1. Regulation of Gene Expression
Chapter 18 (18.1 & 18.2)

2. Gene Expression All cells contain the entire genome
In theory, every cell could make every single protein encoded in the genome
But that doesn’t happen!
e.g. Lymphocytes of immune system are the only cells that make antibodies while red blood cells are the only cells that make the oxygen transport protein hemoglobin
Or sometimes the environment will influence which proteins are made and when
The spatial (space) and temporal (time) select transcription of certain genes is called gene expression

3. Gene Expression
So how do different cells know which genes to use and which genes to ignore?
It’s a little complicated so let’s start with the prokaryotes!

4. Regulation in Prokaryotes
Bacterial cells only make gene products when they are needed
Bacteria can use 2 methods to regulate this production:
Feedback regulation of enzymes
Gene expression regulation by the operon model

5. Regulation by Operons: Enzyme Production
Basic idea: Clusters of functionally related genes are under coordinated control
A single “on-off switch”
The switch is a DNA segment called an operator
Usually found inside the promoter
An operon is everything involved: operator, promoter and genes the operon controls

6. Regulation by Operons
Operons can be switched off by proteins called repressors
Repressors prevent gene transcription by binding to operators and blocking RNA polymerase
Repressors are gene products of separate regulatory genes
Repressors can be active or inactive
Corepressors make repressors active so they can turn operons off (allosteric regulation)

7. Regulation by Operons If repressor present and active:
No transcription
No gene products made
If repressor absent or inactive:
Transcription occurs
Gene products made

8. Negative Gene Regulation
Operons can be repressible or inducible
Repressible operons are usually ON
An active repressor can shut them off
Inducible operons are usually OFF
Inducer molecule inactivates repressor and therefore turns on operon
Ex. lac operon is an inducible operon

9. Negative Gene Regulation
Repressible operons usually found in anabolic pathways (i.e. Pathways that make bigger molecules)
When enough product made, operon turns off
Inducible operons usually found in catabolic pathways (i.e. Pathways that break down molecules)
Presence of reactant molecule will turn on operon when necessary

10. Example of Inducible Operon: lac operon
E.coli can metabolize both glucose or lactose as energy sources but it prefers glucose
If environmental glucose is low and lactose is high, lac operon transcribes genes for enzymes used to break down lactose
3 genes needed for lactose metabolism:
lacZ gene codes for b-galactosidase: Breaks lactose into glucose and galactose
lacY gene codes for permease: Allows lactose into cell
lacA gene codes for transacetylase: Unknown role

11. lac operon: If no lactose
Since the lac operon is inducible, it is normally OFF when no lactose
Would be a waste to make enzymes to break down lactose if no lactose available
lac operon normally switched off by ACTIVE repressor which binds to operator
Repressor made by regulatory gene lacI
Repressor prevents RNA polymerase from transcribing operon’s genes (lacZ, lacY, lacA)

12. Lactose absent, repressor active, operon off
Regulatory gene
Promoter
Operator
lacI
lacZ
No RNA made 3
RNA polymerase 5
Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes.
Active repressor
Lactose absent, repressor active, operon off
i.e. No genes transcribed for lactose metabolism

13. lac operon: Lactose present
If lactose present, b-galactosidase breaks down lactose into allolactose first
Allolactose binds to repressor, making it inactive
Inactive repressor can’t block RNA polymerase so operon transcribes genes (lacZ, lacY, lacA)
Allolactose acts as an inducer in this role

14. (b) Lactose present, repressor inactive, operon on
lac operon
lacI
lacZ
lacY
lacA
RNA polymerase 3 5
-Galactosidase
Permease
Transacetylase
Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes.
Inactive repressor
Allolactose (inducer)
(b) Lactose present, repressor inactive, operon on
i.e. Genes transcribed to metabolize lactose

15. Regulation by Inducible Operons
If lactose present:
Allolactose (inducer) made
Inducer makes repressor INACTIVE
Operon ON
Genes for lactose metabolism transcribed
If lactose absent:
No allolactose (no inducer) made
Repressor stays ACTIVE
Operon stays OFF
Genes for lactose metabolism NOT transcribed

16. Eukaryotic Gene Regulation
Remember that all cells are genetically identical
Yet each typical human cell only expresses 20% of its genes on average at a given time
How do cells have such differential gene expression?

17. Eukaryotic Gene Regulation
Remember that all cells are genetically identical
Yet each typical human cell only expresses 20% of its genes on average at a given time
How do cells have such differential gene expression?
Lots of levels of eukaryotic gene expression control, from transcription to translation and beyond!

18. Summary of stages of gene expression in eukaryotes
Signal
NUCLEUS
Chromatin
Summary of stages of gene expression in eukaryotes
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)

19. Regulation of Chromatin Structure
Recall from Chapter 16 that eukaryotic DNA is wrapped around histone proteins, forming chromatin
Chromatin does more than just compact DNA to fit in nucleus
Amount of wrapping can control whether or not gene expression occurs
Tight wrapping = reduced transcription
Looser wrapping = more transcription
Heterochromatin regions very tightly wrapped with little transcription (telomere & centromere regions)

20. Regulation of Chromatin Structure
Chemical modification to histones and DNA can also influence gene expression
Acetylation (adding acetyl groups, -COCH3) to positively charged histone tails reduces attraction to negatively charged DNA
Acetylation loosens chromatin = more transcription
Deacetylation (removing acetyl groups) = less transcription

21. Acetylation of Histones
Histone tails (+ charged)
DNA double helix (- charged)
Nucleosome
(a) Positive tails attracted to negative DNA; Prevents transcription
Figure 18.7 A simple model of histone tails and the effect of histone acetylation.
Unacetylated histones
Acetylated histones
(b)
Acetylation makes histones less positive so less attraction
to DNA; Permits transcription

22. DNA Methylation
Methylation (adding methyl groups, -CH3) can occur on DNA itself, usually to cytosine nucleotides
In general, more methylation = less transcription
i.e. Methylation turns genes “off”
Ex. Inactivated genes on mammalian X chromosome that becomes Barr body are heavily methylated
Remember those tortoise shell
cats?

23. DNA Methylation
Methylation patterns can be passed on from cell to cell
Helps cells differentiate into cell types during embryonic development
But methylation patterns can ALSO change, thus altering gene expression

24. Epigenetics But wait a minute! That’s a big deal!
Think about this: You inherit your gene sequence BUT the expression of those genes could change over time with DNA methylation changes

25. Epigenetics Epigenetics has profound implications for human health
Inheritance of traits not involving the nucleotide sequence is called epigenetic inheritance
Epigenetics has profound implications for human health
Ex. Could explain why one identical twin has a particular disease and one doesn’t
Ex. Some cancers shown to have a connection to epigenetic modification