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Control of Gene expression - Prokaryotes (Gene terminology (Constitutive…
Control of Gene expression - Prokaryotes
Prokaryotes and eukaryotes alter gene expression in response to their changing environment.
In multicellular eukaryotes, gene expression regulates development and is responsible for difference in cell types: Cell differentiation
RNA molecules play many roles in regulating gene expression in eukaryotes
Gene --Transcription--> mRNA --Translation--> Protein -- Post translation --> Function protein
Bacteria often respond to environmental change by regulating transcription
Natural selection has favoured bacteria that produce only the products needed by that cell
A cell can regulate the production of enzymes by feedback inhibition or by gene regulation
Gene expression in bacteria is controlled by the operon model
Operon theory -Genes that function together are regulated together and are known as an operon
Gene terminology
Constitutive genes: Are genes that are constantly switched on, so are transcribed continuously.
Regulation: Are expressed at a fixed rate
Facultative genes : genes that are transcribed only when needed
Structural genes: Code for proteins and RNA molecules required for normal enzymatic functions
Regulator genes: Code for proteins and RNA molecules which regulate the expression of structural genes
Cistron: region of DNA that encodes for a single polypeptide
Polycistronic RNA - A single mRNA enconding several different polypeptide chains
Operon Theory
An operon is several distinct genes situated in tandem, all controlled by a common regulatory region
The message produced from an operon is polycistronic in that the information for all of the structural genes will reside on one mRNA molecule
Regulation of these genes is coordinated since their transcription depends on a common regulatory region
Operons: The Basic Concept
An operan is the stretch of DNA that includes the operator, the promoter and the genes that they control
A cluster of functionally related genes 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
RNA Polymerase must bind to the promoter site and continue past the operator site to transcribe mRNA
The operon can be switched off by a protein repressor
The repressor prevents gene transcription by binding to the operator and blocking RNA polymerase
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
Inducible system
The lac operon is an inducible operon and contains genes that code for enzymes used in the hydrolysis and metabolism of lactose
Generally the lac operon is switched off
A molecule called an inducer inactivates the repressor to turn the lac operon on
A repressible operon is one that is usually on: binding of a repressor to the operator shuts off transcription - trp is an example
An inducable operon is one that is usually off: a molecule called an inducer inactivates the repressor and turns on transcription
Glucose represses the lac operon:
If the cell has sufficient glucose, it will not metabolise lactose even if it is present
Called catabolite repression
There is a second binding site just upstream of the promoter site CAP site
The Catabolite activator protein
Ability of glucose to control the expression of a number of different inducible operons is accomplished through the catabolite activator protein (CAP)
Binds to cAMP
Inverse relationship between glucose levels and cAMP levels in bacterial cells
GLUCOSE LEVELS HIGH= cAMP LEVELS LOW
Genes only expressed as needed: Repressible gene, Inducible Genes, Catabolite repression
lac operon is under dual control: Negative lac repressor + Positive control CAP(CAP controls rate of transcripton)
In the absence of glucose - Cells have alot of cAMP which complexes tot the CAP= enhances the binding or RNA polymerase and rate of transcription
cAMP molecule and the CAP protein
free CAP protein cannot bind the promoter when cAMP binds to the CAP in bacterial cell:
The cAMP-CAP complex binds to a site on teh DNA in the promoter region
The binding of the complex enhances the activity of the promoter, more transcripts are initiated so this is an example of positive regulation