REGULATION OF GENE
EXPRESSION
Chapter 18
Gene expression
• A gene that is expressed is “turned
on”.
• It is actively making a product
(protein or RNA).
• Gene expression is often regulated at
transcription.
• Newly discovered roles of RNA in
gene expression
Regulation of a metabolic pathway
Prokaryotic Gene Regulation
• Adjust activity of enzymes already
present
– Often through negative feedback
• Adjust production level of certain
enzymes
OPERONS
• Regulation in prokaryotes
– Operator – switch segment of DNA
in promoter
– Operon – the promoter, the
operator, and the genes they
control
– Regulatory gene – long distance
from gene that is regulated
The trp operon: regulated synthesis of repressible enzymes
trp animation
trp tutorial
The trp operon: regulated synthesis of repressible enzymes
The lac operon: regulated synthesis of inducible enzymes
lac operon animation
lac operon tutorial
The lac operon: regulated synthesis of inducible enzymes
• Regulatory gene makes protein
(repressor) that inhibits
operator
• Regulatory protein has inactive
and active shape
– Corepressor – makes
repressor active
– Inducer – inactivates repressor
• Repressible enzymes usually
used when cell makes something
(ex. tryptophan)
• Inducible enzymes usually used
when cell breaks something down
(ex. lactose)
Positive control: cAMP receptor protein
POSITIVE GENE REGULATION
an example…
• Cyclic AMP (cAMP) accumulates
when low sugar
• cAMP receptor protein (CRP)
attaches to cAMP and changes
shape so it becomes and activator
• CRP binds to DNA at lac operon so
cell can break down lactose
Eukaryotic Gene Regulation
• Expression can be regulated at any
stage
• Differential gene expression –
different cells in an organism express
different genes from the genome
• Much regulation occurs at
transcription like prokaryotes, but
even more possibilities in eukaryotes
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CHROMATIN
Composed of DNA and proteins
called histones
Nucloesome – DNA wrapped
around a histone
Forms looped domains
Heterochromatin – highly
compacted DNA so generally is
not transcribed
Levels of chromatin packing
GENOME ORGANIZATION
• 1.5% of DNA in humans codes for
protein
• 24% introns and regulatory
• Most is repetitive DNA (59%)
• Unique noncoding is 15%
Opportunities for the control of gene expression in eukaryotic cells
Eukaryotic Regulation
• At DNA level
– Chromatin modification, DNA unpacking
with histone acetylation and DNA
demethylation
• At RNA level
– Transcription, RNA processing,
transport to cytoplasm
• At protein level
– Translation, protein processing,
transport to cellular destination, protein
degradation
GENE EXPRESSION
• Not all genes are turned on all of
the time!
GENE REGULATION
• Regulation of chromosome structure
– Histone acetylation (-COCH3) loosens
chromatin so transcription can occur
– DNA methylation (-CH3) inactivates DNA
• Responsible for X-inactivation
• Genomic imprinting – in mammals,
methylation turns off paternal or
maternal allele of certain genes at start
of development
• Epigenetic inheritance – inheritance of
traits not directly involving DNA sequence
(all of the above)
• Regulation of transcription
– Control elements– upstream of
promoter; help regulate
transcription by binding certain
transcription factors
– Transcription factors – mediate the
binding of RNA polymerase to the
promoter
– Enhancers – far upstream of gene;
bind to transcription factors; called
distal control element
Figure 19.8 A eukaryotic gene and its transcript
– Activator – transcription factors
bound to enhancer that stimulate
transcription
– Not many different control
elements so the combination of
control elements regulates gene
action
• Different combos of activators
makes different genes turned on
• Different genes can be turned on by
same activator
Cell-type specific transcription based on
available activators
• Coordinate gene expression
– Genes that should be turned on
together have same enhancers so
that same transcription factor(s)
is(are) needed
• Ex. estrogen activates multiple
genes to prepare the uterus for
pregnancy
• Post transcription regulation
– RNA processing (alternative
splicing)
– Lifespan of mRNA in cell controls
expression
– Removal of caps leads to mRNA
destruction
• Translation Regulation
– Translation prevented by
regulatory proteins by not letting
ribosome to attach to mRNA
– Once a protein is made,
ubiquitin can be added to signal
its destruction
– Proteasomes – degrade proteins
with ubiquitin
Degradation of a protein by a proteasome
Noncoding RNAs and gene expression
• Discovering more about RNA’S that do not
make protein
• MicroRNAs (miRNA) – small, single stranded
RNA generated from a hairpin on precursor
RNA; associates with proteins that can
degrade or prevent translation of mRNA with
complementary sequence
• Small interfering RNAs (siRNA) – like
miRNA, but made from longer sections of
double stranded RNA (not hairpins)
• Other small RNA’s are involved in remodleing
chromatin structure and other regulatory
processes
DIFFERENTIAL GENE EXPRESSION =
DIFFERENT CELL TYPES
• Cell differentiation – process by
which cells become specialized in
structure and function
• Morphogenesis – process that gives
an organism its form (shape)
• How do different sets of activators
come to be present in two cells?
– Cytoplasmic determinants (materials in
cyctoplasm)
– Environment surrounding a cell
Sources of Developmental Info for early embryo
Pattern formation
• Pattern formation – development of
spatial organization in which tissues
and organs are in their correct places
• Positional information – molecular
cues that control pattern formation
• Homeotic genes – control pattern
formation
CANCER
• Oncogenes- cancer causing genes
in retroviruses
• Proto-oncogenes – normal genes
that code for proteins that
stimulate cell growth and division
• Tumor suppressor genes - make
proteins that help prevent
uncontrolled cell growth
Converting proto-oncogene into oncogene
Converting proto-oncogene into oncogene
• Movement of DNA within a
chromosome
– May place a more active promoter
near a proto-oncogene (= more cell
division)
• Amplification of a proto-oncogene
• Point mutations in control element or
proto-oncogene (= more expression or
makes abnormal protein that doesn’t
get degraded or is more active)
GENES INVOLVED IN CANCER
• Ras gene – makes ras (G) protein that starts
cascade reactions that initiate cell division
– Mutations in Ras gene cause ~30% cancers
• p53 tumor suppressor gene – “guardian of
genome”
– Activates p21 which halts cell cycle
– Turns on genes to repair DNA
– Activates suicide proteins that cause cell death
(apoptosis)
– Mutations in P53 gene cause ~50% cancer
Multistep Model of Cancer Development
• Approximately half dozen changes have
to occur at the DNA level for cancer to
develop.
• Need at least one oncogene and loss of
tumor suppressor gene(s)
• Most oncogenes are dominant and most
tumor suppressor genes recessive so
must knock out both alleles
• Typically telomerase is activated
A multi-step model for the development of colorectal cancer
Inherited Predisposition to Cancer
• 15% colorectal cancers are inherited
– Most from mutated APC gene
(tumor suppressor gene)
• 5-10% breast cancers are inherited
– Most with mutated BRCA1 and
BRCA2
– A woman with one mutant BRCA1
gene (tumor suppressor gene) has a
60% chance of getting breast
cancer by age 50