Bacterial gene regulation: Why do many structural genes in prokaryotes operate together as a single unit (an operon) rather than as isolated, independently controlled genes?

Introduction / Context:
In prokaryotic molecular biology, many structural genes are arranged into operons. An operon is a cluster of genes under the control of a single promoter and operator, producing one polycistronic mRNA. This question tests why evolution favored such grouped control in bacteria.

Given Data / Assumptions:

• Bacteria often need to turn an entire pathway on or off quickly (for example, lactose utilization or tryptophan biosynthesis).
• One promoter drives transcription of multiple adjacent genes as a single transcript.
• Environmental resources and energy are limited; regulatory efficiency is advantageous.

Concept / Approach:
Coordinated regulation is the key idea. When genes encode proteins that function together in a pathway, regulating them together ensures stoichiometric expression, minimizes waste, and improves response speed to environmental changes.

Step-by-Step Solution:
Identify the problem bacteria solve: synchronizing enzymes of one pathway. Mechanism: a single promoter–operator region controls transcription of several contiguous genes. Outcome: a single regulatory input (repressor/activator) simultaneously modulates all pathway components. Select the explanation emphasizing coordinated regulation for common function.

Verification / Alternative check:
The lac operon (lacZYA) and trp operon are classic examples where all enzymes needed for sugar utilization or amino acid biosynthesis are co-expressed under single-point control, validating the efficiency rationale.

Why Other Options Are Wrong:
“Accident of evolution” ignores clear adaptive benefits; chromosome size does not force unrelated genes to share promoters; operons are primarily about transcriptional control, not DNA replication speed.

Common Pitfalls:
Assuming operons randomly group genes; confusing gene proximity with functional coordination.

Final Answer:
It allows coordinated regulation of multiple genes that serve one pathway or common function.