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Control of Gene Expression Prokaryote (Gene a hereditary unit…
Control of Gene Expression
Prokaryote
Gene
a hereditary unit occupying a specific position (locus) within the genome.
Set of segments of nucleic acid that contains the information necessary to produce a functional RNA product in a controlled manner
Structure of a gene
Promoter regions are base pair sequences that specify where transcription beings.
Coding sequence is a base pair sequence that include coding information for the RNA specified by the gene
The terminator sequence that specifies the end of the RNA transcript
Phenotype = the observed characteristic of the
organism
Determined by an individuals genotype and expressed genes
Genotype = the genetic makeup of an individual
organism
Through interactions through the environment a genotype is expressed in a phenotype
Gene expression - The process by which DNA direct protein synthesis.
Includes 2 stages
Transcription
Translation
Conditions inside the cell and its surroundings can influence the cells DNA
Genetic Orchestra
Prokaryotes and eukaryotes alter gene expression in response to their changing environment
In multicellular eukaryotes gene expression regulate development and is responsible for differences in cell types - cell differentiation
RNA molecules play many roles in regulating gene expression in eukaryotes
Bacteria respond to environmental change by regulating transcription
Natural selection has favoured bacteria that produce only products needed by that cell.
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 gene that function together are regulated together and are known as an operon
Gene terminology
Constitutive genes
these are genes that are constantly switched on. Transcribed continuously
genes for glucose metabolism
Facultative genes
genes that are transcribed only when needed
Operon theory
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
Transcription depends on a common regulatory region
Structural genes - codes for proteins and RNA molecules required for normal enzymatic function
Regulator gene - code for proteins and RNA molecules which regulate the expression of structural genes
Cistron - region of DNA that encodes for a single polypeptide (or functional RNA molecule)
Polcistronic RNA - a single mRNA encoding several different polypeptide chains
Operons
Entire 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
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
Can be switched off by a protein repressor
Repressor prevents gene transcription by bind to the operator and blocking RNA polymerase
-Repressor is the product of a separate regulatory gene
Repressor can be in active or inactive form, depending on the presence of other molecules
Glucose is the main fuel used in E.coil
Used as a carbon and energy source
If glucose not available lactose is used instead
3 proteins important for lactose metabolism
These 3 proteins are all encoded in one single expressible unit called the lac operon
Galactoside permease
Transport protein that pumps lactose into the cell
B-galactosidase
Intracellular enzyme that breaks down lactose into galactose and glucose
B-galactoside transacetylase
Enzyme that transfers an acetyl group from acetyl-CoA to B-galactosides
2 Types of Negative Gene Regulation
Repressible operon is one that is usually on
Binding of repressor to the operator shuts off transcription
The trp operon is a repressible operon
Inducible operon is one that is usually off
Molecule called an inducer inactivates the repressor and turns on transcription
What happens in the presence of lactose
Lactose enters E.coli small amounts of allolactose an isomer of lactose are formed
Allolactose bind to an allosteric site on the repressor protein causing a conformational change
As a result of this change the repressor can no longer bind to the operator region and falls off
RNA polymerase can then bind to the promoter and transcribe the lac genes
Allolactose is called an inducer because it turns on or induces the expression of the lac genes
If the lac I gene becomes mutated such that the repressor encoded no longer binds to allolactose the repressor would bins to the operator regardless of the presence or absence allolactose and the operon would never be transcribed at high levels.
Catabolite repression - Cell will not metabolise lactose if there is sufficient glucose
CAP = Catabolite Activator Protein site
Catabolite Activator Protein (CAP) is also known as Cyclic AMP Receptor Protein (CRP) and is coded for by the crp gene in E. coli.
CAP - ability of glucose to control the expression of a number of different inducible operons.
CAP bind to cAMP
Inverse relationship between glucose levels and cAMP levels in bacterial cells
Glucose levels high = cAMP levels low (and visa versa)
cAMP-CAP complex binds to a site on the DNA in the promoter region
Binding of the complex enhances the activity of the promoter, more transcripts are initiated, this is called positive regulation
Regulation
Constitutive genes are expressed at a fixed rate
Other genes are expressed only as needed
Repressible genes
Inducible genes
Catabolite repression
