Today…
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Turn in Transcription Pogils
Homework – Genetics in the News (due
Sunday)
Transcription/Translation – notes on
website.
Virus day we will cover any knowledge
gaps on TNT & DNA replication
Lecture on Gene Regulation
Lac Operon activity
AP Biology
Control of
Eukaryotic Genes
AP Biology
2007-2008
The BIG Questions…
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How are genes turned on & off
in eukaryotes?
How do cells with the same genes
differentiate to perform completely
different, specialized functions?
AP Biology
Evolution of gene regulation
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Prokaryotes
single-celled
 evolved to grow & divide rapidly
 must respond quickly to changes in
external environment
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Gene regulation
turn genes on & off rapidly
 adjust levels of enzymes
for synthesis & digestion
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AP Biology
Evolution of gene regulation
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Eukaryotes
multicellular
 evolved to maintain constant internal
conditions while facing changing
external conditions
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regulate body as a whole
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AP Biology
Homeostasis
growth & development
specialization
Points of control
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“Decoupling” of
transcription and translation
allows for more control.
1. Access to the DNA
2. Start of Transcription
3. mRNA processing
4. mRNA transport
5. Translation
6. How protein is processed
7. Do we degrade/destroy the
protein
AP Biology
1. Access to DNA: DNA packing
If access to DNA
is prevented by
it’s packaging,
NO TRANSCRIPTION
But we HAVE to
package it to get it to
fit in the nucleus!
What’s a cell to do?
AP Biology
How Does DNA Pack So Tightly?
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Nucleosomes – Like Beads on a String
1st level of DNA packing
 histone proteins
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AP Biology
8 protein molecules
positively charged amino acids
bind tightly to negatively charged DNA
DNA packing as gene control
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Degree of packing of DNA regulates transcription
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tightly wrapped around histones
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no transcription
genes turned off
 heterochromatin
darker DNA (H) = tightly packed
 euchromatin
lighter DNA (E) = loosely packed
H
AP Biology
E
Histone acetylation
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Acetylation of histones unwinds DNA
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attachment of acetyl groups (–COCH3) to histones
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AP Biology
enables transcription
genes turned on
Histone proteins are less positive, so they hug the
DNA less tightly
Transcription factors have easier access to genes
So, if you see Histone Acetylation
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THINK TRANSCRIPTION!
AP Biology
DNA methylation- STOP TRANSCRIPTION
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Methylation of DNA blocks transcription factors
no transcription
 genes turned off
 attachment of methyl groups (–CH3) to cytosine
 nearly permanent inactivation of genes
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AP Biology
ex. inactivated mammalian X chromosome = Barr body
Cancer & DNA Methylation
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Does DNA Methylation start or promote
transcription?
Hypermethylation (p53)
Hypomethylation (oncogenes)
Inherited patterns of methylation
(heredity predisposition to cancers)
AP Biology
2. Transcription initiation
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Control regions on DNA
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Promoter
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Enhancer
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AP Biology
nearby control sequence on DNA
binding of RNA polymerase & transcription
factors
distant control
sequences on DNA
binding of activator
proteins
“enhanced” rate (high level)
of transcription
Model for Enhancer action
AP Biology
3. Post-transcriptional control
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Alternative RNA splicing
variable processing of exons creates
different mRNA molecules
 Proteins manufactured are related
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AP Biology
Ex: muscle cell producing two proteins actin and
myosin
4. Regulation of mRNA degradation
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Why would we need to limit how long
mRNA can last in the cytoplasm?
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5’cap and 3’ tail prevent degredation
AP Biology
RNA interference
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Small interfering RNAs (siRNA)
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short segments of RNA (21-28 bases)
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binds to mRNA
“death” tag for mRNA
 triggers degradation of mRNA
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causes gene “silencing”
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post-transcriptional control
turns off gene = no protein produced
siRNA
AP Biology
siRNA in the News
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Link
AP Biology
5. Control of translation
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Block initiation of translation stage
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regulatory proteins attach to 5' end of mRNA
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AP Biology
prevent attachment of ribosomal subunits &
initiator tRNA
block translation of mRNA to protein
6-7. Protein processing & degradation
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Protein processing
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folding, cleaving, adding sugar groups,
targeting for transport
Protein degradation
ubiquitin tagging (marked for destruction)
 proteasome degradation (dismantles protein)
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AP Biology
6
7
Gene Regulation
protein
processing &
degradation
1 & 2. transcription
- DNA packing
- transcription factors
5
4
initiation of
translation
mRNA
processing
3 & 4. post-transcription
- mRNA processing
- splicing
- 5’ cap & poly-A tail
- breakdown by siRNA
5. translation
- block start of
translation
1 2
initiation of
transcription
AP Biology mRNA splicing
3
6 & 7. post-translation
- protein processing
- protein degradation
4
mRNA
protection
TABOO!
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Get up (ah – come on, you can do it)
Find a partner at a DIFFERENT table
The person who ate cake the most
recently will guess first
Guesser, face the back of the room and
close your eyes
AP Biology
WORD YOUR PARTNER MUST GUESS:
PROMOTER
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1.
2.
3.
