Operons and the Lac Operon
Operons are sections of DNA found in prokaryotes that contain a cluster of structural genes transcribed together. These genes typically code for useful proteins such as enzymes. An operon consists of several key elements:
- Structural genes
- Promoter controlelement
- Regulator activatororrepressor
- Operator
The lac operon, found in E. coli, is a classic example of an operon system. It controls the expression of genes necessary for lactose metabolism when glucose is unavailable.
Example: The lac operon in E. coli contains three structural genes: lac Z, lac Y, and lac A, which produce proteins that help bacteria digest lactose.
When lactose is absent, the lac repressor producedbytheregulatorgenelacI binds to the operator site, blocking transcription. When lactose is present, it binds to the repressor, changing its shape and allowing RNA polymerase to begin transcription of the structural genes.
Highlight: The lac operon demonstrates how prokaryotes can efficiently regulate gene expression in response to environmental changes.
Post-transcriptional modifications also play a role in gene expression control. For example, when mRNA is first made pre−mRNA, it contains both exons and introns. Through a process called splicing, introns are removed, and exons are joined together to produce mature mRNA.