Question

Select the two statements out of the four (A-D) given below about lac operon :
(i) Glucose or galactose may bind with the repressor and inactive it
(ii) In the absence of lactose, the repressor binds with the operator region
(iii) The z-gene codes for permease
(iv) This was elucidated by francois Jacob and Jacques Monod

The correct statements are :

Answer 1

Statement ii: In the absence of lactose the repressor binds with the operator region and prevents transcription of the lac operon. Most of the effect of Lac repressor’s binding to the operator is to block the progression of RNA polymerase transcription. In this sense, the Lac repressor acts as a roadblock on the DNA. Consequently, all of the structural genes of the lac operon (the Z, Y, and A genes) are repressed, and there is no $beta$-galactosidase, $beta$-galactoside permease, or transacetylase in the cell.

Statement iv: In this mechanisms of regulation of expression of genes responsible for coding the involved enzymes was first explained by Francois Jacob and Jacque Monad. The z-gene does not codes for permease, instead it codes for beta galactosidase in lac operon.

In lac operon, lactose acts as the inducer that binds with the repressor and inactivates it. This enables the operator to carry out the translation of the genes and produce enzymes. If glucose is present in the system there is no need of lac operon as it is the preferred substrate. Since galactose is not an inducer it cannot bind with the repressor.

In the absence of lactose repressor binds with the operator and does not allow translation of the genes and hence no enzymes will be formed.

The lac z gene codes for galactosidase enzyme. Permease is encoded by lac y gene.

So, the correct statements are only "ii and iv"