Correct option is A
Correct Answer: (1) Polar effect of the mutation
Explanation:
The polar effect refers to the phenomenon where a mutation in one gene of an operon, such as the nonsense mutation in the gene encoding beta-galactosidase, can prevent the expression of downstream genes in the operon, such as those encoding permease and transacetylase.
This occurs because the nonsense mutation causes premature termination of translation, which leads to ribosome stalling on the incomplete peptide. This stalling interferes with the translation of the downstream genes in the operon.
The polar effect results from the fact that the operon genes are co-transcribed into a single mRNA, and a mutation affecting one gene can impact the translation of subsequent genes due to the linked translation mechanism.
Information Booster:
Nonsense mutations introduce a premature stop codon, causing truncation of the protein.
The ribosome stalls at the premature stop codon, preventing further translation along the mRNA.
Polar effect is particularly seen in operons where mutations in the upstream genes affect the downstream genes in the same mRNA transcript.
The lac operon is a classic example of an operon system in which multiple genes are transcribed together as a single mRNA molecule.
The expression of downstream genes in an operon can be disrupted if translation of the upstream gene is compromised, as seen in the case of a nonsense mutation.
The lac operon genes include beta-galactosidase, permease, and transacetylase, all of which are required for lactose metabolism.
Additional Information:
Option 2: Trans-effect of the mutation refers to regulatory effects that occur indirectly through diffusible molecules (such as transcription factors). This is not applicable in this case, as the mutation affects the translation process directly.
Option 3: Binding of the release factor to the nonsense codon prevents translation of the downstream cistrons is inaccurate because the release factor does not directly affect the translation of downstream genes. The release factor is involved in terminating translation at the nonsense codon, but the downstream genes are affected due to the polar effect.
Option 4: Formation of a stem-loop structure in the upstream cistron prevents translation of downstream cistrons is incorrect because stem-loop structures are more relevant in transcriptional regulation and are not directly associated with the impact of a nonsense mutation on translation in this context.
