6.1.3 Gene control: Lac operon Flashcards
What is an example of a regulatory mechanism at the transcriptional level?
The lac operon (occurs during transcription)
What is an operon?
A group or cluster of genes that are controlled by the same promoter
What does the lac operon control?
It controls the production of the enzyme lactate
What is lactase known as
An inducible enzyme (this means it is only synthesised when lactose is present)
Why is it helpful that lactase is an inducible enzyme?
It helps prevent the bacteria from wasting energy and materials
Why does lactase break down the substrate lactose
So that it can be used as an energy source in the bacterial cell
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
Located to the left (upstream) of the lac operon on the bacterium DNA there is also the:
- Promoter for regulatory gene
- Regulatory gene lacl that codes for the lac repressor protein
Describe the process of the lac operon
- It has two binding sites that allow it to bind to the operator in the lac operon and also to lactose (effector molecule)
- When it binds to the operator it prevents the transcrption 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 is the process that takes place when lactose is absent in the medium that the bacterium is growing in?
The regulatory gene is transcribed and translated to produce lac respressor 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 synthesised
What is the process that takes 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
Describe the genetic cotrol of a repressible enzyme
In this mechanism:
An effector molecule binds to a repressor protein produced by a regulatory gene.
This binding helps the repressor bind to the operator region and prevent transcription of the structural genes.
When there is less of the effector molecule , repressor protein cannot bind to the operator region and the transcription of the structural genes goes ahead (enzyme is produced)
