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Gene Control
Prokaryotes vs. Eukaryotes
Gene regulation

Two types of genes:
1)Structural genes – code specific
proteins
2)Regulatory genes – control activity of
structural genes (gene expression)
Gene regulation in prokaryotes

Operons control rate of transcription.
 group of genes working together  code for
enzymes regulating specific metabolic
pathway.
OPERON
Regulator
gene
Produces the
repressor
Promoter
Operator
RNA polymerase Repressor
binding site
binding site
Structural
gene A
Structural
gene B
The Operon Model

Operon – group of genes with
related functions

Genes – code for specific proteins
Promoter – RNA pol binding site

– controls transcription of ALL genes in
operon
– Single mRNA produced
These 3
make up an
operon!

Operator – binding site of
repressor protein (turns off gene)
Quick Review in Metabolic regulation

Allosteric inhibitors – provide feedback
inhibition (enzyme regulators)

Product of pathway
signals continuation of
path to STOP

Making all these
enzymes is wasteful
Induction: Ex-The Lac Operon
Repressor protein
**NO lactose
present**
RNA Polymerase
Codes for
Regulator
gene
Promoter
Operator
1
2
3
*Presence or absence of lactose regulates production of enzymes
from structural genes of the lac operon*
**Lactose present**
Lactose molecules
Repressor protein
Codes for
Regulator
gene
Promoter
Operator
1
2
3
Lactose is an inducer
*Why do cells have these “on/off ” switches?*
Animation
Repression: Ex – Tryp operon
Excess tryptophan present, binds to tryp
repressor protein triggering repressor to bind
to DNA
– blocks (represses) transcription
– tend to be anabolic pathways

RNA
polymerase
RNA
trp repressor
TATA
polymerase
gene1
gene2
gene3
gene4
1
2
3
4
enzyme1
enzyme2
enzyme3
enzyme4
mRNA
promoter
DNA
trp
operator
trp
trp
trp
trp
trp
repressor
repressor protein
trp
trp
Tryp is an effector - activates
repressor
trp
trp repressor
tryptophan
trp
tryptophan – repressor protein
complex
Gene regulation in
prokaryotes - summary

Genes for metabolic pathways linked together in
operons with a common switch mechanism (operator).

No introns – no RNA processing

Structural genes undergo transcription & translation
simultaneously.

Regulation occurs by switching all genes of a pathway
on or off.
What about Eukaryotes? How are they
different? What might the process have to
accommodate for? How might it do this?
 Eukaryotes
often multicellular
Battle changing
environment
 Must
maintain homeostasis
 Coordinate

body as a whole
Differentiated & specialized cells
When does gene control occur???
1.
2.
3.
4.
5.
6.
Packing/unpacking DNA
Transcription
mRNA processing
Translation
Protein processing
Protein degradation
1. DNA Packing
If all 46 of your chromosomes were lined up in a row,
your DNA would be over 3 feet long. How can your
cells contain this large amount of material when
cells are microscopic?

Coils & Folds
–
–
–
–
–
Double helix
Nucleosomes
Chromatin fiber
Looped domains
Chromosomes
from DNA double helix to
condensed chromosome

Degree of DNA packing regulates transcription
– Tightly packed = no transcription = genes OFF
“Dark” DNA = tight
 “Light” DNA = loose


Repressors - (ex: adding -CH3’s) block
transcription factors  no transcription 
genes OFF!

Activators (ex: adding –COCH3’s) unwind DNA 
coils loosen  transcription  genes ON!
2. Transcription Initiation

Control regions on DNA
– Promoter

nearby control sequence –
“standard” rate
 bind RNA pol
 bind transcription factors
– Enhancer

distant control sequence –
“enhanced” rate
 bind activator proteins
Transcription complex…
Activator Proteins
• regulatory proteins bind to DNA at
Enhancer Sites
distant enhancer sites
• increase the rate of transcription
regulatory sites on DNA
distant from gene
Activator
Activator
Activator
Coactivator
B
A
TFIID
E
F
RNA polymerase II
H
Core promoter
and initiation
complex
Initiation Complex - transcrip activated when “hairpin loop” brings TF’s
on enhancer sequence (activators) to TF’s bound to RNA pol on promoter
 protein-protein interactions KEY!
3. Post-transcriptional control

Alternate splicing pattern
– ↑ variation in protein family
4. Regulation of mRNA degradation

Lifespan of mRNA controls amt of protein
synthesized
– mRNA can last from hrs to weeks!
What if degradation is interfered with???
RNAi
 Small

interfering RNAs (siRNA)
short RNA (21-28 bases)
bind to mRNA
 create sections of double-stranded
mRNA
 “death” tag for mRNA
 triggers degradation


gene “silencing”
post-transcriptional control
 turns off gene = no protein

5. Control of Translation

Block initiation of translation
– Regulatory proteins attach to 5’ end
Prevent attachment of ribosome & initiator tRNA
 Synthesis turned OFF

6/7. Protein Processing & Degradation

Protein processing
– Folding, cleaving, adding sugar groups,
targeting for transport

Protein degradation
– “death tags” (ubiquitin -76 aa’s)
– Proteasomes – degradation machinery
Regulation
Prokaryotes
1.
Genes for metabolic pathways
linked together in operons
w/common switch mechanism
(operator).
Eukaryotes
1.
Pathways separated, no
operons.
2.
Genes switched on
separately.
3.
Introns on genes removed in
RNA processing.
4.
Transcription & translation
do not occur simultaneously.
5.
Large number of control
2. No introns - no RNA processing
3. Structural genes undergo
transcription & translation
simultaneously.
4. Regulation occurs by switching
all genes in pathway on or off.