TABOO WORDS (YOU CANNOT USE)
Promotes
Transcription or Transcription Factor
Starts
AP Biology
Now Guessers will give clues…
AP Biology
WORD YOUR PARTNER MUST GUESS:
SPLICING
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1.
2.
3.
TABOO WORDS (YOU CANNOT USE)
Cutting
Removing
RNA
AP Biology
FIND A NEW PARTNER
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Person who has the most pets guesses
first.
Get ready… Guessers, turn to the back
and close your eyes
AP Biology
WORD YOUR PARTNER MUST GUESS:
siRNA
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1.
2.
3.
4.
TABOO WORDS (YOU CANNOT USE)
small
interfering
RNA
silencing
AP Biology
SWITCH ROLES
AP Biology
WORD YOUR PARTNER MUST GUESS:
ENHANCER
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1.
2.
3.
TABOO WORDS (YOU CANNOT USE)
Distant
Makes Better
Promotes
AP Biology
GOOD JOB! Back to your seats…
AP Biology
Control of
Prokaryotic (Bacterial) Genes
AP Biology
2007-2008
Gene regulation in bacteria
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Cells vary amount of specific enzymes
by regulating gene transcription
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turn genes on or turn genes off
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Depends on what the cell needs
AP Biology
Bacteria group genes together
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Operon
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genes grouped together with related functions
Two types: INDUCIBLE and REPRESSIBLE
Structures of an Operon (PROG)
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promoter = RNA polymerase binding site
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single promoter controls transcription of all genes in
operon
transcribed as one unit & a single mRNA is made
repressor = protein that binds to operator & stops
trascription
 operator = DNA binding site of repressor protein
 genes = sequences that code for proteins
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AP Biology
Operon model
Operon:
operator, promoter & genes they control
serve as a model for gene regulation
RNA
polymerase
RNA repressor
polymerase
gene1
gene2
gene3
gene4
1
2
3
4
enzyme1
enzyme2
enzyme3
enzyme4
mRNA
promoter
DNA
operator
Repressor protein turns off gene by
blocking
AP BiologyRNA polymerase binding site.
repressor
= repressor protein
Repressible operon: tryptophan
When excess tryptophan is present, it binds to tryp
repressor protein & triggers repressor to bind to DNA
RNA
polymerase
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blocks (represses) transcription
RNA
trp repressor
polymerase
gene1
gene2
gene3
gene4
1
2
3
4
enzyme1
enzyme2
enzyme3
enzyme4
mRNA
promoter
DNA
trp
operator
trp
trp
repressor
repressor protein
trp
trp
trp
trp
trp
trp
conformational change in
AP Biologyprotein!
repressor
trp
repressor
tryptophan
trp
tryptophan – repressor protein
complex
Tryptophan operon
What happens when tryptophan is present?
Don’t need to make tryptophan-building enzymes
Tryptophan
AP Biology
is allosteric regulator of repressor protein
Inducible operon: lactose
lac
lac
RNA
polymerase
lac
Digestive pathway model
lac
When lactose is present, binds to
lac repressor protein & triggers
repressor to release DNA
lac
lac
lac
RNAlac repressor
polymerase
induces transcription
gene1
gene2
gene3
gene4
1
2
3
4
enzyme1
enzyme2
enzyme3
enzyme4
mRNA
promoter
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operator
repressor
lac
conformational change in
AP Biologyprotein!
repressor
lac
repressor
DNA
repressor protein
lactose
lactose – repressor protein
complex
Lactose operon
What happens when lactose is present?
Need to make lactose-digesting enzymes
Lactose is allosteric regulator of repressor protein
AP Biology
UpRegulation – increase the rate!
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Ex: CAP/cAMP system
When lactose is
present and glucose is
low:
 cAMP is high
 cAMP activates
Catabolite Activator
Protein (CAP)
 Increases the rate of
transcription by
100x!
AP Biology
Operon summary
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Repressible operon
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usually functions in anabolic pathways
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synthesizing end products
when end product is present in excess,
cell allocates resources to other uses
Inducible operon
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usually functions in catabolic pathways,
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produce enzymes only when nutrient is
available
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AP Biology
digesting nutrients to simpler molecules
cell avoids making proteins that have nothing to do,
cell allocates resources to other uses
1961 | 1965
Jacob & Monod: lac Operon
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Francois Jacob & Jacques Monod
first to describe operon system
 coined the phrase “operon”
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AP Biology
Jacques Monod
Francois Jacob
Don’t be repressed!
How can I induce you
to ask Questions?
AP Biology
2007-2008
Lac Operon
Turning on your genes
AP Biology
The model
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You will be using a simple model to
illustrate how and when the lac operon
is turned on and off.
Complete the worksheet questions on a
separate piece of paper as you work
through the concepts identified.
AP Biology
Vocabulary
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Operon/ Lac operon: (Entire labeled
dowel)
Promoter: Green portion
Regulator: Blue Portion
Operator: Red Portion
RNA polymerase: Clear Tube with Red
Tape
Repressor Protein: Clear Tubing with Split
Lactose/Inducor: Paperclip
AP Biology
Lab Time
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You will have the remainder of class to
complete the simulation.
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Please follow in order, as to understand
the process.
AP Biology
Operon Review
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https://www.youtube.com/watch?v=10Y
WgqmAEsQ
AP Biology