Question

You have created a fusion between trp operon and lac operon which encodes the enzymes for tryptophan biosynthesis, under the regulatory control of the lac operator. Under which of the following conditions will tryptophan synthase be induced in the strain that carries the chimeric operator fused operons?

• A. Only when both lactose and glucose are absent.
• B. Only when both lactose and glucose are present.
• C. Only when lactose is absent and glucose is present.
• D. Only when lactose is present and glucose is absent.

Answer 1

Answer: (D)

The operon model of prokaryotic gene regulation was proposed by Jacob and Monod. Operons are groups of genes, coding for related proteins and are arranged in units. It is the part of the DNA which act as a single regulated unit having one or more structural gene, an operator, a promoter and a regulator gene.
The regulator or i-gene synthesises a repressor protein to regulate the functioning of the structural gene. To control the regulation of structural genes in lac operon, repressor protein binds to the operator region and prevents the movement of RNA polymerase. However, the repressor protein in lac operon gets deactivated by an inducer that is lactose.
In the absence of glucose, the repressor protein binds to the operator gene and does not allow the promoter to bind to the RNA polymerase enzyme by not allowing the structural genes to get expressed. In the presence of lactose, the repressor protein binds to the lactose and the operator site is free for the binding of RNA polymerase enzyme and allows the expression of the structural genes. Thus, lactose acts as an inducer and hence, lac operon is named inducible operon.