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Copyright Pearson Prentice Hall

12-5 Gene Regulation


Fruit fly chromosome

Fruit fly embryo

Adult fruit fly

Mouse chromosomes

Mouse embryo

Adult mouse


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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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How are lac genes turned off and on?


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The lac genes are turned off by repressors and turned on by the presence of lactose.


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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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• The other region is the operator (O).


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When the lac repressor binds to the O region, transcription is not possible.


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When lactose is added, sugar binds to the repressor proteins.


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The repressor protein changes shape and falls off the operator and transcription is made possible.


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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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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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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 product is not consumed by some other process.



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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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What are examples of Positive Feedback?

• Clotting mechanism in blood

• Panic in cattle herds



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Eukaryotic Gene Regulation

How are most eukaryotic genes controlled?


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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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Many eukaryotic genes have a sequence called the TATA box.

Promotersequences

Upstreamenhancer

TATAbox Introns

Exons

Direction of transcription


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The TATA box seems to help position RNA polymerase.

Promotersequences

Upstreamenhancer

TATAbox

Introns

Exons



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Eukaryotic promoters are usually found just before the TATA box, and consist of short DNA sequences.

Promotersequences

Upstreamenhancer

TATAbox

Introns

Exons



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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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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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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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Hox Genes

Fruit fly chromosome

Fruit fly embryo

Adult fruit fly

Mouse chromosomes

Mouse embryo

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.

END OF SECTION

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Documents

gene regulation slide

gene expression

positive feedback

negative feedback lac

lac genes

kind of feedback

feedback mechanisms

lac repressor