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?
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AIt allows coordinated regulation of multiple genes that serve one pathway or common function
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BIt is merely an accident of evolution with no selective advantage
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CBecause the chromosome is small, many unrelated genes must share the same promoter
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DNone of the above explanations applies in living bacteria
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EOperons exist only to speed up DNA replication, not transcription control
Answer
Correct Answer: It allows coordinated regulation of multiple genes that serve one pathway or common function
Explanation
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.