Question

In an inducible operon, the genes are
A. Usually expressed unless a signal turns them off
B. Never expressed
C. Always expressed
D. Usually not expressed unless a signal turns them on

Answer 1

Hint: A gene system or an operon is a system which encodes a synchronised group of enzymes involved in a catabolic pathway. An operon can be inducible or repressible based on the method of its function.

Complete step by step answer: An inducible operon is a type of operon that is switched off in its normal state. It only becomes operational when an inducer is present, for example, lactose in the lac operon. The inducible operons normally operate in catabolic pathways in the presence of an inducer such as lactose. The repressor possesses a greater affinity for the inducer rather than for the operator. When the operon is not working, a small amount of lactose permease enzyme is present in the cell even. When the lactose is present in the cell, the enzyme lactose permease carries some molecules of lactose inside the cell. The lactose molecules are then transformed into allolactose molecules by the enzyme. Allolactose acts as an inducer and then it binds to the repressor gene. When the repressor-inducer complex does not attach to the operator gene, it gets turned on.
So, the correct answer is option D.

Additional information: The lac operon is an inducible operon which contains the following components: an operator, a promoter, and three structural genes. These structural genes are transcribed together and they are controlled by the catabolite activator protein (CAP) or the repressor gene.

Note: The tryptophan operon is a repressible operon. It is found in E. coli bacteria that consists of a group of genes which encode biosynthetic enzymes for the amino acid tryptophan. The trp operon, in normal conditions, is expressed or turned on. When the tryptophan levels are low in the cell, the operon is repressed or turned off when the levels are high.