Question

In lac operon, structural gene 'Z' synthesizes
a. $\beta -$galactosidase
b. Galactosidase permease
c. Galactosidase transacetylase
d. None of the above

Answer 1

Hint: Lac operon is responsible for the lactose catabolism in E.coli. An operon consists of the control genes and structural genes. The structural gene 'Z' synthesizes an enzyme that is responsible to hydrolyze the lactose into glucose and galactose by breaking a glycosidic bond.

Complete answer:
The famous operon model was proposed by Jacob and Monod in 1961. It functions for lactose catabolism in bacteria. Lac operon consists of a control gene (regulator, operator, promoter) and structural genes. These structural genes are regulated by promoter and regulator genes.

Structural genes: These are the segments of DNA that determine the primary structure of polypeptide to be synthesized under the control of an operator which in turn is controlled by a regulator. The three types of structural genes are
• Z gene: It synthesizes an enzyme β-galactosidase that functions to break down the lactose into glucose and galactose.
• Y gene: It synthesizes an enzyme lac permease that functions to transport the lactose into the cell.
• A gene: It synthesizes an enzyme transacetylase that functions to protect the cell from toxins.
These three structural gene codes for certain enzymes necessary for lactose catabolism.

Hence, the correct answer is option (A).

Note: In this Lac operon, lactose is a substrate for the enzyme β-galactosidase and also it controls the switching on and off of the operon. This is why lactose is also called an inducer. In the presence of this inducer transcription and translation occurs and also necessary enzymes are formed.