Question

Select the two correct statements out of the four given below about the lac operon.
(1) Glucose and 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

Hint: An operon, or group of genes with a single promoter (transcribed as a single mRNA), is the lac operon. The genes in the operon encode proteins that allow lactose to be used as an energy source by bacteria. In E.coli and many other enteric bacteria, the lactose operon is an operon necessary for the transport and metabolism of lactose.

Complete answer:
In Escherichia coli and other bacteria, the lac operon is an operon responsible for the transport and metabolism of lactose. Francois Jacob and Jacque Monad first explained the mechanisms of regulation of the expression of genes responsible for coding the enzymes involved. For the transport of lactose into the cell, the beta-galactoside permease gene is responsible and is encoded by the lac y-gene, not the z-gene.

The repressor protein binds to the operator in the absence of lactose sugar and thus does not permit the transcription of the three lac genes. Allolactose, an isomer of lactose, binds to the repressor and induces an allosteric change in the presence of lactose. The altered repressor does not bind to the operator, allowing the lac genes to be transcribed by RNA polymerase. The chosen sugars are glucose/galactose and their presence inhibits the expression of lac operon by catabolite repression. They do not bind the sugar to the repressor. The enzyme beta-galactosidase, encoded by the gene lacZ, transforms allolactose and lactose.
So, the correct answer is ‘(c) 2and 4’.

Note: A protein that represses (inhibits) transcription of the lac operon is the lac repressor. The lac repressor binds tightly to the operator when lactose is not available, preventing transcription by RNA polymerase. However, when lactose is present, its ability to bind DNA is lost by the lac repressor.