Download AP Bio Ch 15

Regulation of Gene
Expression
AP Biology
Ch 15
Gene  Protein Control
• Feedback inhibition –
enough product is made
the system shuts down
– More product is made
when needed
– The product shuts down
the process
• Gene Expression –
genes are only expressed
when needed. Often
regulated at
transcription.
Gene Expression: Prokaryotes
• Operon – grouped genes that are transcribed together –
code for functionally similar proteins
• Key Players
– Promoter – section of DNA where RNA polymerase binds
– Operator – Controls activation of transcription
• on off switch
• between promoter and genes for proteins – structural genes
– Repressor protein – binds to operator to block RNA polymerase
and shut down transcription
• Turns off the operon
• Corepressor – keeps the repressor protein on the operator
– Trp operon
• Inducer – pulls repressor off the operator
– Turns on the operon – lactose on the lac operon
– Regulatory gene – produces the repressor protein
– Structural genes – code for proteins
Positive and Negative Gene Regulation
• Negative
• Positive
– Repressible: usually on but
can be inhibited trp operon,
allosteric inhibition,
tryptophan present prevents
its own production.
(anabolic)
– Inducible: usually off, but
can be turned on, an inducer
(a specific small molecule,
allolactose in the lac operon)
inactivates the repressor and
allows transcription
(catabolic)
– E. coli prefer to use glucose for
energy, they will only use lactose
when glucose is in short supply
–
glucose
cAMP binds to
regulatory protein “CAP” & stimulates
gene transcription
Positive gene regulation!
– The cAMP & CAP combination allow
RNA polymerase to bind to the
promoter sequence more efficiently.
– Remember cAMP is regulating the
gene expression in the bacteria
Trp operon: repressible,
always making tryptophan,
repressed if tryptophan is
“eaten” tryptophan is
necessary for the cell to
function
Lac operon: inducible,
only turned on if lactose is
“eaten” lactose is not
necessary for the cell to
function
Eukaryotic Chromosome
• Chromosomes – tightly coiled DNA
around proteins during cell division
• Chromatin – loosely packed DNA
around proteins
• Histones – protein which the DNA
wraps around
• Nucleosomes – grouped histones
together
– Heterochromatin – tighter packed
chromatin
• Not transcribing
– Euchromatin – looser packed chromatin
• Transcription occurring
Gene Expression: Eukaryotes
• Cell Differentiation –
cell specialization
• All cells contain the
same genes
• The genes that are
expressed determines
the type of cell
– Ex: Skin cell vs. a nerve
cell
Chromatin Regulation
• Histone acetylation –
allows transcription
factors to bind to DNA
allowing transcription to
occur
– Creates loosely packed DNA
- euchromatin
• DNA Methylation – occurs
after DNA synthesis has
occurred
– Lower transcription rates
– One X in females is highly
methylated
– Works w/ a deacetylation
enzyme in some spp.
Epigenetic inheritance
• Not controlled by base
sequences.
• DNA methylation
(deactivates one
homologous chromosome)
may explain abnormal or
unexpected DNA
expression as is often seen
in identical twins.
http://images.the-scientist.com/content/images/general/55342-1.jpg
Regulation of Transcription
• Transcription involves RNA Polymerase II and
transcription factors
• RNA polymerase II attaches to the promoter
(TATA box) sequence to begin transcription
• Control elements – non coding sequences of
DNA where the transcription factors attach
Regulation of Transcription
..\..\..\AP Bio 15-16\Genetics\15_10TranscripInitiation_A.swf
• Enhancer – control element far from a gene
or intron
• Activator – bind to enhancers to turn on
transcription of a gene
• Transcription factors + enhancer + activator
+ RNA Polymerase II = transcription
initiation complex
– Needed for transcription to begin
• Repressors – inhibit gene expression
– Turn off transcription
– Block activators from binding to enhancers
Distal control
element
Promoter
Activators
Gene
Enhancer
TATA
box
General
transcription
factors
1
Activator proteins bind
to distal control elements
grouped as an enhancer in
the DNA. This enhancer has
three binding sites.
DNA-bending
protein
Group of
Mediator proteins
2
A DNA-bending protein
brings the bound activators
closer to the promoter.
Other transcription factors,
mediator proteins, and RNA
polymerase are nearby.
RNA
Polymerase II
Chromatin changes
3
The activators bind to
certain general transcription
factors and mediator
proteins, helping them form
an active transcription
initiation complex on the promoter.
Transcription
RNA processing
mRNA
degradation
RNA
Polymerase II
Translation
Protein processing
and degradation
Transcription
Initiation complex
RNA synthesis
RNA Processing Regulation
• Alternative RNA Splicing – different regions of the
pre-mRNA serve as introns or exons creating different
mRNA strands depending on what is removed &
spliced together.
mRNA Degredation
• Prokaryotes
– Short Life span
– Degraded in seconds
– Allows rapid response to
environmental changes
• Eukaryotes
– Survive from hours to
weeks
– Internal conditions
constant, no need for
rapid response
ncRNA: 1000’s of RNA’s, current
research
• miRNA’s - micro RNA hat can
degrade mRNA or block
translation
• Causes mRNA to fold on
itself and base pair to create
dsRNA which is then
digested with an enzyme
• Short interferring RNA
(siRNA) – also degrade
mRNA or block translation
(blocking by siRNA is called
RNAi, or RNA interferance)
Protein Degradation
18_12ProteinDegradation_A.swf
• Proteosomes – break apart proteins in to
smaller peptide units
Chromatin changes
Transcription
RNA processing
mRNA
degradation
Proteasome
and ubiquitin
to be recycled
Ubiquitin
Translation
Proteasome
Protein processing
and degradation
Protein to
be degraded
Protein
fragments
(peptides)
Ubiquinated
protein
Protein entering a
proteasome
Single Gene Expression
• Different cells express different genes,
therefore they make different mRNA’s
• We can detect mRNA in a cell using nucleic
acid hybridization, by pairing it to a nucleic
acid probe
• Each probe is labeled with a fluorescent tag to
allow visualization
• The technique allows us to see the mRNA in
place (in situ) in the intact organism and is
thus called in situ hybridization
Figure 15.16
Technique
1 cDNA synthesis
mRNAs
cDNAs
Primers
2 PCR amplification
-globin
gene
3 Gel electrophoresis
Results
Embryonic stages
1 2 3 4 5
6
Figure 15.15-5
DNA in nucleus
1 Test tube containing
reverse transcriptase
and mRNA
2 Reverse transcriptase
makes the first
DNA strand.
mRNAs in
cytoplasm
mRNA
5
Reverse
transcriptase
A A A A A A 3
T T T T T 5
3
DNA Primer
strand
3 mRMA is degraded.
5
3
4 DNA polymerase
synthesizes the
second strand.
Poly-A tail
A A A A A A 3
T T T T T 5
5
3
3
5
DNA
polymerase
5 cDNA carries complete
coding sequence
without introns.
5
3
cDNA
3
5
Groups of Gene Expression
• Recall Microarray assays:
• Used to pinpoint differences in gene
expression between 2 different cell types
• How it’s done:
– Sequence a genome
– Use PCR to copy the genes (verification steps
here)
– Split the genes into single strands
– Place the single stranded DNA onto microscope
slides in spots (robots & computers do all this)