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 structure resulting in Rho-independent 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 the ‘nonsense’ codon generated by the mutation diminishes the consequences of the polar effects.

Choose the option that represents all correct statements.

Correct option is C

Explanation-

Statement 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 - Incorrect
Claims transcription is unaffected, which is wrong — the polar effect often involves transcription termination due to Rho factor action when ribosomes fall off early.

Statement 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 structure resulting in Rho-independent termination -  Incorrect
This statement says the polar effect occurs only in operons involving Rho-independent termination and stem-loop structures, which is incorrect. In fact, Rho-dependent termination is the main mechanism for the polar effect in bacteria.

Statement 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  - Correct
It accurately describes that presence of a termination codon in the upstream ORF due to a nonsense mutation depletes ribosomes traveling downstream. This permits Rho factor to bind and causes premature transcription termination, which is the hallmark of a polar effect.

Statement D - Presence of the suppressor tRNA reading the ‘nonsense’ codon generated by the mutation, is essential for causing a polar effect of the mutation -  Incorrect
Says suppressor tRNA is essential for causing the polar effect, which is false. Suppressor tRNAs prevent or reduce the polar effect, not cause it.

Statement E - Presence of the suppressor tRNA reading the ‘nonsense’ codon generated by the mutation diminishes the consequences of the polar effects - Correct
It correctly states that the presence of suppressor tRNA reading the nonsense codon reduces the consequences of the polar effect — because translation can continue, downstream gene expression is preserved.

Since the correct statements are C and E so, the correct answer is option c - C and E