Question

The lac operon consists of

• A. Three regulatory genes and three structural genes.
• B. Four regulatory genes only.
• C. One regulatory gene and three structural genes.
• D. Two regulatory genes and two structural genes.

Answer 1

The correct option is C One regulatory gene and three structural genes.

Lac operon is made up of one regulator, one operator and three structural genes. The three structural genes are LacZ, LacY and LacA. LacZ encodes β-galactosidase (LacZ), an intracellular enzyme that cleaves the disaccharide lactose into glucose and galactose. LacY encodes beta permease which pumps the lactose inside the cell. Lac A encodes beta transacetylase which removes toxic substances from the cell.

The regulator gene produces m-RNA which translates into the repressor. The repressor attaches to the operator and blocks the expression of the structural gene in absence of inducer. In presence of inducer, the repressor attaches to the inducer and operator is not blocked so it allows the expression of the structural gene. Operator genes are a region of DNA sequence which attaches to the repressor molecule and stops the expression of the structural gene.

So, the correct answer is option C.