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Chapter 15
I. Prokaryotic Gene Control
 A.
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Conserves Energy and Resources by
1. only activating proteins when necessary
a. don’t make tryptophan if it can be
absorbed from environment
2. only producing proteins when needed
a. don’t need lactose digesting enzymes
if no lactose is present
 B.
control enzymes already in cell : posttranslation control
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1. allosteric enzymes
a. activated
b. inhibited
2. feed back inhibition
a. end product of
anabolic path is
the inhibitor
3. adjustment to short
term changes
 C.
control production of enzymes:
transcription control
 1. control transcription of genes
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a. repressors block promoters to stop
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transcription
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b. enhancers bind to promotor to speed
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transcription
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2. slower/longer lasting effects: more stable
environment
 D.
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Negative control
1. negative slows or stops function
2. feed back inhibition
3. repressors blocking transcription
positive control
1. SPEEDS up production
2. just allowing production does not count!!
3. Enhancers bound to promoter
4. allosteric enhancers
 F.
Operon model : clusters of functionally
related genes controlled as a group (3 parts)
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1. DNA code for the genes
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2. promotor – stretch of DNA before genes
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a. attracts RNA polymerase
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b. needed to start transcription
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3. operator – DNA sequence near promotor
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a. binding site for repressor protein
 G.
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Regulatory Genes : make repressors
found up stream from operon they regulate
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trp Operon : trp = tryptophan amino acid
1. Repressible operon bcs it is normally active
2. genes make trp
3. low trp level in cell : operon active
a. repressor is inactive
b. promoter is open to RNA polymerase
c. genes to make trp are copied
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4. High trp level in cell : operon repressed
a. trp repressor is allosteric
1. binding to trp activates repressor
2. active repressor binds to operator
3. blocks RNA polymerase
4. genes not transcribed
 I.
Lac operon : lactose (galactose + glucose)
 1. Inducible operon : usually off
 a. repressor is active unless lactose bound to it
 2. genes make
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a. β-galactosidase cleaves lactose in 1/2
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b. permiase membrane transport protein
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c. third gene
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unknown
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3. presence of lactose:
a. lactose binds to allosteric repressor
i. inactivates repressor
ii. Repressor releases operator
iii. RNA polymerase copies all 3 genes
http://biology-animations.blogspot.com/2007/11/lac-operon-animation.html
J. Positive control of lac operon
 1.
lac operon also has positive control
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a. inducible = can be activated
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b. positive control = can be speeded up
 2. If glucose is present E. coli prefer to use it
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a. lack of glucose causes E. coli to speed up
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use of lactose
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b. lack of glucose causes build up of cAMP
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(cyclic AMP) = signal molecule
 c.
cAMP binds to regulatory protein CAP
 i. catabolite activator protein becomes active
 ii. CAP binds to start of promotor
 iii. Makes promotor more attractive to RNA
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polymerase
 iv. speeds up translation
 d. build up of glucose in cell causes lack of
cAMP so CAP becomes inactivated
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https://smartsite.ucdavis.edu/access/content/user/00002950/bis10v/flashvideo/lac_positive.html
 Repressible
operons
 a) repressor inactive w/o allosteric binding
 b) normally on
 c) usually anabolic
 Inducible operons
 a) repressor active unless bound
 b) normally off
 c) usually catabolic
II. Eukaryotic Gene Control
 A.
Gene expression regulated at many stages
 1. Transcription control
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a. chromatin structure regulation
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b. transcription initiation control
 2. Post-transcriptional control
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a. RNA processing
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b. mRNA degradation
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c. Translation initiation
 3. Post-translational control
a.allosteric P, b. P processing, c. P degradation
B. Chromatin structure control
Heterochromatin – Chromatin that remains
tightly compacted even in interphase
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a. genes not transcribed
 2. acetylation –
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a. acetyl group (-COCH3) bonded to histone
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b. loosens up chromatin winding
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c. promotes transcription
 3. DNA methylation –
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a. –CH3 bonds to DNA blocking transcription
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b. methylated regions passed on to daughter
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cells (x - chromosome inactivation)
 1.
C. Initiation control
 1.
control elements : non-coding DNA upstream from promotor that bind transcription
factor proteins
 a. distal control elements are far up-stream
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i. often act as enhancers
 b. proximal control elements : near promotor
 2.
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transcription factors: proteins
a. needed for transcription initiation
b. general transcription factors (GTF)
needed for all transcription of genes
i. GTFs bind each other & RNA Polym. II
to form initiation complex
ii. Initiation complex binds to control
elements near promotor: start transcription
iii. One protein of the GTF will bind to a
section of promotor called the TATA box.
(fig 14.9)
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iv. General Transcription Factor complexes
allow slow transcription of gene
 f. Specific Transcription Factors
needed for
rapid transcription of gene
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 3.
Vocabulary in order to have a clue on 15.2
 a. Things that are part of the DNA
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1. control elements : binding site for
transcription factors
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2. Enhancers : distal (far) control elements,
can be activated or repressed by transcription
factor proteins
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3. TATA box : section of the promoter’s code
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4. promoter : just upstream from start of
gene, where RNA polymerase binds to start
transcription
b. Things that are proteins
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1. Transcription factor : regulatory protein
binds control elements on
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a. general transcription factors
allow transcription
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b. activators speed transcription
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c. repressors slow transcription
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2. Mediator proteins : form link between
regulatory proteins and DNA
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 4.
Distal control elements = enhancers
 a. may be up or down stream
 b. each gene can have many enhancers
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i. each active under different conditions
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ii. Or active in different cell types
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iii. Each enhancer works with only one gene
 c. transcription factors called activator
proteins bind to enhancer control elements
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i. fold DNA so that the activator
protein/enhancer complex binds to initiation
complex to speed up translation
d. repressor transcription factors interfere with
the activator transcription factors to slow
transcription
 i. by binding to distal control elements and
keeping activators out
 ii. By binding to activator proteins
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5.coordination of functionally related genes
 a. related genes have same control element
sequences
 b. bind same transcription factors
 c. environmental signal triggers TF and the bind
to all the matching control elements in the genome
 d. activate all the related genes
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D. Post-translational Control
1. RNA processing
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a. alternative splicing
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b. poly A tail length
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c. cap designation
 2. mRNA degradation
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a. specialized RNAs can degrade mRNA
 3. Translation initiation control
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a. proteins bond to mRNA prevent ribosome
binding
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