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

Lactose (milk sugar) is a β-galactoside that E. coil can use for energy and as a carbon source. It is broken down into glucose and galactose. It is metabolized by the products of lactose (lac) operon which includes an operator, a promoter and three structural genes.

Complete answer:
The lac operator is a short region of DNA that interacts with a regulatory protein lac repressor, which negatively controls the transcription of the operon. The three structural genes are lac z, lac y, and lac a. Lac z codes for the enzyme β-galactosidase that cleaves the lactose molecule to yield glucose and galactose. This enzyme also converts lactose to allolactose. There are very few molecules of beta-galactosidase in a wild-type E. coli cell grown in the absence of lactose. After adding lactose to the medium, however, this enzyme increases in concentration within the bacterial cell. Lac y codes for the permease, which transports lactose into the cell. Lac a code for acetyltransferase (or β-galactoside transacetylase). This operon is controlled by both positive and negative regulation.
So the correct answer to the given question is option C, i.e., One regulatory gene and three structural genes.

Note: The acetyltransferase enzyme is not essential for lactose metabolism but appears to play a role in the detoxification of compounds by transferring an acetyl group from acetyl-CoA to β-galactoside.