Chapter 18

1. Chapter 18 Regulation of Gene Expression

2. Classify these things as occurring in prokaryotes, eukaryotes, or both. • Single loop of DNA • Chromosomes wound around histones • Telomeres • Multiple sites of origin in DNA replication • Single origin site for DNA Replication • Uses DNA polymerase • RNA is processed after transcription • Transcription/translation can be coupled • Translation occurs at the ribosome • Uses codons and anticodons

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

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

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

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

7. Gene Structures

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

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

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

11. Gene Structures

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

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

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

15. If Lactose is absent

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

17. If Lactose is present

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

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

20. trp Operon • Makes Tryptophan. • Repressible Operon.

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

22. Tryptophan absent

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

24. If Tryptophan present

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

27. Wednesday, February 18 • Predict what would happen in the lac operon for each of these scenarios. • Lactose is present, glucose is scarce • A mutation in the operator so the repressor cannot bind • Lactose is absent • Glucose is present • The repressor has a mutation so that it always binds to the operator • CAP and cAMP levels are high

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.

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

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

34. 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.

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

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

39. 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.

41. Another Example

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

43. 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).

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

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

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

47. Protein Degradation • By Proteosomes using Ubiquitin to mark the protein.

48. Noncoding RNA • Small RNA molecules that are not translated into protein. • Whole new area in gene regulation. • Ex - RNAi

49. Types of RNA • MicroRNAs or miRNAs. • RNA Interference or RNAi using small interfering RNAs or siRNAs. • Both made from RNA molecule that is diced into double stranded (ds) segments.

50. RNAi • siRNAs or miRNAs can interact with mRNA and destroy the mRNA or block translation. • A high percentage of our DNA produces regulatory RNA.