Question

The lac operon consists of
A)4 regulatory genes only.
B)1 regulatory gene and 3 structural genes.
C)2 regulatory genes and 2 structural genes.
D)3 regulatory genes and 3 structural genes.

Answer 1

Hint: Lac genes that encode enzymes are lacZ, lacY, and lacA. The fourth lac gene is lacI, which encodes the lactose repressor—"I" stands for inductibility. One can differentiate between structural genes that encode enzymes and regulatory genes that encode proteins that affect gene expression.

Complete answer:
Lac operon consists of 1 regulator, one operator and three structural genes. LacZ encodes β-galactosidase, an intracellular enzyme that binds disaccharide lactose to glucose and galactose. LacY encodes the beta permease that pumps the lactose within the cell. Lac A encodes beta transacetylase that eliminates toxic substances from the cell.

The factor produces m-RNA that's passed to the repressor. Within the absence of an inducer, the repressor binds the operator to suppress the structural expression of the gene. within the presence of the inducer, the repressor binds the inducer and therefore the operator isn't inhibited so the factor may be expressed. Operator genes are a locality of the DNA sequence that binds to the repressor molecule and prevents the expression of the cistron.

Hence, the correct answer is option (B)

Note: Allolactose is the inducer present in the lac operon. When lactose is present in the medium, it is converted into allolactose (small amount) by enzyme β-galactosidase found in the cell. The allolactose then binds to the repressor and decreases the affinity of the repressor to the operator site.