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 Gene Expression AP Biology  Gene Expression AP Biology

Gene Expression AP Biology - PowerPoint Presentation

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Gene Expression AP Biology - PPT Presentation

Questions to Ponder How do your cells know what kind of cell they are How do your cells know when to make a particular protein When to stop making it How does the environment affect your cells ID: 776556

operon repressor gene tryptophan operon repressor gene tryptophan protein expression lac genes mrna cells dna active enzymes cell fig

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Slide1

Gene Expression

AP Biology

Slide2

Questions to Ponder…..

How do your cells “know” what kind of cell they are?How do your cells “know” when to make a particular protein? When to stop making it?How does the environment affect your cells?ANSWER: Gene Expression

Slide3

Liver Cells

Cartilage Cells

Red Blood Cells

Stem Cells

What makes cells from the same individual look different?

DNA sequence in each cell is the same, but different cell types have different “GENE EXPRESSION PATTERNS”

Slide4

Slide adapted from Genetic Science Learning Center, University of Utah

 2013

When a gene is “on” and its protein or RNA product is being made, scientists say that the gene is being EXPRESSED.The on and off states of all of a cell’s genes is known as a GENE EXPRESSION PROFILE.Each cell type has a unique gene expression profile.

InsulinDNA?Protein?Muscle CellXPancreatic Cell

Slide5

Gene Expression in Bacteria

Bacteria are single-celled organisms who are surrounded on all sides by their environment. They must be able to regulate expression of their genes in response to environmental changes.

Slide6

Bacteria Respond by Regulating Transcription

Bacteria cells that can conserve resources and energy have a selective advantage over cells that cannot do so.Natural selection has favored bacteria that express only the genes they need.

Slide7

E. Coli Regulation of Tryptophan

An individual E. coli cell living in the erratic environment of the human colon, is dependent for its nutrients on the whimsical eating habits of its host—you!If the environment is lacking in the amino acid tryptophan, which the bacterium needs to survive, it responds by activating a metabolic pathway that makes tryptophan from another compound. If tryptophan becomes available, it shuts down this pathway.

Slide8

Regulation of a Metabolic Pathway

In the pathway for tryptophan synthesis, an

abundance

of tryptophan can both inhibit the activity of the first enzyme (a rapid response) OR repress expression of the genes encoding the enzymes in the pathway (a longer response).

This is an example of

feedback inhibition

. It allows for a cell to adapt to short-term fluctuation in the supply of a substance it needs.

Slide9

Fig. 18-2

Regulation

of geneexpression

trpE

gene

trpD gene

trpC gene

trpB gene

trpA gene

(b) Regulation of enzyme production

(a) Regulation of enzyme activity

Enzyme 1

Enzyme 2

Enzyme 3

Tryptophan

Precursor

Feedbackinhibition

Slide10

Gene Expression Controls Which Enzymes are Made and When

In many cases, this occurs in the process of transcription.Many genes may be switched on or off by changes in the metabolic status of the cell.One example was discovered in 1961 by Francois Jacob and Jacques Monod at the Pasteur Institute in Paris. This method is called the Operon Model.

Slide11

Operons: The Basic Concept

A cluster of functionally related genes

that 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.

Slide12

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

Slide13

The repressor can be in an active or inactive form, depending on the presence of other

molecules.

A

corepressor

is a molecule that cooperates with a repressor protein to switch an operon

off.

Slide14

Bacteria can synthesize tryptophan by utilizing the

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

The repressor is active only in the presence of its

co-repressor

tryptophan; thus the

trp

operon is turned off (repressed) if tryptophan levels are high

Slide15

Fig. 18-3a

Polypeptide subunits that make up

enzymes for tryptophan synthesis

(a) Tryptophan absent, repressor inactive, operon on

DNA

mRNA 5

Protein

Inactiverepressor

RNApolymerase

Regulatorygene

Promoter

Promoter

trp operon

Genes of operon

Operator

Stop codon

Start codon

mRNA

trpA

5

3

trpR

trpE

trpD

trpC

trpB

A

B

C

D

E

Slide16

Fig. 18-3b-1

(b) Tryptophan present, repressor active, operon off

Tryptophan

(corepressor)

No RNA made

Activerepressor

mRNA

Protein

DNA

Slide17

Fig. 18-3b-2

(b) Tryptophan present, repressor active, operon off

Tryptophan

(corepressor)

No RNA made

Activerepressor

mRNA

Protein

DNA

Slide18

Video for Gene Expression

http://education-portal.com/academy/lesson/regulation-of-gene-expression-transcriptional-repression-and-induction.html

Slide19

Different Types of Operons

A repressible operon is one that is usually ON—binding a repressor to the operator turns off transcription. (The

trp

operon is like this)

An inducible operon is one that is usually OFF—a molecule called an inducer inactivates the repressor and starts transcription. (The

lac

operon is this type)

Slide20

The lac Operon

The

lac

operon is an inducible operon (usually off) and contains genes that code for enzymes that break down the sugar lactose (found in dairy products)

By itself, the

lac

repressor is active and therefore shuts the

lac

operon off most of the time.

A molecule called an inducer inactivates this repressor which turns the

lac

operon on.

Slide21

Fig. 18-4

(b) Lactose present, repressor inactive, operon on

(a) Lactose absent, repressor active, operon off

mRNA

Protein

DNA

DNA

mRNA 5

Protein

Active

repressor

RNApolymerase

Regulatorygene

Promoter

Operator

mRNA

5

3

Inactiverepressor

Allolactose(inducer)

5

3

NoRNAmade

RNApolymerase

Permease

Transacetylase

lac operon

-Galactosidase

lacY

lacZ

lacA

lacI

lacI

lacZ

Slide22

Fig. 18-4a

(a) Lactose absent, repressor active, operon off

DNA

Protein

Active

repressor

RNApolymerase

Regulatorygene

Promoter

Operator

mRNA

5

3

NoRNAmade

lacI

lacZ

Slide23

Fig. 18-4b

(b) Lactose present, repressor inactive, operon on

mRNA

Protein

DNA

mRNA 5

Inactive

repressor

Allolactose(inducer)

5

3

RNApolymerase

Permease

Transacetylase

lac operon

-Galactosidase

lacY

lacZ

lacA

lacI

Slide24

Inducible Enzymes

Inducible enzymes (such as those found in the

lac

operon) are usually catabolic enzymes, which means they break things apart.

Their synthesis is usually induced by some kind of signal.

In the

lac

operon, the signal is the presence of the lactose sugar molecule.

Slide25

Repressible Enzymes

Repressible enzymes (such as those in the

trp

operon) usually function in anabolic pathways which build things or put things together.

Since these are almost always ON, they are repressed (shut down) when there are high levels of the

end-product present.