1. Ch. 18 Regulation of Gene Expression
Objectives:
LO 3.18 The student is able to describe the connection between the regulation of gene expression and observed differences between different kinds of organisms.
LO 3.19 The student is able to describe the connection between the regulation of gene expression and observed differences between individuals in a population.
LO 3.20 The student is able to explain how the regulation of gene expression is essential for the processes and structures that support efficient cell function.
LO 3.21 The student can use representations to describe how gene regulation influences cell products and function.
LO 3.22 The student is able to explain how signal pathways mediate gene expression, including how this process can affect protein production..
LO 3.23 The student can use representations to describe mechanisms of the regulation of gene expression.

2. 18.1 Bacteria Often Respond to Environmental Change by Regulating Transcription
Conserve resources 1 of 2 ways:
Feedback inhibition (discussed in Ch. 8)
Regulation of gene expression (discussed here)
Precursor
Feedback inhibition
Enzyme 1
Enzyme 2
Enzyme 3
Tryptophan
(a)
(b)
Regulation of enzyme activity
Regulation of enzyme production
Regulation of gene expression 
trpE gene
trpD gene
trpC gene
trpB gene
trpA gene

3. Operons: The Basic Concept and Negative Gene Regulation
Operator (“on/off switch”), promoter, and genes.
Repressible (anabolic) operons: Always “on” until repressor is bound. (inhibited)
Corepressor is like feedback inhibition (product works with repressor)
Ex: tryptophan producing genes
Promoter
DNA
Regulatory gene
mRNA
trpR 5 3
Protein
Inactive repressor
RNA polymerase
trp operon
Genes of operon
Operator
mRNA 5
Start codon
Stop codon
trpE
trpD
trpC
trpB
trpA E D C B A
Polypeptide subunits that make up enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on
(b) Tryptophan present, repressor active, operon off
Tryptophan (corepressor)
Active repressor
No RNA made
Trp operon – always on unless repressed by the repressor + tryptophan. Repressor is always available but in inactive form until tryptophan binds to it.

4. Inducible (catabolic) operons are usually off but can be induced.
Inducer inactivates the repressor
Ex: lac (lactose) operon
(a) Lactose absent, repressor active, operon off
(b) Lactose present, repressor inactive, operon on
Regulatory gene
Promoter
Operator
DNA
lacZ
lacI
mRNA 5 3
No RNA made
RNA polymerase
Active repressor
Protein
lac operon
lacY
lacA
mRNA 5
Inactive repressor
Allolactose (inducer)
-Galactosidase
Permease
Transacetylase
Lac Operon is always off unless induced by inducer. Lac repressor is active once made.

5. Positive Gene Regulation
Gene is always on but activator stimulates transcription.
Ex: cAMP
Promoter
DNA
CAP-binding site
lacZ
lacI
RNA polymerase binds and transcribes
Operator
cAMP
Active CAP
Inactive CAP
Allolactose
Inactive lac repressor
(a)
Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized
Promoter
DNA
CAP-binding site
lacZ
lacI
Operator
RNA polymerase less likely to bind
Inactive lac repressor
Inactive CAP
(b)
Lactose present, glucose present (cAMP level low): little lac mRNA synthesized

6. 18.2 Eukaryotic Gene Expression is Regulated at Many Stages
Signal
NUCLEUS
Chromatin
Chromatin modification: DNA unpacking involving histone acetylation and DNA demethylation
DNA
Gene
Gene available for transcription
RNA
Exon
Primary transcript
Transcription
Intron
RNA processing
Cap
Tail
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
mRNA in cytoplasm
Translation
Degradation of mRNA
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)
Each cell of multicellular organisms contain all genetic info; only some is expressed (differential gene expression).
Each process has the potential for regulation.

7. Regulation of Chromatin Structure
Amino acids available for chemical modification
Histone tails
DNA double helix
Nucleosome (end view)
(a) Histone tails protrude outward from a nucleosome
Unacetylated histones
Acetylated histones
(b)
Acetylation of histone tails promotes loose chromatin structure that permits transcription
Histone Modifications: acetylation loosens chromatin  easier protein access.
DNA Methylation: addition of methyl group to gene turns it off.
Epigenetic Inheritance: gene regulation passed on to offspring.

8. Regulation of Transcription Initiation
Control elements/enhancers upstream from a gene can activate or repress transcription factors to regulate gene expression.
Combination of control elements and their activators.
Like genes use similar control elements and activators.

9. Mechanisms of Post-Transcriptional Regulation
mRNA degradation
Alternative RNA splicing: different intron/exons spliced together.
Exons
DNA
Troponin T gene
Primary RNA transcript
RNA splicing or
mRNA 1 2 3 4 5

10. Animation: Blocking Translation
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.

11. Animation: Protein Processing
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.

12. 18.3 Noncoding RNAs Play Multiple Roles in Controlling Gene Expression
Parts of DNA that make very small RNA (ncRNA) but not proteins; regulate gene expression.
Bind to a complementary sequence of mRNA, blocking translation.
Bind to DNA changing chromatin structure
microRNAs (miRNA): begins as hairpin
Small interfering RNAs (siRNA): begins as double strand
(a) Primary miRNA transcript
Hairpin
miRNA
Hydrogen bond
Dicer
miRNA- protein complex
mRNA degraded
Translation blocked
(b) Generation and function of miRNAs 5 3

13. Embryonic development: division  differentiation  morphogenesis
18.4 A Program of Differential Gene Expression Leads to the Different Cell Types in a Multicellular Organism
Embryonic development:
division  differentiation  morphogenesis
Cytoplasmic Determinants
RNA and proteins from mom’s cell unevenly distributed giving rise to different cells during 1st divisions.
(a) Cytoplasmic determinants in the egg
Unfertilized egg
Sperm
Fertilization
Zygote (fertilized egg)
Mitotic cell division
Two-celled embryo
Nucleus
Molecules of two different cytoplasmic determinants

14. Morphogens (proteins) establish an embryo’s axes
Induction is how embryonic cells effect one another due to cell-surface molecules or growth factors.
Determination due to the expression of genes for tissue-specific proteins.
Pattern Formation puts determined cells in their “proper places” for the resulting organism.
Morphogens (proteins) establish an embryo’s axes
(b) Induction by nearby cells
Early embryo (32 cells)
NUCLEUS
Signal transduction pathway
Signal receptor
Signaling molecule (inducer)

15. Animation: Development of Head-Tail Axis in Fruit Flies
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.