1. Regulation of Gene Expression
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.

11. Structural Genes
Make the enzymes for the metabolic pathway.

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

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

14. If Lactose is absent

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

16. If Lactose is present

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

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

19. Animation

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

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

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

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

32. Chromatin Structure Histone Modifications DNA Methylation
Epigenetic Inheritance

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

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

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

37. 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.

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

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

42. 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.

44. Another Example

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

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

47. 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).

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

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

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

51. Protein Degradation
By Proteosomes using Ubiquitin to mark the protein.

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

53. 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.

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

56. Morphogensis
The generation of body form is a prime example of gene expression control.
How do cells differentiate from a single celled zygote into a multi-cellular organism?

57. Clues? Some of the clues are already in the egg.
Cytoplasmic determinants – chemicals in the egg that signal embryo development.
Made by Maternal genes, not the embryo’s.

59. Induction
Cell to cell signaling of neighboring cells gives position and clues to development of the embryo.

61. Fruit Fly Studies
Have contributed a great deal of information on how an egg develops into an embryo and the embryo into the adult.

62. Homeotic (Hox) Genes
Any of the “master” regulatory genes that control placement of the body parts.
Usually contain “homeobox” sequences of DNA (180 bases) that are highly conserved between organisms.

63. Comment Evolution is strongly tied to gene regulation. Why?
What happens if you mutate the homeotic genes?
Stay tuned for more “evo-devo” links in the future.

66. When things go wrong

67. Example case
Bicoid (two tailed) – gene that controls the development of a head area in fruit flies.
Gene produces a protein gradient across the embryo.

70. Result Head area develops where Bicoid protein levels are highest.
If no bicoid gradient – get two tails.

71. Other Genes
Control the development of segments and the other axis of the body.

72. Gene Expression and Cancer
Cancer - loss of the genetic control of cell division.
Balance between growth-stimulating pathway (accelerator) and growth-inhibiting pathway (brakes).

73. Proto-oncogenes
Normal genes for cell growth and cell division factors.
Genetic changes may turn them into oncogenes (cancer genes).
Ex: Gene Amplification, Translocations, Transpositions, Point Mutations

74. Proto-oncogenes

75. Tumor-Suppressor Genes
Genes that inhibit cell division.
Ex - p53, p21

76. Cancer Examples RAS - a G protein.
When mutated, causes an increase in cell division by over-stimulating protein kinases.
Several mutations known.

78. Cancer Examples
p53 - involved with several DNA repair genes and “checking” genes.
When damaged (e.g. cigarette smoke), can’t inhibit cell division or cause damaged cells to apoptose.

80. Carcinogens Agents that cause cancer. Ex: radiation, chemicals
Most work by altering the DNA, or interfering with control or repair mechanisms.

81. Multistep Hypothesis
Cancer is the result of several control mechanisms breaking down (usually).
Ex: Colorectal Cancer requires 4 to 5 mutations before cancer starts.

82. Colorectal Cancer

83. News Flash
Severe damage to a chromosome that causes it to “shatter” can lead to immediate cancer.
Doesn’t always take a long time and multiple steps.

84. Can Cancer be Inherited?
Cancer is caused by genetic changes but is not inherited.
However, oncogenes can be inherited.
Multistep model suggests that this puts a person “closer” to developing cancer.

85. Example – BRAC1
BRAC1 is a tumor suppressor gene linked with breast cancer.
Normal BRAC1 – 2% risk.
Abnormal BRAC1 – 60% risk.
Runs in families. Some will have breasts removed to avoid cancer risk.

86. Homework Read Chapter 20 Lab – Gel Electrophoresis. Lab report – 2/9
New Discussion Forum – articles found under “labs”.
Chapter 18 – Fri. 2/10
No broadcast Mon. 2/6

87. Summary
Know Operons
Be able to discuss several control mechanisms of gene expression.
Be familiar with gene expression and development of organisms.

88. Summary
How control of DNA can lead to cancer.