Question

In lac operon concept of gene expression, allolactose acts as-
A. Co-repressor
B. Repressor
C. Coenzyme
D. Inducer

Answer 1

Hint: The lac operon is the commonly known form of the lactose operon which is a combination of genes that are specific for uptake and metabolism of lactose. Lac operon is present in E. coli and some other bacteria. The lac operon is made up of three structural genes, that are lacZ, lacY, and lac A.

Complete answer:
An operon is denoted as a functioning unit of genomic DNA that contains a group of genes. The lactose operon, commonly known as lac operon, is an operon which is required for the transport and metabolism of lactose. This operon is found in E.coli and many other bacteria. This operon contains a set of genes coding for proteins in response to transportation of lactose into the cytoplasm and further its digestion into glucose. This glucose is then used for generation of energy in the form of ATP. The lac operon works only when lactose is available and glucose is absent in the cell.
The lac operon is made up of three structural genes that are lacZ, lacY, and lacA as well as a promoter and operator. Each structural gene produces coding needed for processing of enzymes such as β-galactosidase, β-galactoside permease and β-galactoside transacetylase by lac Z, lacY and lac A, respectively. The expression of these genes is regulated by the two regulators which turn the operon on and off in response to lactose and glucose levels; they are the lac repressor and catabolite activator protein (CAP). The lac repressor acts as a lactose sensor.
When the lactose is present in the operon, the lac repressor protein combines with the lactose and forms allolactose. Lactose gets transformed in the cell to allolactose by β-galactosidase which is encoded by LacZ. Allolactose binds the lac repressor and prevents the lac repressor from inhibiting the expression of the lac operon. As it turns on or induces the expression of the lac genes, allolactose is called an inducer.

Therefore, the correct option is option D.

Note: The lac operon is the cluster of genes found in prokaryotes like E.coli and many other enteric bacteria. This operon is involved in lactose metabolism. When the lactose is present, the lac repressor protein is attached to the lactose and forms allolactose. The allolactose is known as the inducer because it turns on, or induces the expression of the lac genes.