7. An bacterial operon contains three structural genes A, B, and C in the same order. If polar mutation occurs in gene B, then the effect in protein would be observed in
(1) In all proteins A, B, and C
(2) Only IN B and C
(3) Only in B
(4) Complete loss of all proteins
The correct answer is: The effect in protein would be observed in proteins B and C only, due to polarity in operon gene expression caused by the mutation in gene B.
Introduction
In bacterial genetics, operons are clusters of genes transcribed together that allow coordinated expression. A polar mutation in one gene of a polycistronic operon can impact not only that gene but also the expression of other genes downstream. In CSIR NET and similar exams, understanding the impact of polar mutations helps clarify gene regulation concepts and prepares for advanced studies.
Analysis of Exam Options
Option 1: In all proteins A, B, and C
This is incorrect. A polar mutation in gene B will not affect gene A because A is upstream (before) B in the operon. Polar effects are specific to the mutated gene and the downstream genes; upstream gene expression remains unaffected.
Option 2: Only IN B and C
This is correct. A polar mutation (like a nonsense or frameshift mutation) in gene B not only disrupts the translation of its own product, but also often causes premature transcription or translation termination for downstream genes—in this case, gene C. Therefore, both protein B and protein C are lost or greatly reduced, while protein A is produced normally. This classic polarity effect in bacterial polycistronic messages means protein synthesis is lost beyond the site of mutation.
Option 3: Only in B
This is incorrect. Loss of protein in gene B alone would suggest no polar effect, but polarity always involves the downstream gene (gene C) whose expression is reduced or abolished because of the mutation in B and transcription termination mechanisms (such as Rho factor binding).
Option 4: Complete loss of all proteins
This is incorrect. Complete loss would mean that even gene A’s protein is lost, which cannot happen with a mutation in gene B, as gene A lies upstream and will be transcribed and translated normally in the absence of mutations.
Molecular Mechanism and Explanation
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Polar mutations (typically nonsense mutations) introduce a premature stop codon in a gene (here, B), causing early termination of translation.
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Ribosome stalling can lead to Rho-dependent termination, aborting transcription of downstream genes (C), so both B and C products are depleted.
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Upstream genes (A) are unaffected because transcription and translation begin with them and continue until the site of mutation.
Understanding these effects is fundamental for genetic engineering, functional genomics, and bacterial gene regulation studies.
For exam preparation, remember: in a bacterial operon with a polar mutation in a middle gene (B), the loss of protein will be observed in both B and C, while A remains unaffected. This matches option (2), which is the correct answer.
