Question

Describe how the lac operon operates, both in the presence and absence of an inducer in E.coli.

Answer 1

Hint: Operons can be defined as regions of DNA in which clusters of related genes are present. There are two types of operons and one of them is - Lac operon, it consists of a regulator gene, promoter gene, operator gene and structural gene.

Complete Answer:
- The lac operon contains a promoter, an operator, and three closely related structural genes- z, y, a that codes for enzymes B-galactosidase, B-galactoside permease, and B-galactoside transacetylase respectively.
- B-galactoside permease is responsible for pumping lactose into the cell whereas B-galactosidase catalyses the conversion of lactose into glucose and galactose. It is to be noted that these genes are not expressed in the absence of lactose. In the lac operon, the presence of lactose acts as an inducer.
- The promoter (p) is the site at which RNA polymerase binds to initiate the transcription of the structural genes. The operator (O) is the site at which the repressor protein (product of regulator gene) binds. In absence of an inducer, the repressor protein binds to the operator region and prevents RNA polymerase from transcribing the structural gene. Thus, the lac operon gene is inactivated.
- Lac operon in presence of an inducer binds with repressor protein and inactivates it. This allows RNA polymerase access to promoter and transcription proceeds.

Note: It is to be noted that operons occur primarily in prokaryotes but may also be seen in some eukaryotes, including nematodes such as C. elegans and the fruit fly, Drosophila melanogaster.