Question

Inducible operons differ from repressible operons in that
A. Operator of inducible operon is switched off by a repressor protein.
B. Inducible operons have an operator that controls the binding of RNA polymerase to the promoter region.
C. Inducible operons use activators and positive control to regulate their transcription.
D. Inducible operons usually include structural genes that function in catabolic pathways.

Answer 1

Hint: Linked genes in bacteria are mostly located in a cluster on a chromosome. They are transcribed as a single unit with the help of a promoter called the RNA polymerase binding site. Such a gene cluster under the control of a single promoter is referred to as an operon.

Complete answer:
Operons can be of two types. They are inducible operons and repressible operons. The naturally ‘off’ operons can be turned ‘on’ with the help of a molecule called inducer. Such operons are referred to as inducible operons. The naturally ‘on’ operon can be turned ‘off’ with the use of a molecule called corepressor. Such types of operons are called repressible operons. Typically, the inducible operon is off and can be induced in the presence of a lactose-like inducer. After being induced, the operon is turned on, and to catabolize these inducers, transcription takes place. The inducible operon, therefore, is usually involved in catabolism.
Although repressible operon is generally on and results in the transcription of structural genes, tryptophan. It, therefore, works at times when the bacterial cell does not need such products as tryptophan and this operon is turned off by the presence of tryptophan from an external source. Therefore, option D is the correct answer as structural genes are involved in inducible operons that function in the catabolic pathway.

So, option D) is the correct option.

Note: Operons are commonly seen in bacteria, but they are uncommon in eukaryotes, such as humans. The lac operon is an inducible operon that encodes sugar lactose metabolism enzymes. It only activates when sugar lactose is present (and other, preferred sugars are absent). Allolactose, a modified form of lactose, is the inducer in this instance. The trp operon is a repressible operon that codes for the amino acid tryptophan synthesis enzymes. By default, this operon is expressed but can be repressed when there are elevated levels of tryptophan amino acid. In this case, the corepressor is tryptophane.