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AP Biology Discussion Notes
2/25/2015
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Goals for Today
• Be able to describe regions of DNA and how they are important to gene expression in Bacteria (Prokaryotes) & Eukaryotes
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Question of the Day 2/25What two things would be found on a DNA sequence before (______ stream of) a coding region or genes?
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Conducting the Genetic Orchestra CH18
• Prokaryotes and eukaryotes alter gene expression in response to their changing environment
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18.1: Bacteria often respond to environmental change by regulating transcription
• Natural selection has favored bacteria that produce only the products needed by that cell
• A cell can regulate the production of enzymes by feedback inhibition or by gene regulation
• Gene expression in bacteria is controlled by the operon model
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Repressible and Inducible Operons: Two Types of Negative Gene Regulation
An inducible operon is one that is usually ____; a molecule called an inducer inactivates the repressor and turns on transcription
What is the “inducer” in the Lac Operon?
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Figure 18.4a
(a) Lactose absent, repressor active, operon off
Regulatorygene
Promoter
Operator
DNA lacZlacIDNA
mRNA5
3
NoRNAmade
RNApolymerase
ActiverepressorProtein
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Figure 18.4b
(b) Lactose present, repressor inactive, operon on
lacI
lac operon
lacZ lacY lacADNA
mRNA5
3
Protein
mRNA 5
Inactiverepressor
RNA polymerase
Allolactose(inducer)
-Galactosidase Permease Transacetylase
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Positive Gene Regulation
• Some operons are also subject to positive control through a stimulatory protein, such as catabolite activator protein (CAP), an activator of transcription
• When glucose (a preferred food source of E. coli) is scarce, CAP is activated by binding with cyclic AMP (cAMP)
• Activated CAP attaches to the promoter of the lac operon and increases the affinity of RNA polymerase, thus accelerating transcription
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• When glucose levels increase, CAP detaches from the lac operon, and transcription returns to a normal rate
• CAP helps regulate other operons that encode enzymes used in catabolic pathways
Positive Gene Regulation
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Figure 18.5a
Promoter
DNA
CAP-binding site
lacZlacI
RNApolymerasebinds andtranscribes
Operator
cAMPActiveCAP
InactiveCAP
Allolactose
Inactive lacrepressor
(a) Lactose present, glucose scarce (cAMP level high):abundant lac mRNA synthesized
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Figure 18.5b
Promoter
DNA
CAP-binding site
lacZlacI
OperatorRNApolymerase lesslikely to bind
Inactive lacrepressor
InactiveCAP
(b) Lactose present, glucose present (cAMP level low):little lac mRNA synthesized
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Fill in the blanks
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Repressible and Inducible Operons: Two Types of Negative Gene Regulation
• A repressible operon is one that is usually ____; binding of a repressor to the operator shuts off transcription
• The trp operon is a repressible operon
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• E. coli can synthesize the amino acid tryptophan
• Think of these questions and think about energy and natural selection:–When would “want” to do this?
–When would “not want” to do this?
trp OPERON
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• By default the trp operon is on; and the genes for tryptophan synthesis are transcribed
• When tryptophan is present, it binds to the trp repressor protein, which turns the operon off
trp OPERON
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• The repressor can be in an active or inactive form, depending on the presence of other molecules
• The repressor is active only in the presence of its corepressor tryptophan; – A corepressor is a molecule that cooperates
with a repressor protein to switch an operon off
• thus the trp operon is turned off (repressed) if tryptophan levels are high
trp OPERON
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Figure 18.3a
Promoter
DNA
Regulatory gene
mRNA
trpR
5
3
Protein Inactive repressor
RNApolymerase
Promoter
trp operon
Genes of operon
Operator
mRNA 5
Start codon Stop codon
trpE trpD trpC trpB trpA
E D C B A
Polypeptide subunits that make upenzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on
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Figure 18.3b-1
(b) Tryptophan present, repressor active, operon off
DNA
mRNA
Protein
Tryptophan (corepressor)
Activerepressor
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Figure 18.3b-2
(b) Tryptophan present, repressor active, operon off
DNA
mRNA
Protein
Tryptophan (corepressor)
Activerepressor
No RNAmade
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Precursor
Feedbackinhibition
Enzyme 1
Enzyme 2
Enzyme 3
Tryptophan
(a) (b)Regulation of enzymeactivity
Regulation of enzymeproduction
Regulationof geneexpression
trpE gene
trpD gene
trpC gene
trpB gene
trpA gene
Figure 18.2
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Promoter
DNA
Regulatory gene
mRNA
trpR
5
3
Protein Inactive repressor
RNApolymerase
Promoter
