Two different mutations are identified in a haploid strain of yeast. The first prevents the synthesis of adenine by a nonsense mutation of the ade-1 gene. In this mutation, a base-pair substitution changes a tryptophan codon (UGG) to a stop codon (UGA). The second affects one of several duplicate tRNA genes. This base-pair substitution mutation changes the anticodon sequence of a tRNAᵀʳᵖ from


   3′−ACC−5′ to 3′−ACU−5′


Do you consider the first mutation to be a forward mutation or a reversion? Why?

Verified step by step guidance

1. Understand the definitions: A forward mutation is a change in the DNA sequence that creates a new mutant allele, while a reversion (or reverse mutation) restores the original function of a gene, either by correcting the original mutation or by compensating for its effects.
2. Analyze the first mutation: The nonsense mutation in the ade-1 gene changes a tryptophan codon (UGG) to a stop codon (UGA). This prevents the synthesis of adenine by prematurely terminating translation, which is a forward mutation because it introduces a new mutant allele.
3. Consider whether the mutation restores function: Since the nonsense mutation disrupts the function of the ade-1 gene and does not restore the original function, it cannot be classified as a reversion.
4. Evaluate the context of the question: The first mutation is a base-pair substitution that introduces a stop codon, which is consistent with the definition of a forward mutation. It does not reverse or compensate for any prior mutation.
5. Conclude: The first mutation is a forward mutation because it introduces a new change that disrupts the normal function of the ade-1 gene, rather than restoring its original function or compensating for a previous mutation.

Key Concepts

Forward Mutation

A forward mutation refers to a change in the DNA sequence that alters a gene's function, typically resulting in a loss of function. In the context of the ade-1 gene mutation, the nonsense mutation introduces a premature stop codon, leading to the inability to synthesize adenine. This type of mutation is considered forward because it creates a new phenotype that differs from the wild type.

Reversion Mutation

A reversion mutation, or back mutation, occurs when a mutated gene returns to its original sequence, restoring the original function. This can happen through various mechanisms, such as a second mutation that corrects the first. In the case of the ade-1 gene, if a subsequent mutation restored the ability to synthesize adenine, it would be classified as a reversion.

Nonsense Mutation

A nonsense mutation is a specific type of point mutation where a single nucleotide change results in a premature stop codon in the mRNA sequence. This leads to the production of a truncated protein that is often nonfunctional. In the example provided, the change from a tryptophan codon to a stop codon in the ade-1 gene exemplifies a nonsense mutation, which significantly impacts the gene's function.