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Chapter 18

Chapter 18. Regulation of Gene Expression. Regulation of Gene Expression. Important for cellular control and differentiation. Understanding “expression” is a “hot” area in Biology. General Mechanisms. 1. Regulate Gene Expression 2. Regulate Protein Activity. Operon Model.

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Chapter 18

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  1. Chapter 18 Regulation of Gene Expression

  2. Regulation of Gene Expression • Important for cellular control and differentiation. • Understanding “expression” is a “hot” area in Biology.

  3. General Mechanisms 1. Regulate Gene Expression 2. Regulate Protein Activity

  4. Operon Model • Jacob and Monod (1961) - Prokaryotic model of gene control. • Always on the National AP Biology exam !

  5. Operon Structure 1. Regulatory Gene 2. Operon Area a. Promoter b. Operator c. Structural Genes

  6. Gene Structures

  7. Regulatory Gene • Makes Repressor Protein which may bind to the operator. • Repressor protein blocks transcription.

  8. Promoter • Attachment sequence on the DNA for RNA polymerase to start transcription.

  9. Operator • The "Switch”, binding site for Repressor Protein. • If blocked, will not permit RNA polymerase to pass, preventing transcription.

  10. Structural Genes • Make the enzymes for the metabolic pathway.

  11. Lac Operon • For digesting Lactose. • Inducible Operon - only works (on) when the substrate (lactose) is present.

  12. If no Lactose • Repressor binds to operator. • Operon is "off”, no transcription, no enzymes made

  13. If Lactose is absent

  14. If Lactose is present • Repressor binds to Lactose instead of operator. • Operon is "on”, transcription occurs, enzymes are made.

  15. If Lactose is present

  16. Enzymes • Digest Lactose. • When enough Lactose is digested, the Repressor can bind to the operator and switch the Operon "off”.

  17. Net Result • The cell only makes the Lactose digestive enzymes when the substrate is present, saving time and energy.

  18. Animation • http://www.biostudio.com/d_%20Lac%20Operon.htm

  19. trp Operon • Makes Tryptophan. • Repressible Operon.

  20. If no Tryptophan • Repressor protein is inactive, Operon "on” Tryptophan made. • “Normal” state for the cell.

  21. Tryptophan absent

  22. If Tryptophan present • Repressor protein is active, Operon "off”, no transcription, no enzymes • Result - no Tryptophan made

  23. If Tryptophan present

  24. Repressible Operons • Are examples of Feedback Inhibition. • Result - keeps the substrate at a constant level.

  25. Positive Gene Regulation • Positive increase of the level of transcription. • Uses CAP - Catabolite Activator Protein • Uses cAMP as a secondary cell signal.

  26. CAP - Mechanism • Binds to cAMP. • Complex binds to the Promoter, helping RNA polymerase with transcription.

  27. Result • If the amount of glucose is low (as shown by cAMP) and lactose is present, the lac operon can kick into high gear.

  28. Eukaryotic Gene Regulation • Can occur at any stage between DNA and Protein. • Be prepared to talk about several mechanisms in some detail.

  29. Chromatin Structure • Histone Modifications • DNA Methylation • Epigenetic Inheritance

  30. Histone Acetylation • Attachment of acetyl groups (-COCH3) to AAs in histones. • Result - DNA held less tightly to the nucleosomes, more accessible for transcription.

  31. DNA Methylation • Addition of methyl groups (-CH3) to DNA bases. • Result - long-term shut-down of DNA transcription. • Ex: Barr bodies genomic imprinting

  32. Epigenetics • Another example of DNA methylation effecting the control of gene expression. • Long term control from generation to generation. • Tends to turn genes “off”.

  33. Do Identical Twins have Identical DNA? • Yes – at the early stages of their lives. • Later – methylation patterns change their DNA and they become less alike with age.

  34. Transcriptional Control • Enhancers and Repressors • Specific Transcription Factors • Result – affect the transcription of DNA into mRNA

  35. Enhancers • Areas of DNA that increase transcription. • May be widely separated from the gene (usually upstream).

  36. Posttranscriptional Control • Alternative RNA Processing Ex - introns and exons • Can have choices on which exons to keep and which to discard. • Result – different mRNA and different proteins.

  37. Another Example

  38. Results • Bcl-XL – inhibits apoptosis • Bcl-XS – induces apoptosis • Two different and opposite effects!!

  39. DSCAM Gene • Found in fruit flies • Has 100 potential splicing sites. • Could produce 38,000 different polypeptides • Many of these polypeptides have been found

  40. Commentary • Alternative Splicing is going to be a BIG topic in Biology. • About 60% of genes are estimated to have alternative splicing sites. (way to increase the number of our genes) • One “gene” does not equal one polypeptide (or RNA).

  41. Other post transcriptional control points • RNA Transport - moving the mRNA into the cytoplasm. • RNA Degradation - breaking down old mRNA.

  42. Translation Control • Regulated by the availability of initiation factors. • Availability of tRNAs, AAs and other protein synthesis factors. (review Chapter 17).

  43. Protein Processing and Degradation • Changes to the protein structure after translation. • Ex: Cleavage • Modifications • Activation • Transport • Degradation

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