16. The effect of nonsense mutation could be nullified by reversion as well as suppression. Which of the following processes will help to distinguish between the two kinds of revertants?
(1) Complementation (2) Transgenesis
(3) Test for allelism (4) Recombination
Nonsense mutations introduce premature stop codons, halting protein synthesis, but their effects can be nullified by reversion or suppression, producing revertants that restore function. The correct process to distinguish these revertants is (3) Test for allelism, as it determines if the second mutation resides in the same gene (true reversion) or a different gene (suppressor mutation).
Understanding Revertants
Revertants restore the wild-type phenotype after a nonsense mutation. True reversion changes the original stop codon back to a sense codon within the same gene, directly repairing the mutation. Suppression, often by suppressor tRNAs, occurs via a second mutation in a different gene (e.g., tRNA genes), bypassing the stop codon without altering the original site. Distinguishing them requires testing if mutations are allelic (same gene) or non-allelic (different genes).
Option Analysis
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(1) Complementation: Crosses two mutants to check if wild-type function is restored; indicates mutations in different genes. It fails to distinguish revertants since both types already show wild-type phenotype and tests intergenic complementation, not intragenic changes.
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(2) Transgenesis: Introduces foreign DNA (e.g., wild-type gene via vectors). Irrelevant here, as it bypasses natural mutations rather than classifying revertant mechanisms.
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(3) Test for Allelism: Crosses revertant with original mutant or known allele; no wild-type progeny (non-complementation) confirms same-gene reversion, while complementation indicates suppression in another gene. This cis-trans test precisely differentiates.
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(4) Recombination: Exchanges DNA segments between homologs, potentially separating mutations if linked. It cannot reliably distinguish same-gene (no separation) from different-gene events without prior allelism knowledge.
CSIR NET Application
In exams like CSIR NET Life Sciences, test for allelism is key for inheritance biology questions on mutations. Revertants from suppression show complementation with original mutants (different loci), unlike true revertants. Practice crosses: for suppression, F1 shows wild-type; for reversion, mutant phenotype persists.
