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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
Adult mouse
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2. 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.
The lac genes are turned off by repressors and turned on by the presence of lactose.
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3. 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.
The lac genes in E. coli are turned off by repressors and turned on by the presence of lactose. When lactose is not present, the repressor binds to the operator region, preventing RNA polymerase from beginning transcription. Lactose causes the repressor to be released from the operator region.
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4. Gene Regulation: An Example
The other region is the operator (O).
When the lac repressor binds to the O region, transcription is not possible.
The lac genes in E. coli are turned off by repressors and turned on by the presence of lactose. When lactose is not present, the repressor binds to the operator region, preventing RNA polymerase from beginning transcription. Lactose causes the repressor to be released from the operator region.
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5. Gene Regulation: An Example
When lactose is added, sugar binds to the repressor proteins.
The lac genes in E. coli are turned off by repressors and turned on by the presence of lactose. When lactose is not present, the repressor binds to the operator region, preventing RNA polymerase from beginning transcription. Lactose causes the repressor to be released from the operator region.
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6. Gene Regulation: An Example
The repressor protein changes shape and falls off the operator and transcription is made possible.
The lac genes in E. coli are turned off by repressors and turned on by the presence of lactose. When lactose is not present, the repressor binds to the operator region, preventing RNA polymerase from beginning transcription. Lactose causes the repressor to be released from the operator region.
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7. Gene Regulation: An Example
Many genes are regulated by repressor proteins.
Some genes use proteins that speed up transcription.
Sometimes regulation occurs at the level of protein synthesis.
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8. 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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9. Eukaryotic Gene Regulation
Many eukaryotic genes have a sequence called the TATA box.
The TATA box seems to help position RNA polymerase.
Many eukaryotic genes include a sequence called the TATA box that may help position RNA polymerase. Eukaryotic genes have regulatory sequences that are more complex than prokaryotic genes.
Eukaryotic promoters are usually found just before the TATA box, and consist of short DNA sequences.
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10. 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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11. 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.
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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12. Development and Differentiation
Hox Genes
Fruit fly chromosome
Mouse chromosomes
Fruit fly embryo
Mouse embryo
n fruit flies, a series of hox genes along a chromosome determines the basic structure of the fly’s body. Mice have very similar genes on four different chromosomes. The color bars along the mouse’s back show the approximate body area affected by genes of the corresponding colors.
Adult fruit fly
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?
start codon, regulatory site, promoter, stop codon
regulatory site, promoter, start codon, stop codon
start codon, promoter, regulatory site, stop codon
promoter, regulatory site, start codon, stop codon
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12–5
A group of genes that operates together is a(an)
promoter.
operon.
operator.
intron.
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12–5
Repressors function to
turn genes off.
produce lactose.
turn genes on.
slow cell division.
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12–5
Which of the following is unique to the regulation of eukaryotic genes?
promoter sequences
TATA box
different start codons
regulatory proteins
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12–5
Organs and tissues that develop in various parts of embryos are controlled by
regulation sites.
RNA polymerase.
hox genes.
DNA polymerase.
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Hoxgene Mutations
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Normal Fruit Fly
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29. Human Hoxgene mutations
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Resources
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