6.1.3 Gene Control: Lac Operon Flashcards
What do regulatory genes control?
Regulatory genes control structural genes and their levels of protein production
Regulatory genes sometimes have control over several structural genes at once
If the structural genes being controlled are in any way involved in the process of transcription, then gene control is occurring at the transcriptional level
What does the lac operon provide an example of?
The lac operon provides an example of a regulatory mechanism at the transcriptional level (i.e. a regulatory mechanism that occurs during transcription)
What can form an operon in prokaryotes?
Structural genes
What is an operon?
A group or a cluster of genes that are controlled by the same promoter
What does the lac operon do?
The lac operon controls the production of the enzyme lactase (also called β-galactosidase) and two other structural proteins
What does lactase do?
Lactase breaks down the substrate lactose so that it can be used as an energy source in the bacterial cell
What is lactase, as an enzyme, known as and what does this help prevent?
It is known as an inducible enzyme (this means it is only synthesized when lactose is present)
This helps prevent the bacteria from wasting energy and materials
What are the components of the lac operon?
Promoter for structural genes
Operator
Structural gene lacZ that codes for lactase
Structural gene lacY that codes for permease (allows lactose into the cell)
Structural gene lacA that codes for transacetylase
What is a promoter?
Region of DNA required to allow transcription of the gene to take place.
What is an operator?
Segment of DNA to which a repressor binds to inhibit the transcription of a gene.
What is also located to the left (upstream) of the lac operon on the bacterium’s DNA?
Promoter for regulatory gene
Regulatory gene lacI that codes for the lac repressor protein
What do the two binding sites on the lac repressor protein bind to?
the operator in the lac operon and also to lactose (the effector molecule)
When it binds to the operator it prevents the transcription of the structural genes as RNA polymerase cannot attach to the promoter
When it binds to lactose the shape of the repressor protein distorts and it can no longer bind to the operator
What happens when lactose is absent?
The regulatory gene is transcribed and translated to produce lac repressor protein
The lac repressor protein binds to the operator region upstream of lacZ
Due to the presence of the repressor protein RNA polymerase is unable to bind to the promoter region
Transcription of the structural genes does not take place
No lactase enzyme is synthesized
What happens when lactose is present?
The following processes take place when lactose is present in the medium that the bacterium is growing in:
There is an uptake of lactose by the bacterium
The lactose binds to the second binding site on the repressor protein, distorting its shape so that it cannot bind to the operator site
RNA polymerase is then able to bind to the promoter region and transcription takes place
The mRNA from all three structural genes is translated
Enzyme lactase is produced and lactose can be broken down and used for energy by the bacterium
