Biology Chap 8 Presentation

8/4/2019 Biology Chap 8 Presentation

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Introduction Proteins play an important role in the structures and

functions of living cells. Proteins form enzymes whichcontrol many cell metabolisms.

Prokaryotes (e.g :bacteria) live in direct contact with

their environment which may vary at different times.

The bacteria have evolved regulating mechanisms to

control gene transcription to adapt to the different

levels of nutrients in the changing environment.


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How do Organisms Control the Level of

Gene Expression?

Gene expression (transcription, translation)takes up large amounts of cellular energy andresources

Cells live frugal lifestyles they conserveenergy and resources

Cells must only express genes when needed

So genes will only be expressed when theirproducts are needed


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What is Gene Expression?

It is the process by which information from a

gene is used in the synthesis of a functional

gene product.

These products are often proteins, but in non-

protein coding genes such as rRNA genes or

tRNA genes, the product is a functional RNA.
http://www.news-medical.net/health/What-is-RNA.aspxhttp://www.news-medical.net/health/What-is-RNA.aspx


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Terminologies,components of an operon

and function of each component A operon system consists of a regulator gene and

an operon.

A typical operon consists of a functional complexon the same length of DNA. The operon consistsof a cluster ofstructural genes, operator, and apromoter.

a) Structural gene (S) : is a gene which has genetic

information for the synthesis of a polypeptide. Ifmore than one structural genes are present inan operon, then all the genes will be activated atthe same time and function as a single

transcription unit.


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b) Operator (O) : short region of DNA situated

between the promoter and the first

structural gene. It acts as a switch for startingand stopping the transcription process. The

operator is the binding site for the

regulatory protein.

c) Promoter (P) : short sequence of DNA next

to the operator. The promoter is the binding

site to which the RNA polymerase molecule

first attaches to initiate transcription of the

structural genes.


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d) Repressors are protein molecules that can

bind to the operator and block the

transcription of structural genes.e) Activators can bind to specific sites on DNA

to stimulate the initiation of transciption.


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Experiments of Jacob and Monod

Francois Jacob and Jacques Monod carriedout a research on gene regulation for protein

synthesis in the human gut bacterium,

Escherichia coli. The DNA ofE. Coliis sufficient

to encode about 4000 proteins, but only a

fraction of these are made at any one time. E.

Coliregulated the expression of many of its

genes according to the substrates that areavailable to it.


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Principles of Gene Regulation:

Most prokaryotic genes are regulated in

units

called operons.

Francois Jacob & Jacques Monod, 1961.

This is largely based on regulation of

lactose metabolism. By intestinal bact.E.coli.


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A regulator gene codes for a repressor

protein.

Transcription of the regulator gene producesmRNA. Translation of mRNA produces the

repressor molecule.

In the presence of an inducer (allolactosefrom lactose), it binds to the repressor

molecule to form a repressor-allolactose

complex. The conformation of the repressor isaltered so that it does not fit into the operator

site. The repressor molecule is inactivated.


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The RNA polymerase binds to the promoter.

The operon is switched on thereby initiatingthe transcription of the structural genes lac Z,

lac Yand lac A. Enzymes ofor lactose metabolism -

galactosidase, permease and transacetylase

are produced. The lactose operon is an inducible operon.

The regulatory switch is turned on in thepresence of lactose and turned off in its

presence.

This is an adaption by E. colitowards itsenvironment, to conserve its cell nutrients and

to prevent energy wastage.


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General structure of an OPERON


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Explanation of the lactose (lac) operon as

a model for control of gene expression

Effect of the absence of lactose on the lactose

operon

A certain DNA segment in the bacterium

Escherichia colifunctions as the lac operon.

Lac operon is an example of an inducible system.

The repressor molecule does not affect the

structural genes directly. When lactose is absent,the active repressor molecule which has a strong

affinity for the operator binding site, binds to the

operator and also covers part of the promoter.


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This prevents the binding of PNA polymerase

to the promoter site.

The operator acts as a switch and the lactose

operon is switched off.

There is no transcription of the genes oflac Z,

lac Y and lac A. Enzymes -galactosidase ,

lactose permease and transacetylase are not

produced.


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Absence of lac operon:


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Effect of the presence of lactose on the

lactose operon

When lactose (milk sugar) is present in theenvironment, a small amount is converted toits isomer, allolactose.

Allolactose acts as an inducer and binds to

the allosteric site of the lactose repressormolecule produced by the regulator gene.

This causes a change in the conformation of

the repressor. The repressor- allolactose complex is unable

to attach to the operator.


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Presence of lac preron:


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Lac OPERON an inducible Operon

In the absence of

lac

In the presence

of lac


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Catabolic repression (the glucose effect)

The presence of glucose in the surrounding

prevents the induction of the lac operon andother operons, for example, the galoperon,which regulates the enzymes involved in thecarbohydrates metabolism.

This glucose effect or catabolic repressionoccurs because the bacterial cellspreferentially metabolise glucose as the main

energy source. In catabolic repression, the lac operon is

repressed while glucose is being used.


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High levels of glucose cause a sharp decreasein the levels of cAMP in the cell and vise versa.

The cAMP acts as the effector molecule whichregulates the effect of CAP on transcription inthe lac operon.

When concentration of cAMP is low, there is

insufficient cAMP to bind to CAP to form thecAMP-CAP complex.

When the cAMP=CAP complex does not bind

to the CAP site on the DNA, the RNApolymerase cannot bind well to the lac operonpromoter.This prevents activation of the lacoperon.


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When glucose is absent (or low

concentration) , there is a high concentration

of cAMP. cAMP is abundant and binds to CAPto form cAMP-CAP complex.

The cAMP-CAP complex then binds to the

CAP binding site (catabolic activator proteinbinding site). The binding causes the DNA to

bind around the complex to stimulate more

efficient binding of RNA polymerase to

promoter.


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CRP or CAP is positive regulator of Lac and some

other catabolic Operons:

CRP= Catabolic gene regulatory Protein

CRP= cAMP receptor Protein

CAP= Catabolic gene Activating Protein


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Tryptophan (trp) operon

Tryptophan operon in E. coliis an example of

a repressible operon.

Tryptophan acts as a repressor and attaches

to the allosteric site of the repressor. This

alters its conformation and the active

repressor binds to the operator site.

The RNA polymerase is unable to attach to the

promoter. The trp operon is turned off and theenzymes are not produced. (Look at the figure

in the next slide)

Th O ibl


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The trp Operon - repressible operon

The trp operon is always on and is turned off only by end product

tryptophan. This is why it is called a repressible operon.


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Tryptophan therefore switches off its own

synthesis (end-product repression) andprevents expenditure of energy to synthesise

excessive proteins which are not required by

the cells.


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There is repression of the synthesis of the tryptophan

biosynthetic enzymes when the surrounding environment

contains concentrations of the tryptophan amino acid

sufficient to sustain optimal growth of the bacterial cells.


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Biology Chap 8 Presentation

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