- A ‘ nonsense’ mutation in the protein coding region of an upstream gene of a group of genes in an operon often leads to depletion of the downstream gene products. This is a classic example of the phenomenon of “polar effect” of the mutation.
Following statements are being made about this phenomenon.
(A) It occurs primarily because the termination codon generated in the upstream open reading frame (ORF) leads to termination of protein synthesis depleting the ribosomes for translation of the downstream ORFs but it does not affect the process of transcription.
(B) The phenomenon of polar effects of mutation occurs only in the operons where the point mutation leading to creation of ‘nonsense’ mutation also leads to formation of a stem-loop resulting in Rho-independent structure termination.
(C) While the presence of termination codon in the upstream ORF may deplete the ribosomes that travel down to the downstream ORF, the depletion of ribosomes downstream of the ‘nonsense’ mutation allows loading of the Rho factor that then results in premature transcription termination.
(D) Presence of the suppressor tRNA reading the ‘nonsense’ codon generated by the mutation, is essential for causing a polar effect of the mutation.
(E) Presence of the suppressor tRNA reading thenonsense’ codon generated by the mutation diminishes the consequences of the polar effects.Choose the option that represents all correct statements.
(1) AandC(2) B and D
(3) C and E (4) A and DUnderstanding the Polar Effect of Nonsense Mutations in Operons
A polar mutation typically refers to a mutation in an upstream gene of an operon that affects the expression of downstream genes. A classic example is a nonsense mutation in the coding region of an upstream gene, which can lead to depletion of downstream gene products, a phenomenon known as the polar effect.
Key Concepts About the Polar Effect
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Polar effect occurs primarily in polycistronic mRNA operons, where multiple genes are transcribed as a single mRNA molecule.
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A nonsense mutation introduces a premature stop codon in the upstream gene, causing early termination of translation.
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This premature termination can reduce ribosome density downstream, exposing the mRNA to Rho-dependent termination and degradation, thereby reducing transcription and translation of downstream genes.
Evaluating the Statements
Statement Explanation Correctness (A) The termination codon in the upstream ORF terminates translation, depleting ribosomes for downstream ORFs but does not affect transcription. Incorrect — transcription is also affected due to Rho-dependent termination. (B) Polar effects occur only when the nonsense mutation leads to formation of a stem-loop causing Rho-independent termination. Incorrect — polar effects often involve Rho-dependent termination, not only Rho-independent. (C) Termination codon depletes ribosomes downstream, allowing Rho factor to bind and cause premature transcription termination. Correct — this is a key mechanism of polarity. (D) Presence of suppressor tRNA reading the nonsense codon is essential for causing polar effect. Incorrect — suppressor tRNAs reduce polar effects by allowing readthrough. (E) Presence of suppressor tRNA diminishes the consequences of polar effects. Correct — suppressor tRNAs help alleviate polar effects by suppressing premature termination.
Correct Combination of Statements
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(C) and (E) are correct.
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(A) is incorrect because transcription is indeed affected.
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(B) is incorrect because polar effects are not limited to Rho-independent termination.
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(D) is incorrect because suppressor tRNAs reduce, not cause, polar effects.
Keywords for SEO Optimization
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Polar effect in operons
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Nonsense mutation polarity
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Rho-dependent transcription termination
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Suppressor tRNA and polar mutation
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Translation-transcription coupling in bacteria
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Premature translation termination effects
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Operon gene expression regulation
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Polycistronic mRNA translation
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Molecular mechanism of polarity
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Genetic mutations and operon expression
Conclusion
The polar effect of a nonsense mutation in an upstream gene of an operon involves premature translation termination that depletes ribosomes downstream, allowing Rho factor to induce premature transcription termination. Suppressor tRNAs reduce the polar effect by enabling readthrough of the nonsense codon. Therefore, the correct statements describing this phenomenon are:
(3) C and E
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1 Comment
Kajal
November 4, 2025Correct answer is C and E