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 second mutation to be a forward mutation or a reversion? Why?

Verified step by step guidance

1. Understand the concept of forward mutation and reversion: A forward mutation is a genetic change that alters the wild-type phenotype, while a reversion restores the wild-type phenotype or function.
2. Analyze the first mutation: The nonsense mutation in the ade-1 gene changes a tryptophan codon (UGG) to a stop codon (UGA), preventing the synthesis of adenine. This is a forward mutation because it disrupts the normal function of the gene.
3. Examine the second mutation: The second mutation alters the anticodon sequence of a tRNAᵀʳᵖ from 3′−ACC−5′ to 3′−ACU−5′. This change allows the tRNA to recognize the stop codon (UGA) and insert tryptophan during translation, potentially restoring the synthesis of adenine.
4. Determine whether the second mutation is a reversion: Since the second mutation compensates for the effect of the first mutation by restoring the ability to synthesize adenine, it is considered a reversion. However, it is not a true reversion of the original mutation in the ade-1 gene but rather a suppressor mutation that restores function indirectly.
5. Conclude: The second mutation is classified as a reversion because it restores the wild-type phenotype (adenine synthesis), even though it does so through a compensatory mechanism rather than directly reversing the original 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 or a new function. In the context of the question, the second mutation changes the anticodon of a tRNA, which can affect its ability to pair with the corresponding mRNA codon, potentially leading to altered protein synthesis. This type of mutation is generally considered to be a forward mutation if it results in a new or different phenotype.

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 a second mutation that counteracts the effects of the first. In the context of the question, if the second mutation in the tRNA gene restores the original anticodon sequence, it would be classified as a reversion mutation, as it would revert the tRNA's function back to its original state.

tRNA and Anticodon Function

Transfer RNA (tRNA) molecules play a crucial role in translating mRNA sequences into proteins by carrying specific amino acids to the ribosome. Each tRNA has an anticodon that pairs with a corresponding codon on the mRNA. The mutation described in the question alters the anticodon of a tRNA, which can affect its ability to recognize and bind to the correct mRNA codon, thereby influencing the overall protein synthesis process and potentially leading to phenotypic changes.