Distinguish between constitutive and inducible genes and give an example.

• A. Constitutive genes are expressed continuously; inducible genes are activated under specific conditions (e.g., lac operon is inducible by lactose).
• B. Constitutive genes are activated only under stress; inducible genes are always on; lac operon is constitutive.
• C. Constitutive genes are expressed only in prokaryotes; inducible genes only in eukaryotes.
• D. Inducible genes are expressed continuously; constitutive genes require lactose.

Answer: (A)

Some genes are kept on all the time because their products are needed for basic cellular functions, while others are turned on only when a specific signal or condition arises to meet a need. Constitutive genes are expressed continuously, providing essential proteins at steady levels. Inducible genes are activated only in response to a signal, conserving energy until their products are required. The lac operon is a classic example of inducible regulation: in the absence of lactose, a repressor binds to the operator and prevents transcription; when lactose is present, it is converted to an inducer that binds the repressor, releasing it and allowing transcription of the lacZ, lacY, and lacA genes to metabolize lactose. This illustrates how inducible systems switch on in response to environmental cues, in contrast to constitutive genes that remain expressed regardless of such conditions.