1. Regulation of Gene Expression
Prokaryotes vs. Eukaryotes

2. Gene Expression
is the process by which information from a gene is used in the synthesis of a functional protein or other gene product such as tRNA or rRNA.
As you can imagine, not all genes are expressed at the same time in an organism.

3. Regulation of Gene Expression
Both prokaryotic and eukaryotic cells, have the ability to turn gene expression on or off depending on what the cell or organism needs at any given time.
Prokaryotic cells have evolved to be able to regulate gene expression in response to different environmental conditions.
Eukaryotes regulate gene expression to maintain different cell types, in embryonic development, etc.

4. Why Regulate Gene Expression?
Bacterial cells that can conserve energy have the selective advantage over cells that are unable to do so.
Natural selection has favored bacteria who express genes only when their products are needed by the cell.

5. Why regulate gene expression?
E. coli can adjust their metabolism to changing environments i.e.
They can adjust the activity of enzymes already present in the cell by Feedback inhibition where the product inhibits the first enzyme in a metabolic pathway.
Cells can adjust the production level of the enzyme and only produce it as needed.

7. The operon model was dicovered in 1961
The mechanism by which bacterial cells can switch genes on or off
Synthesis or metabolism of molecules (metabolism) occurs in steps (pathway)
Each reaction in a pathway is catalyzed by a specific enzyme.
The genes for the enzymes that work together are usually clustered on the same chromosome.

8. The Lac Operon
Promoter – a DNA segment that signals the beginning of a gene (RNA polymerase attaches)
Operator – a DNA segment that controls the access of RNA polymerase to the genes
Operon – a DNA segment that includes a cluster of genes, the promoter and the operator
Repressor – a protein that binds to a specific operator, blocking attachment of RNA polymerase & thus turning the operon off

9. The Lac Operon
Regulatory genes – code for repressor proteins

10. The Lac Operon
Genes
LacI – regulatory gene, codes for the lac repressor
LacZ – codes for βgalactosidase (hydrolysis lactose)
LacY – codes for permease (membrane protein that transport lactose into the cell)
LacA – codes for transacetylase ( transfers an acetyl group from acetyl CoA to βgalactosidase but function unclear)

11. How it works
The dissacharide lacose is available to E.coli in the human colon if the host drinks milk.
Lactose metabolism begins with hydrolysis into ___ and _____.
This reaction is catalyzed by β-galactosidase.
In the absence of glucose, only a few molecules are present in the E.coli cell.
If lactose is added to the environment, the number of molecules of enzyme increase a thousandfold

12. E. coli uses 3 enzymes to metabolize lactose
E.coli uses 3 enzymes to metabolize lactose. The genes for these enzymes are all clustered together on a molecule of DNA in the lac operon .

13. How it Works

14. How it Works

15. Matching β-galactosidase Allolactose Operator Promoter Regulator gene
Repressor
Gene in operon
Is inactivated when attached to allolactose
Codes for repressor protein
Hydrolyzes lactose
Repressor attaches here
RNA polymerase attaches here
Acts as an inducer that inactivates repressor
Usually codes for an enzyme

16. Prokaryotes vs. Eukaryotes
Control expression by transcription.
No Nucleus to separate transcription and translation
Nucleus provides opportunity for post transcription control.
Transcription
RNA processing
mRNA transport
mRNA translation
mRNA degradation
Protein degradation

17. Figure 18.6
Signal
NUCLEUS
Chromatin
Chromatin modification: DNA unpacking involving histone acetylation and DNA demethylation
DNA
Gene available for transcription
Gene
Transcription
RNA
Exon
Primary transcript
Intron
RNA processing
Tail
Cap
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
mRNA in cytoplasm
Degradation of mRNA
Translation
Figure 18.6 Stages in gene expression that can be regulated in eukaryotic cells.
Polypeptide
Protein processing, such as cleavage and chemical modification
Active protein
Degradation of protein
Transport to cellular destination
Cellular function (such as enzymatic activity, structural support)

18. Control Points of Gene Expression
Chromatin Modification
Transcription
RNA Processing (Post transcriptional)
mRNA degradation (Post transcriptional)
Translation
Protein Degradation (Post translational)

19. Chromatin modification
Chromatin is the DNA – protein complex in eukaryotic cells
Each chromosome consists of 1 long DNA strand bound and wound around positively charges proteins called histones.
DNA is wound around 4 histones to form a nucleosome
Some areas of chromatin are coiled more tightly than others

20. Amino acids available for chemical modification
Figure 18.7
Histone tails
DNA double helix
Amino acids available for chemical modification
Nucleosome (end view)
(a) Histone tails protrude outward from a nucleosome
Figure 18.7 A simple model of histone tails and the effect of histone acetylation.
Unacetylated histones
Acetylated histones
Acetylation of histone tails promotes loose chromatin structure that permits transcription

21. Genes within regions where chromatin is tightly coiled (heterochromatin) are usually not transcribed or expressed.
Histones can be modified to regulate gene expression
DNA can be modified to regulate gene expression

22. Histone tails protrude from the nucleosome and are accessible to enzymes that catalyse the addition or removal of certain chemical groups.
Acetyl, methyl, phosphate groups
Acetylation – promotes transcription (loosens chromatin)
Methylation – prevents transcription (tightens chromatin)

23. Epigenetic inheritance
Once methylated, genes usually stay that way through successive generations
Enzymes methylate daughter strands according to the template strand in each round of replication.
Chromatin modification such as methylation can be passed from generation to generation.
Epigenetics is the inheritance of traits transmitted by mechanisms not involving the nucleotide sequence.

24. Changes in nucleotide sequences (mutations)are permanent.
Changes in chromatin (methylation, acetylation) is reversible.
Alterations in normal patterns of DNA methylation are seen in some cancers (inappropriate gene expression)

25. Transcriptional Control
Includes transcription factors
Enhancers
Promoters
Hormones

26. A Typical Eukaryotic Gene
Consists of a promoter sequence – a sequence where RNA polymerase binds and starts transcription
Introns
Exons

27. Upstream from the promoter sequence are:
Enhancer (distal control elements)
Proximal Control Elements

28. RNA polymerase attaches to the promoter sequence

29. Distal control element TATA box Enhancer General transcription factors
Figure
Promoter
Activators
Gene
DNA
Distal control element
TATA
box
Enhancer
General transcription factors
DNA- bending protein
Group of mediator proteins
RNA polymerase II
Figure A model for the action of enhancers and transcription activators.
RNA polymerase II
Transcription initiation complex
RNA synthesis