6.1.3 Gene Control: Lac Operon Flashcards
Transcription level gene crontrol
- If the regulatory gene is controlling any structural gene in the process of transcription, then it is classified as this
Lac operon
- Transcription level regulation
- Structural genes in prokaryotes form an operon (cluster of genes controlled by the same promoter)
- Lac operon controls production of enzyme lactase
- Lactase breaks down lactose so it can be used as an energy source in bacteria
- It is an inducible enzyme (only synthesised when lactose is present)
- Helps prevent bacteria from wasting energy and materials
Components of Lac Operon
- Promoter for structural genes (region of DNA which allows transcription to take place)
- Operator (segment of DNA where a repressor binds to inhibit transcription of gene)
- LacZ structural gene coding for lactase
- LacY (codes for permease, letting lactose into cell)
- LacA coding for transacetylase
Top left of lac operon is the :
- Promoter for the regulatory gene
- Regulatory gene LacL coding for lac repressor protein
Repressor can bind to 2 sites, 1 being the operator which when occurs, prevents transcription of structural genes because RNA cannot attach to promoter. The other is lactose, which when binds, changes the shape of the repressor protein, so it cannot bind to operator.
What happens when lactose is absent?
- Regulatory gene transcribed and translated to produce lac repressor proteins
- Lac repressor protein binds to operator
- Presence of repressor protein means RNA polymerase cannot bind to promoter region
- Transcription of structural gene does not take place
- No lactase is synthesised
What happens when lactose is present?
- Uptake of lactose by bacteria
- Lactose binds to 2nd binding site of repressor protein
- Repressor protein changes structure
- So cannot bind to operator region
- So RNA polymerase can bind to promoter region
- So transcription takes place
- Lactase is produced and lactose can be broken down and used for energy by bacteria
