Gene ExpressionAP Biology

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

Liver Cells

Cartilage CellsRed 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”

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.

Insulin DNA? Protein?

Muscle Cell X

Pancreatic Cell

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.

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.

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.

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.

Fig. 18-2

Regulationof 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

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.

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.

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

• 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.

• 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

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

codonStart codon

mRNA

trpA

5

3

trpR

trpE

trpD

trpC

trpB

ABCDE

Fig. 18-3b-1

(b) Tryptophan present, repressor active, operon off

Tryptophan(corepressor)

No RNA made

Activerepressor

mRNA

Protein

DNA

Fig. 18-3b-2

(b) Tryptophan present, repressor active, operon off

Tryptophan(corepressor)

No RNA made

Activerepressor

mRNA

Protein

DNA

Video for Gene Expression http://education-portal.com/academy/les

son/regulation-of-gene-expression-transcriptional-repression-and-induction.html

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)

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.

Fig. 18-4

(b) Lactose present, repressor inactive, operon on

(a) Lactose absent, repressor active, operon off

mRNA

Protein

DNA

DNA

mRNA 5

Protein

Activerepressor

RNApolymerase

Regulatorygene

Promoter

Operator

mRNA 5

3

Inactiverepressor

Allolactose(inducer)

5

3

NoRNAmade

RNApolymerase

Permease

Transacetylase

lac operon

-Galactosidase

lacYlacZ lacAlacI

lacI lacZ

Fig. 18-4a

(a) Lactose absent, repressor active, operon off

DNA

Protein

Activerepressor

RNApolymerase

Regulatorygene

Promoter

Operator

mRNA 5

3

NoRNAmade

lacI

lacZ

Fig. 18-4b

(b) Lactose present, repressor inactive, operon on

mRNA

Protein

DNA

mRNA 5

Inactiverepressor

Allolactose(inducer)

5

3

RNApolymerase

Permease

Transacetylase

lac operon

-Galactosidase

lacYlacZ lacA

lacI

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.

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.