Q69. A mutant phenotype due to a nonsense mutation can be rescued by a mutation in tRNA gene. This rescue is an example of
- Induced mutation
- Suppressor mutation
- Spontaneous mutation
- Deletion mutation
Suppressor mutations rescue mutant phenotypes caused by nonsense mutations through tRNA alterations, allowing translation to bypass premature stop codons.
Correct Answer
Suppressor mutation
A nonsense mutation introduces a premature stop codon (UAG, UAA, or UGA), truncating the protein and causing a loss-of-function phenotype. A second mutation in a tRNA gene creates a suppressor tRNA with an altered anticodon that recognizes the stop codon, inserts an amino acid, and allows full-length protein production, restoring function. This is classic nonsense suppression, often called translational or informational suppression.
Option Explanations
Induced Mutation
These are deliberate DNA changes caused by external mutagens like UV radiation, gamma rays, or chemicals such as base analogs (e.g., bromouracil). They differ from natural errors and are used in labs for genetic studies or crop breeding, but they do not specifically rescue existing phenotypes via tRNA.
Suppressor Mutation
Correct choice. This second-site mutation counteracts the original mutation’s effect without reverting the original site. In tRNA suppressor cases, it enables readthrough of nonsense codons, as seen in bacterial and eukaryotic systems for studying gene function.
Spontaneous Mutation
These occur naturally during DNA replication or repair without external agents, due to errors like tautomerization or deamination. Rates are low (e.g., 10<sup>-9</sup> per base pair per generation), and they cause the initial phenotype but do not rescue it.
Deletion Mutation
This removes DNA segments, often shifting the reading frame or eliminating genes, leading to nonfunctional proteins. Unlike suppressor mutations, deletions disrupt rather than restore translation and cannot mimic tRNA-based rescue.
