Gene control in prokaryotes (lac operon)
In both prokaryotes and eukaryotes, transcription of a gene is controlled
by transcription factors.
Transcription factors are:
• proteins that bind to a specific DNA sequence
• control the formation of mRNA --> control the flow of information from
DNA to RNA
Structural genes code for proteins required by a cell.
Regulatory genes code for proteins that regulate the expression of other
genes.
The synthesis of repressible enzymes can be prevented by binding
a repressor protein to the operator (a specific site) on the bacterium's DNA.
The synthesis of inducible enzymes occur when its substrate is present.
Prokaryotic lac operon
operon: a length of DNA making up a unit of gene expression in a bacterium.
The lac operon consists of a length of DNA
with operator and promoter regions and a cluster of 3 structural genes:
• lacZ - coding for β-galactosidase (hydrolyses lactose to glucose +
galactose)
• lacY - coding for permease (allows lactose to enter cell)
• lacA - coding for transacetylase
Close to the promoter, although not part of the operon, is the regulatory
gene for the lac operon.
Lactose absent
• regulatory gene codes for repressor protein*
• repressor binds to operator region, close to lacZ
• RNA polymerase can't bind to DNA at promoter region
• no transcription of the 3 structural genes
*repressor protein is allosteric (has 2 binding sites). The repressor binds to DNA
with one site and binds to lactose with another site.
In both prokaryotes and eukaryotes, transcription of a gene is controlled
by transcription factors.
Transcription factors are:
• proteins that bind to a specific DNA sequence
• control the formation of mRNA --> control the flow of information from
DNA to RNA
Structural genes code for proteins required by a cell.
Regulatory genes code for proteins that regulate the expression of other
genes.
The synthesis of repressible enzymes can be prevented by binding
a repressor protein to the operator (a specific site) on the bacterium's DNA.
The synthesis of inducible enzymes occur when its substrate is present.
Prokaryotic lac operon
operon: a length of DNA making up a unit of gene expression in a bacterium.
The lac operon consists of a length of DNA
with operator and promoter regions and a cluster of 3 structural genes:
• lacZ - coding for β-galactosidase (hydrolyses lactose to glucose +
galactose)
• lacY - coding for permease (allows lactose to enter cell)
• lacA - coding for transacetylase
Close to the promoter, although not part of the operon, is the regulatory
gene for the lac operon.
Lactose absent
• regulatory gene codes for repressor protein*
• repressor binds to operator region, close to lacZ
• RNA polymerase can't bind to DNA at promoter region
• no transcription of the 3 structural genes
*repressor protein is allosteric (has 2 binding sites). The repressor binds to DNA
with one site and binds to lactose with another site.
