Distinguish constitutive expression from inducible expression.

• A. Constitutive expression requires a stimulus; inducible is constant
• B. Constitutive expression is turned on by a stimulus; inducible is always on
• C. Constitutive expression is constant; inducible expression requires a stimulus to be turned on or off
• D. Constitutive expression is never turned on; inducible is always on

Answer: (C)

Constitutive expression means a gene is expressed at a steady, ongoing level regardless of most environmental conditions, while inducible expression is regulated and changes in response to specific signals, turning on or off when a stimulus is present or absent. The statement that constitutive expression stays constant and inducible expression requires a stimulus to be turned on or off captures this difference precisely. For example, housekeeping genes are typically expressed continuously, whereas inducible genes respond to environmental cues or substrates, becoming active only when the signal arrives. The other options mix up which expression type is stimulus-dependent or constant, so they don’t fit as well.