Name the lac operon concept and how it regulates gene expression in E. coli.

Name the lac operon concept and how it regulates gene expression in E. coli.

• A. A DNA sequence encoding tRNA modification enzymes; lac operon is activated by glucose.
• B. A cluster of genes under a single promoter; lac operon is inducible by lactose, repressed by lac repressor; presence of lactose leads to repressor inactivation, allowing transcription.
• C. A single gene regulated by multiple promoters; lac operon is constitutively expressed regardless of lactose.
• D. A eukaryotic system with chromatin remodeling.

Answer: (B)

Regulation of a prokaryotic operon by a repressor and inducer, yielding inducible gene expression. The lac operon in E. coli groups lacZ, lacY, and lacA under a single promoter and is kept off by the lac repressor binding to the operator when lactose isn’t present. When lactose is available, it is converted to allolactose, which binds the repressor and inactivates it, lifting the block and allowing RNA polymerase to transcribe the operon. This classic example shows how a cluster of genes can be turned on by a small molecule signal, producing the enzymes needed to metabolize lactose. It’s not about tRNA modification enzymes, it isn’t a single-gene system with multiple promoters, and it’s not a eukaryotic chromatin-based setup.

Regulation of a prokaryotic operon by a repressor and inducer, yielding inducible gene expression. The lac operon in E. coli groups lacZ, lacY, and lacA under a single promoter and is kept off by the lac repressor binding to the operator when lactose isn’t present. When lactose is available, it is converted to allolactose, which binds the repressor and inactivates it, lifting the block and allowing RNA polymerase to transcribe the operon. This classic example shows how a cluster of genes can be turned on by a small molecule signal, producing the enzymes needed to metabolize lactose. It’s not about tRNA modification enzymes, it isn’t a single-gene system with multiple promoters, and it’s not a eukaryotic chromatin-based setup.