trp operon
Genes of operon
Operator
mRNA 5
Start codon Stop codon
trpE trpD trpC trpB trpA
E D C B A
Polypeptide subunits that make upenzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on
(b) Tryptophan present, repressor active, operon off
DNA
mRNA
Protein
Tryptophan (corepressor)
Activerepressor
No RNAmade
Figure 18.3
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Repressible and Inducible Operons: Two Types of Negative Gene Regulation
• A repressible operon is one that is usually ____
• An inducible operon is one that is usually ____
• The trp operon is a(n) ____________ operon• The Lac operon is a(n) ____________ operon
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Conducting the Genetic Orchestra CH18
• Prokaryotes and eukaryotes alter gene expression in response to their changing environment
• In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types
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Figure 18.1
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Eukaryotic gene expression:regulated at many stages
• All organisms must regulate which genes are expressed at any given time
• In multicellular organisms regulation of gene expression is essential for cell specialization
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Figure 11.22
Interdigital tissueCells undergoing
apoptosisSpace between
digits1 mm
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Differential Gene Expression
• Almost all the cells in an organism are genetically identical
• Differences between cell types result from differential gene expression,– the expression of different genes by cells with
the same genome• Abnormalities in gene expression can lead to
diseases including cancer• Gene expression is regulated at many stages
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Figure 18.6 Signal
NUCLEUSChromatin
Chromatin modification:DNA unpacking involvinghistone acetylation and
DNA demethylationDNA
Gene
Gene availablefor transcription
RNA ExonPrimary transcript
Transcription
Intron
RNA processing
Cap
Tail
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
mRNA in cytoplasm
TranslationDegradationof mRNA
Polypeptide
Protein processing, suchas cleavage and
chemical modification
Active proteinDegradation
of proteinTransport to cellular
destination
Cellular function (suchas enzymatic activity,structural support)
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Figure 18.6a Signal
NUCLEUSChromatin
Chromatin modification:DNA unpacking involvinghistone acetylation and
DNA demethylationDNA
Gene
Gene availablefor transcription
RNA ExonPrimary transcript
Transcription
Intron
RNA processing
Cap
TailmRNA in nucleus
Transport to cytoplasm
CYTOPLASM
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Figure 18.6b
CYTOPLASM
mRNA in cytoplasm
TranslationDegradationof mRNA
Polypeptide
Protein processing, suchas cleavage and
chemical modification
Active proteinDegradation
of proteinTransport to cellular
destination
Cellular function (suchas enzymatic activity,structural support)
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DNA Methylation
• DNA methylation, the addition of methyl groups to certain bases in DNA, is associated with reduced transcription in some species
• DNA methylation can cause long-term inactivation of genes in cellular differentiation
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DNA Methylation
• In genomic imprinting, methylation regulates expression of either the maternal or paternal alleles of certain genes at the start of development
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Histone Modifications
• In histone acetylation, acetyl groups are attached to positively charged lysines in histone tails
• This loosens chromatin structure, thereby promoting the initiation of transcription
• The addition of methyl groups (methylation) can condense chromatin; the addition of phosphate groups (phosphorylation) next to a methylated amino acid can loosen chromatin
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Figure 18.7
Amino acidsavailablefor chemicalmodification
Histone tails
DNA double helix
Nucleosome(end view)
(a) Histone tails protrude outward from a nucleosome
Unacetylated histones Acetylated histones
(b) Acetylation of histone tails promotes loose chromatinstructure that permits transcription
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Operon Practice
• If you can draw it you understand it
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Registration Materials• See Ms. Benz in A156
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Thoughts on taking the AP TEST
• You have worked hard all semester, and you should get your reward for all that hard work!
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Thoughts on taking the AP TEST• IF biology major:
– You should take the test which will count as an elective credit and be much cheaper than other credits
– You will still want to take MAJORS biology at your college/university to:• Understand their specific expectations & emphasis• Meet professors and make good impressions for
potential lab/field jobs• Meet your cohort – other students who will be
going through the program with you – and form friendships & study groups
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Thoughts on taking the AP TEST
• IF non-science or non-biology major:–You should take the test for easy credit
while it is all fresh in your mind.• It will be much more difficult and more
expensive to get this same credit as a college class