlled as a unit to produce messenger RNA (mRNA), in the process of transcription by an RNA polymerase 4 Slide 4 of 10
  5. 5. GENE REGULATION 1.1. OperonIntroduction How are genes regulated? They are turned off if there is no need for the enzymes they code for or turned on when the environment changes and the enzymes are once again needed Example E. coli in an environment without lactose does not produce the enzymes for lactose digestion. When lactose is present the enzymes for digestion are produced. 5 Slide 5 of 10
  6. 6. GENE REGULATION1.1. Operon Why is this off/on switch important? Energy is not wasted. It would be similar to having all the electrical appliances in your house on at once. Which of course would be very wasteful. Also unnecessary materials would lead to sluggish functioning. 6 Slide 6 of 10
  7. 7. GENE REGULATION1.1. Operon How are the genes for a particular metabolic pathway turned on or off? On the prokaryote chromosome a combination of genes and regulatory DNA sequences known as the operon accomplishes this. An example of an operon discovered by Jacob and Monod in E. coli The lac operon which is off if no lactose is present but can be induced to turn on in the presence of lactose. 7 Slide 7 of 10
  8. 8. GENE REGULATION1.2 Lac OperonLactose Operon Francois Jacob & Jacques Monod demonstrated how some genes in the E. coli are regulated at the biochemical level. The E. coli contains a set of genes that encodes for three proteins that the bacteria use to obtain energy from the sugar lactose. 8 Slide 8 of 10
  9. 9. GENE REGULATION1.2 Lac Operon Repressor/Regulator Gene - Produces a repressor protein that fits in the operator to turn the operon off Promoter - RNA polymerase attaches here to begin transcription of the genes Operator - The active repressor fits in notches to block RNA polymerase and turn off transcription. Structural Genes - Metabolic pathway genes with code for enzymes to digest lactose 9 Slide 9 of 10
  10. 10. GENE REGULATION1.2 Lac Operon 10 Slide 10 of 10
  11. 11. GENE REGULATION1.2 Lac Operon 11 Slide 11 of 10
  12. 12. GENE REGULATION1.2 Lac Operon 12 Slide 12 of 10
  13. 13. GENE REGULATION1.2 Lac Operon 13 Slide 13 of 10
  14. 14. GENE REGULATION1.2 Lac Operon 14 Slide 14 of 10
  15. 15. GENE REGULATION1.2 Lac Operon 15 Slide 15 of 10
  16. 16. GENE REGULATION1.2 Lac OperonThree enzymes produced β-galactosidase: catalyst the hydrolysis process of lactose into glucose & galactose Lactose permease: absorption of lactose by bacteria Transacetylase: function not clear 16 Slide 16 of 10
  17. 17. GENE REGULATION1.2 Lac Operon 17 Slide 17 of 10
  18. 18. GENE REGULATION1.2 Lac Operon 18 Slide 18 of 10
  19. 19. GENE REGULATION1.3 Trypthon OperonTrp Operon Trp operon – gene activity is repressed when a tryptophan is added unlike Lac operon which is induced when lactose is added 5 structural genes (trp A, trp B, trp C, trp D & trp E) The presence of tryptophan in the cell shut down the operon When Trp is present, it binds to a site on the Trp repressor & enables the Trp repressor to bind to the operator When Trp is not present, the repressor leaves its operator, & transcription of the 5 structural genes begins 19 Slide 19 of 10
  20. 20. GENE REGULATION1.3 Trypthon Operon 20 Slide 20 of 10
  21. 21. GENE REGULATION1.3 Trypthon Operon 21 Slide 21 of 10
  22. 22. GENE REGULATION1.3 Trypthon Operon Enzyme: Tryptophan synthethase 22 Slide 22 of 10
  23. 23. GENE REGULATION1.3 Trypthon Lac OperonRepression The repressor for the trp operon is produced upstream by the trp R gene, which is continually expressed When tryptophan is present, it binds to the tryptophan repressor tetramers, & causes a change in conformation, which allows the repressor to bind the operator, which prevents RNA polymerase from binding or transcribing the operon, so tryptophan is not produced When tryptophan is not present, the repressor cannot bind the operator, so transcription can occur. Therefore, this is called negative feedback mechanism 23 Slide 23 of 10