Transcript operon - RHSAPBiologyJacobs

AP Biology Discussion Notes
Monday 3/14/2016
Goals for Today
• Be able to describe regions of DNA and
how they are important to gene expression
in Bacteria (Prokaryotes) & Eukaryotes
• Be able to understand & describe how our
Transformation Lab works
Question of the Day 3/14
What is a “gene” and what can be produced
from it?
Is this always produced/in-production? Why
or why not
Gene Review
What cellular machinery is necessary to
make proteins?
DNA
mRNA
Protein
Transcription:
Translation:
DNA
RNA Polymerase
Nucleotides
Ribsosome
(mRNA)
tRNA
Amino Acids
Gene Expression/Regulation
• The Nobel Prize in Physiology or Medicine
1965 was awarded jointly to François
Jacob, André Lwoff and Jacques Monod
"for their discoveries concerning genetic
control of enzyme and virus synthesis".
Conducting the Genetic
Orchestra CH18
• Prokaryotes and eukaryotes alter gene
expression in response to their changing
environment
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
Upstream
PROG
Downstream
OPERON
Promoter – helps bind RNA Polymerase
Repressor (Protein)–stops transcription,
binds to OPERATOR
Operator – ON/OFF switch, place where
repressor binds – Control operation
Genes – Code for protein
Regulator – codes for repressor protein
Operons: The Basic Concept
• A cluster of functionally related genes can be
under coordinated control by a single “on-off
switch”
• The regulatory “switch” is a segment of DNA
called an operator usually positioned within the
promoter
• An operon is the entire stretch of DNA that
includes the operator, the promoter, and the genes
that they control
• The operon can be switched off by a protein
repressor
• The repressor prevents gene transcription by
binding to the operator and blocking RNA
polymerase
• The repressor is the product of a separate
regulatory gene
Repressible and Inducible
Operons
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?
Figure 18.4b
What is the INDUCER in the lac
Operon?
lac operon
lacI
DNA
lacZ
lacY
lacA
Permease
Transacetylase
RNA polymerase
3
mRNA
5
mRNA 5
-Galactosidase
Protein
Allolactose
(inducer)
Inactive
repressor
(b) Lactose present, repressor inactive, operon on
Repressible and Inducible
Operons
An inducible operon is one that is usually
OFF_; a molecule called an inducer
inactivates the repressor and turns on
transcription
What is the “inducer” in the Lac Operon?
Figure 18.4a
Regulatory
gene
DNA
Promoter
Operator
lacI
lacZ
No
RNA
made
3
mRNA
5
Protein
RNA
polymerase
Active
repressor
(a) Lactose absent, repressor active, operon off
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
Positive Gene Regulation
• 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
Figure 18.5a
Promoter
DNA
lacI
lacZ
CAP-binding site
cAMP
Operator
RNA
polymerase
Active binds and
transcribes
CAP
Inactive
CAP
Allolactose
Inactive lac
repressor
(a) Lactose present, glucose scarce (cAMP level high):
abundant lac mRNA synthesized
Figure 18.5b
Promoter
DNA
lacI
CAP-binding site
lacZ
Operator
RNA
polymerase less
likely to bind
Inactive
CAP
Inactive lac
repressor
(b) Lactose present, glucose present (cAMP level low):
little lac mRNA synthesized
Questions?
Repressible and Inducible
Operons:
• 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
trp OPERON
• E. coli can synthesize the amino acid tryptophan
• Think of these questions and think about
energy and natural selection:
– When would E. coli “want” to do this?
– When would E. coli “not want” to do this?
trp OPERON
• 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
• 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
Figure 18.3a
trp operon
Promoter
Promoter
Genes of operon
DNA
trpR
Regulatory
gene
mRNA
trpE
3
Operator
RNA
Start codon
polymerase
mRNA 5
trpD
trpC
trpB
trpA
C
B
A
Stop codon
5
E
Protein
Inactive
repressor
D
Polypeptide subunits that make up
enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on
Figure 18.3b-1
DNA
mRNA
Protein
Active
repressor
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off
Figure 18.3b-2
DNA
No RNA
made
mRNA
Protein
Active
repressor
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off
Figure 18.2
Precursor
Feedback
inhibition
trpE gene
Enzyme 1
trpD gene
Enzyme 2
Regulation
of gene
expression
trpC gene

trpB gene

Enzyme 3
trpA gene
Tryptophan
(a) Regulation of enzyme
activity
(b) Regulation of enzyme
production
Figure 18.3
trp operon
Promoter
Promoter
Genes of operon
DNA
trpE
trpR
trpD
trpC
trpB
trpA
C
B
A
Operator
Regulatory
gene
3
RNA
polymerase
Start codon
Stop codon
mRNA 5
mRNA
5
E
Protein
Inactive
repressor
D
Polypeptide subunits that make up
enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on
DNA
No RNA
made
mRNA
Protein
Active
repressor
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off
Repressible and Inducible
Operons:
• 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
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
Figure 18.1
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
Figure 11.22
Interdigital tissue
Cells undergoing
apoptosis
Space between
1 mm
digits
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
Figure 18.6
Signal
NUCLEUS
Chromatin
DNA
Chromatin modification:
DNA unpacking involving
histone acetylation and
DNA demethylation
Gene available
for transcription
Gene
Transcription
RNA
Exon
Primary transcript
Intron
RNA processing
Cap
Tail
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
mRNA in cytoplasm
Degradation
of mRNA
Translation
Polypeptide
Protein processing, such
as cleavage and
chemical modification
Degradation
of protein
Active protein
Transport to cellular
destination
Cellular function (such
as enzymatic activity,
structural support)
Figure 18.6a
Signal
NUCLEUS
Chromatin
DNA
Chromatin modification:
DNA unpacking involving
histone acetylation and
DNA demethylation
Gene available
for transcription
Gene
Transcription
RNA
Exon
Primary transcript
Intron
RNA processing
Cap
Tail
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
Figure 18.6b
CYTOPLASM
mRNA in cytoplasm
Degradation
of mRNA
Translation
Polypeptide
Protein processing, such
as cleavage and
chemical modification
Degradation
of protein
Active protein
Transport to cellular
destination
Cellular function (such
as enzymatic activity,
structural support)
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
DNA Methylation
• In genomic imprinting, methylation regulates
expression of either the maternal or paternal
alleles of certain genes at the start of development
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
Figure 18.7
Histone
tails
Amino acids
available
for chemical
modification
DNA
double
helix
Nucleosome
(end view)
(a) Histone tails protrude outward from a nucleosome
Acetylated histones
Unacetylated histones
(b) Acetylation of histone tails promotes loose chromatin
structure that permits transcription
Operon Practice
• If you can draw it you understand it