16.3 Gene control Flashcards
Transcription Factors
proteins that bind to a specifc DNA sequence and control the flow of information from DNA to RNA by controlling the formation of mRNA.
Structural Genes
Genes that code for proteins required by a cell
Regulatory Genes
Genes that code for proteins that regulate the expression of other genes
Inducible Enzymes
Enzyme production only occurs when its substrate is present
Repressible Enzymes
The presence of a particular substance allows the repressor to bind with the operator and stops the enzyme being produced
Operon
a length of DNA making up a unit of gene expression in a bacterium. It consists of one or more structural genes and also control regions of DNA that are recognised by the products of regulatory genes.
Most studied example of gene expression control
production of the enzyme lactase (β-galactosidase) in the bacterium Escheria Coli
- enzyme β-galactosidase hydrolyses the disaccharide
lactose to the monosaccharides glucose and galactose.
Is lactase an inducible or repressible enzyme?
Inducible
- Bacterium only produces lactase when the sugar lactose is present
Structure of Lac Operon
1) Promoter for regulatory gene
2) Regulatory Gene
3) Promoter for structural genes
4) Operator
5) Structural Genes
Promoter
allows transcription of the structural genes to begin
Operator
allows a molecule called a transcription factor to bind, which in this case stops the structural genes being transcribed
The 3 Structural Genes
- lacZ, coding for β-galactosidase
- lacY, coding for permease (which allows lactose to enter the cell)
- lacA, coding for transacetylase.
Regulatory Gene
codes for a protein called a repressor
Lac Operon when lactose is present
- the lactase enzyme is inducible; its production occurs only when lactose is present
- When there is lactose in the medium, it binds to the repressor protein (allosteric site) and alters its shape.
- This prevents the repressor protein from binding to the operator region.
- Now RNA polymerase can bind to the promoter, and the structural genes can be transcribed.
Lac Operon when lactose is not present
- When there is no lactose in the medium in which the bacterium is growing, the regulatory gene is expressed and therefore the repressor protein is produced.
- This repressor protein binds to the promoter for the structural genes, so they are not transcribed and no lactase is made.
What happens when both glucose and lactose is present in the medium
- Glucose has a higher preference
- When a bacterium f nds both glucose and lactose
in the medium in which it is growing, it represses the use
of lactose by suppressing the lac operon by means of a
dif erent transcription control factor.
Gene control in Eukaryotes
- the bigger the size of the genome, the more transcription factors
- This means that Eukaryotes have many more
ways of regulating gene expression than have Prokaryotes. - The factors may bind to the promoter region of a gene.
- They may increase or decrease the transcription of the
gene. - Whatever the mechanism, their role is to make sure that genes are expressed in the correct cell at the correct time and to the correct extent.
Effects of Transcription Factors
1) Necessary for transcription to occur. They form part of the protein complex that binds to the promoter region of the gene concerned.
2) factors activate appropriate genes in sequence,
allowing the correct pattern of development of body regions.
3) responsible for the determination of sex in mammals.
4) allow responses to environmental stimuli, such as switching on the correct genes to respond to high environmental temperatures.
5) Some transcription factors regulate the cell cycle, growth and apoptosis (programmed cell death)
6) Affected by hormones
Example of Gene expression in Eukaryotes
- The plant hormone, gibberellin, controls seed germination by stimulating the synthesis of amylase
- a good example of how a hormone can influence transcription.
- It has been shown that, in barley seeds, application of
gibberellin causes an increase in the transcription of
mRNA coding for amylase.
Gibberellin Hormone effect in plants
1) production of the enzyme amylase in germinating seeds depends on a transcription factor called PIF binding with the promoter region next to the gene that codes for amylase
2) When no gibberellin is present, PIF is bound to a protein called a DELLA protein. This prevents PIF from binding with the promoter for the amylase gene, so no amylase is made
3) When gibberellin is present, it binds to a receptor and an enzyme. This activates the enzyme, which breaks down the DELLA protein. PIF is now free to bind with the promoter for the amylase gene, and amylase is made
