Question

Select the two correct statements out of the four (a-d) given below about lac operon.

(a) Glucose or galactose may bind with the repressor and inactivate it.

(b) In the absence of lactose the repressor binds with the operator region.

(c) The z gene codes for permease.

(d) This was elucidated by Francois Jacob and Jacques Monod.

The correct statements are :

• A. (a) and (c)
• B. (b) and (d)
• C. (a) and (b)
• D. (b) and (c)

Answer 1

Answer: (B)

(b) and (d)

Statement A is incorrect as in presence of lactose, allolactose; an isomer of lactose, binds to the repressor and causes an allosteric shift. The altered repressor cannot bind to the operator, allowing RNA polymerase to transcribe the lac genes. The repressor, encoded by regulator gene, bind to the operator region which is located just upstream the binding site of the RNA polymerase and hence, prevent the binding of the RNA polymerase and transcription of structural genes is turned off; this makes statement B correct. The lac z, lac y and lac a genes in lac operon are the structural genes that codes for $\beta$-galactosidase (converts lactose into glucose and galactose), permease (transports lactose into the cell) and transacetylase respectively; this makes statement C incorrect. Jacob and Monod, 1961, proposed operon model to explain induction and suppression of enzyme synthesis based on their studies on inducible system for synthesis of $\beta$-galactosidase enzyme in E. coli.