Question

Select the two correct statements out of the four given below about the lac operon.
(1) Glucose or galactose may bind with the repressor and inactivate it.
(2) In the absence of lactose, the repressor binds with the operator region.
(3) The z-gene codes for permease.
(4) This was elucidated by Francois Jacob and Jacques Monod.

• A. 2 and 3
• B. 1 and 3
• C. 2 and 4
• D. 1 and 2

Answer 1

Answer: (C)

2 and 4

The lac operon is an operon responsible for transport and metabolism of lactose in Escherichia coli and other bacteria. The mechanisms of regulation of expression of genes responsible for coding the involved enzymes was first explained by Francois Jacob and Jacque Monad. The beta-galactoside permease gene is responsible for the transport of lactose into the cell and is encoded by lac y-gene, not be z-gene. Statement 3 is incorrect. In absence of lactose sugar, the repressor protein binds to the operator and thereby does not allow the transcription of three lac genes. Statement 2 is correct. 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. Glucose/ galactose are the preferred sugars and their presence inhibit the expression of lac operon by catabolite repression. These sugar does not bind to a repressor. Statement 1 is incorrect. The beta-galactosidase enzyme, encoded by lacZ gene, converts lactose into allolactose.

So, the correct answer is '2 and 4'.