Transcript You Light Up My Life

Control of Gene
Expression
Chapter 16
Contolling Gene Expression
What does that mean?
 Regulating which genes are being
expressed
 transcribed/translated
 i.e.

made into proteins
Not all genes are expressed all of the time
Controlling Gene Expression
Why do cells control gene expression?
 Each cell in an organism contains the
exact same set of DNA (i.e. 6 billion bp,
~30,000 genes)
 What is the difference, then, between a
skin cell and a nerve cell?
 The
proteins found within the cell, i.e. the
genes that are expressed
 Allows for the process of differentiation
Controlling Gene Expression
All organisms regulate when and for how
long a gene is on
 This regulation allows for the conservation
of energy
 In eukaryotes differential gene expression
is what creates the different cell types
 Too much or too little expression can lead to
disease, aging, etc.

Control of Genes
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Regulatory proteins interact with DNA, RNA or
other proteins to control the expression of
genes
Transcription factors are regulatory proteins
which interact with DNA at specific sequences to
regulate gene activity
Two types of control:
 Negative control slows down or stops gene
activity
 Positive control promotes gene activities
Gene Control in Prokaryotes
No nucleus separates DNA from
ribosomes in cytoplasm
 Translation occurs even before mRNA
transcripts are finished
 Control functionally related genes
together by grouping them into units
called Operons

 E.g.
enzymes in a biosynthesis pathway
Operons in Prokaryotes
Consist of a Regulatory Gene, Operator,
Promoter and the Structural genes they
control
 Transcription of these genes is initiated by
one promoter, and controlled by a single
operator
 Transcribes as 1 unit, and a single mRNA
is made, which is later translated into one
polypeptide, which is later cleaves into
individual proteins - polycistronic

Polycistronic Expression
Inducible Operon - Lac Operon
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Encodes genes necessary to process lactose
Not needed unless lactose is present
If there is no lactose:
Lac Operon
Regulatory gene
Operator
RNA Polymerase
Repressor
Structural
genes not
transcribed
Inducible Operon - Lac Operon
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What happens when lactose is present?
Need to make lactose-digesting enzymes
Lactose binds allosterically to regulatory protein:
Inducible vs. Repressible Operons
Inducible operon (e.g. Lac operon)
 usually functions in catabolic pathways, digesting
nutrients to simpler molecules
 produce enzymes only when nutrient is available
 cell avoids making proteins that have nothing to do
Repressible operon (e.g. Tryp operon)
 usually functions in anabolic pathways
synthesizing end products
 when end product is present cell allocates
resources to other uses
Eukaryotic Gene Expression


MUCH more complicated that prokaryotic
control
Most genes in eukaryotic cells are turned
off at any given point
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Only 5-10% of genes are being expressed at any
point
Controlling gene expression occurs at
several different points in the process:
Mechanisms of Gene Control
in Eukaryotes
Chromatin Structure
 Eukaryotic DNA wraps around histones, is
further structured into nucleosomes
• Promoters inaccessible
 Chromatin remodeling makes gene promoters
more accessible
• Activators recruit remodeling complexes that
displace nucleosomes
• Activators recruit enzyme that acetylates and
loosens histone assocation with DNA
Chromatin Remodeling
Eukaryotic gene organization
Enhancer
Promoter
Transcription unit of gene
Exon Intron Exon
Intron
Exon
DNA
Regulatory
sequences
TATA 5' UTR
box
3' UTR
Transcription in Eukaryotes
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Transcription factors bind to the TATA sequence
within the promoter of gene to be transcribed
RNA polymerase binds to the transcription
factors (initiates low levels of transcription)
Activators bind to enhancer sequences (may be
located far from the gene)
Activators bind to RNA polymerase and trigger it
to begin transcription (high level of transcription)
Transcription in Eukaryotes
Initial general
transcription factor
DNA
TATA box
Site where
transcription starts
Promoter
Additional general
transcription factors
1 The first general
transcription
factor recognizes
and binds to the
TATA box of a
protein-coding
gene’s promoter.
RNA polymerase
DNA
Transcription
begins
Transcription complex
2 Additional general
transcription factors
and then RNA
polymerase add to
the complex, and
then transcription
begins.
Controlling transcription in Eukaryotes
Transcription regulation in
Eukaryotes
DNA sequences
 Promoters
 Enhancers
 Other regulatory
sequences
Proteins
 Transcription
factors
 Activators
 Inhibitors
Eukaryotic Gene Expression
Regulates cell cycle
 Controls development (Homeo box genes)
 Controls differentiation
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….and lots, lots more!
Apoptosis
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Programmed cell death
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Signals unleash molecular weapons of
self-destruction
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Cancer cells do not commit suicide on cue