Assertion (A) : In an operon, the regulator and operator genes are not associated with the constitutive genes. Reason (R) : Constitutive genes need not be repressed.

The correct option is: "Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A)."

Step-by-Step Explanation:

1. Understanding Constitutive Genes:
Constitutive genes (also known as housekeeping genes) are genes that are expressed continuously at a relatively constant rate in all cells. They code for proteins that are essential for basic, ongoing cellular functions (such as enzymes for glycolysis or ribosomal proteins). Because their products are constantly required by the cell, their transcription is ongoing and does not need to be turned on or off in response to environmental changes.

2. Analyzing the Assertion (A):
The assertion states: "In an operon, the regulator and operator genes are not associated with the constitutive genes."
In a typical operon system (like the lac operon), the regulator gene produces a repressor protein that binds to the operator gene to turn off transcription of the structural genes when their products are not needed. Since constitutive genes are always active and do not undergo this type of on/off regulation, they do not require regulator or operator genes to control their expression. Therefore, Assertion (A) is true.

3. Analyzing the Reason (R):
The reason states: "Constitutive genes need not be repressed."
Because the products of constitutive genes are continuously needed for the basic survival of the cell, these genes are not subject to repression. There is no biological need to block their transcription under normal physiological conditions. Therefore, Reason (R) is also true.

4. Evaluating the Connection:
Why are regulator and operator genes not associated with constitutive genes (Assertion A)?
It is precisely because constitutive genes must be continuously expressed and thus do not need to be shut down or repressed (Reason R). Because they do not need to be repressed, they do not require the repressor-binding sites (operators) or regulatory proteins (regulators) that govern inducible or repressible operons. Thus, Reason (R) is the correct explanation for Assertion (A).