1. Regulation of Gene Expression
Chapter 13

2. Prokaryotic REgulation
13.1
Prokaryotic REgulation

3. Prokaryotic Regulation
Bacterium don’t always need to express all enzymes and proteins.
Think about the cool lights in the cooler section at a grocery store. They only turn on when you open the door or walk by.
Operons are structural and functional genes that function as a unit to control gene expression.

4. Operon
Regulator Gene- outside of operon, codes for repressor (controls whether operon is active or not)
Promoter- short sequence of DNA where RNA polymerase attaches to start transcription.
Operator- short sequence of DNA where repressor can bind to prevent transcription.

5. Operon
Structural Gene- code for enzymes and proteins involved in pathways of operon, transcribed as a unit.

6. trp Operon Makes enzymes that synthesize tryptophan, an amino acid.
Exists in the “on” rather than the “off”
IF tryptophan is already present, it binds to the repressor, changes it’s shape allowing it to bind to operator and prevent transcription.

7. trp Operon Enzymes are said to be repressible Repressible operon
Tryptophan called corepressor.

9. lac Operon Structural genes called lactose metabolizing genes
Repressor binds to operon and prevents transcription. “off”
ONLY when lactose is present does it need to be broken down.

10. lac Operon
Lactose binds to repressor, changes its shape so it cannot bind to operator to prevent transcription.
Transcription occurs and lactose metabolizing enzymes are made.
Lactose called an inducer.
Inducible operon

12. Further Control of the lac Operon
E. coli would rather break down glucose than lactose.
They make sure that lactose operon is only “on” when glucose is absent.

13. Further Control of the lac Operon
Cyclic AMP (cAMP) accumulates when glucose is absent.
cAMP binds to CAP
CAP binds to site on DNA, making promoter more accessible to RNA polymerase so transcription can occur on lac operon.

14. Further Control of the lac Operon
When glucose is present, little cAMP is in the cell.
Therefore, CAP is inactive, lac operon does not function.
Example of positive control.
When active, promotes activity

15. Further Control of the lac Operon
When using repressors, it is said to be negative control.
When active, operons shut down
**lac operon is only maximally active when glucose is absent and lactose is present.

18. Questions?
Explain the difference between the roles of the promoter and the operator of an operon.
Summarize how gene expression differs in an inducible operon versus a repressible operon.
Describe the difference between positive control and negative control of gene expression.

19. Eukaryotic Regulation
13.2
Eukaryotic Regulation

20. Eukaryotic Regulation
Like prokaryotes, eukaryotes use a variety of mechanisms to regulate gene expression
5 mechanisms: 3 occur in the nucleus
2 occur in the cytoplasm

21. Chromatin Structure Chromatin packing to keep genes turned off.
Not accessible to RNA polymerase.
DNA and Proteins Chromatin
REMEMBER: Histones pack DNA like spools

22. Chromatin Structure
Active genes are associated with loosely packed chromatin called eurochromatin.
Inactive genes associated with tightly packed chromatin called heterochromatin.
What decides whether chromatin is heterochromatin or eurochromatin??

23. Chromatin Structure Histones have tails
Heterochromatin tails have methyl groups
Eurochromatin tails have acetyl groups
Heterochromatin is not transcribed
inaccessible to RNA polymerase
Ex: Barr Body- inactive X chromosome in females

24. Transcriptional Control (critical)
Transcription controlled by DNA binding proteins
Transcription factors, activators, and repressors
Transcription factors- proteins that help RNA polymerase bind to promoter.

25. Transcriptional Control
Even if all tf’s are present, cannot begin without DNA binding proteins called transcription activators.
Bind to enhancers
Makes hairpin loop

28. Mediator proteins Transcription Activators
BRIDGE
Transcription Factor Complex

29. Posttranscriptional Control
Last gene expression controller that occurs in the nucleus.
Includes alternative mRNA splicing and controls the speed mRNA leaves the nucleus.
Introns removed, differential splicing of exons can occur.
Introns and/or exons may be skipped when splicing.

30. Posttranscriptional Control
Cells seemed to have more DNA than needed.
Only 1.5% of DNA transcribed coded for proteins.
Other 98.5% coded to make siRNA (helps regulate gene expression

31. Posttranscriptional Control
How identical twins are different.

32. Translational Control
Begins when processed mRNA molecules reach the cytoplasm.
Presence or absence of cap and tail on mRNA determines whether translation takes place and how long mRNA is active.

33. Posttranslational Control
Begins once a protein has been synthesized and has become active.
If all proteins produced by a cell during its lifetime remained in the cell, serious problems would arise.
May not fold correctly or change shape over time.
Alzheimer's, mad cow disease
Protease-regulate how long proteins are active

34. 13.1
Gene mutations

35. Gene Mutations Permanent change in sequence of bases in DNA Causes:
Spontaneous- arise as a result of abnormalities, happens for no apparent reason
Induced- may result from toxic chemicals or radiation.

36. Spontaneous Mutations
Transposons may “jump” from one location to the other, disrupting genes.
Mispairing during replication, rarely happens.

37. Induced Mutations
Mutagens- environmental factors can alter the base composition.
Radiation and organic chemicals
Carcinogens- cancer causing mutagens
Food we eat
Industrial chemicals
Tobacco smoke
1/3 of all cancer deaths come from smoking
UV rays absorbed by pyrimidines in DNA

38. Effects of Mutations on Protein Activity
Point Mutation- change in single DNA nucleotide
Sickle cell
Frameshift Mutation- occur most often when one or more nucleotides are either added or deleted from DNA.