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12-5 Gene Regulation
Fruit fly chromosome Mouse chromosomes
12-5 Gene Regulation
Fruit fly embryo
Mouse embryo
Adult fruit fly
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Adult mouse
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12-5 Gene Regulation
Gene Regulation: An Example
Gene Regulation: An Example
E. coli provides an example of how gene
expression can be regulated.
An operon is a group of genes that operate
together.
In E. coli, these genes must be turned on so the
bacterium can use lactose as food.
Therefore, they are called the lac operon.
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12-5 Gene Regulation
Gene Regulation: An Example
How are lac genes turned off and on?
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12-5 Gene Regulation
Gene Regulation: An Example
The lac genes are turned off by
repressors and turned on by the
presence of lactose.
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12-5 Gene Regulation
Gene Regulation: An Example
On one side of the operon's three genes are two
regulatory regions.
• In the promoter (P) region, RNA polymerase
binds and then begins transcription.
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12-5 Gene Regulation
Gene Regulation: An Example
• The other region is the operator (O).
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12-5 Gene Regulation
Gene Regulation: An Example
When the lac repressor binds to the O region,
transcription is not possible.
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12-5 Gene Regulation
Gene Regulation: An Example
When lactose is added, sugar binds to the repressor
proteins.
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12-5 Gene Regulation
Gene Regulation: An Example
The repressor protein changes shape and falls off the
operator and transcription is made possible.
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12-5 Gene Regulation
Gene Regulation: An Example
Many genes are regulated by repressor proteins.
Some genes use proteins that speed transcription.
Sometimes regulation occurs at the level of protein
synthesis.
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12-5 Gene Regulation
positive vs. negative feedback
Many molecular and physiological processes are
controlled by feedback mechanisms. In a feedback
loop the product of a process, such as the
breakdown of proteins into amino acids, has an
effect on the rate of the process.
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12-5 Gene Regulation
positive vs. negative feedback
Negative feedback occurs when the rate of the
process decreases as the concentration of the
product increases (or reactant decreases).
Positive feedback occurs when the rate of a process
increases as the concentration of the product
increases (or reactant decreases).
Negative feedback controls the rate of a process to
avoid accumulation of a product. The rate of a
process will continuously accelerate under positive
feedback as long as substrate is available and the Slide
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12-5 Gene Regulation
positive vs. negative feedback
What kind of feedback is the lac operon?
• Negative Feedback (lac operon is turned on,
lactose is digested, lactose is removed, lac operon
turns back off)
What are other examples of Negative Feedback?
• Sweating and temperature regulation
• Predator-Prey interactions
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12-5 Gene Regulation
positive vs. negative feedback
What are examples of Positive Feedback?
• Clotting mechanism in blood
• Panic in cattle herds
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12-5 Gene Regulation
Eukaryotic Gene Regulation
How are most eukaryotic genes
controlled?
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12-5 Gene Regulation
Eukaryotic Gene Regulation
Eukaryotic Gene Regulation
Operons are generally not found in
eukaryotes.
Most eukaryotic genes are controlled
individually and have regulatory
sequences that are much more complex
than those of the lac operon.
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12-5 Gene Regulation
Eukaryotic Gene Regulation
Many eukaryotic genes have a sequence called the
TATA box.
Upstream
enhancer
TATA
box
Promoter
sequences
Introns
Exons
Direction of transcription
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12-5 Gene Regulation
Eukaryotic Gene Regulation
The TATA box seems to help position RNA
polymerase.
Upstream
enhancer
TATA
box
Promoter
sequences
Introns
Exons
Direction of transcription
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12-5 Gene Regulation
Eukaryotic Gene Regulation
Eukaryotic promoters are usually found just before
the TATA box, and consist of short DNA sequences.
Upstream
enhancer
TATA
box
Promoter
sequences
Introns
Exons
Direction of transcription
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12-5 Gene Regulation
Eukaryotic Gene Regulation
Genes are regulated in a variety of ways by enhancer
sequences.
Many proteins can bind to different enhancer
sequences.
Some DNA-binding proteins enhance transcription
by:
• opening up tightly packed chromatin
• helping to attract RNA polymerase
• blocking access to genes
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12-5 Gene Regulation
Development and Differentiation
Development and Differentiation
As cells grow and divide, they undergo
differentiation, meaning they become specialized
in structure and function.
Hox genes control the differentiation of cells and
tissues in the embryo.
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12-5 Gene Regulation
Development and Differentiation
Careful control of expression in hox genes is
essential for normal development.
All hox genes are descended from the genes of
common ancestors.
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Development and Differentiation
Hox Genes
Fruit fly chromosome Mouse chromosomes
Fruit fly embryo
Mouse embryo
Adult fruit fly
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Adult mouse
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12–5
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12–5
Which sequence shows the typical organization
of a single gene site on a DNA strand?
a. start codon, regulatory site, promoter, stop
codon
b. regulatory site, promoter, start codon, stop
codon
c. start codon, promoter, regulatory site, stop
codon
d. promoter, regulatory site, start codon, stop
codon
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12–5
A group of genes that operates together is a(an)
a. promoter.
b. operon.
c. operator.
d. intron.
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12–5
Repressors function to
a. turn genes off.
b. produce lactose.
c. turn genes on.
d. slow cell division.
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12–5
Which of the following is unique to the regulation
of eukaryotic genes?
a. promoter sequences
b. TATA box
c. different start codons
d. regulatory proteins
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12–5
Organs and tissues that develop in various parts
of embryos are controlled by
a. regulation sites.
b. RNA polymerase.
c. hox genes.
d. DNA polymerase.
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END OF SECTION