lac Operon
Biology: Cambridge International A Level, topic 16.3.1
Transcriptional Level
Regulation of gene expression in prokaryotic and sometimes eukaryotic cells occurs at the transcriptional level. Simply, if a cell does not transcribe DNA into mRNA, protein synthesis does not occur.
Operons
- Bacterial genes are often organized into common pathways called operons, for more coordinated regulation of expression.
- All the genes in one operon are transcribed as a long, single piece of mRNA.
- The operon includes a regulatory gene that codes for a repressor protein which binds to the operator, preventing RNA polymerase from transcribing the gene of interest.
Regulatory molecules
- In prokaryotic cells, there are three types of regulatory molecules that can affect the expression of operons:
- Repressors are proteins which suppress transcription of a gene in response to an external stimulus.
- Activators are proteins which increase transcription of a gene in response to an external stimulus.
- Inducers are small molecules which activate or repress transcription depending on the needs of the cell and substrate availability.
Lac operon
- The lac operon is an inducible operon which contains the genes necessary to acquire and process lactose from the environment.
- This is useful for E. coli bacteria because it means it can use lactose as an energy resource when glucose concentrations are low.
Lac operon
- When glucose is absent and lactose is present, the catabolite activator protein (CAP) binds to the operator sequence to activate transcription.
- If lactose is absent, the repressor binds to the operator to prevent transcription.
- Lac operon is only transcribed when glucose is low AND lactose is present.
Comparison to eukaryotes
- Transcription factors are involved in the regulation of gene expression in eukaryotes at the transcriptional level.
- Eukaryote genes are NOT organized into operons.
- However, one transcription factor may regulate a group of genes involved in a particular function.
Test yourself on lac Operon
What are the three types of regulatory molecules in prokaryotes?
The lac operon contains genes necessary to process which sugar?