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?

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 case of the second mutation in the question, if the change in the tRNA anticodon restores the ability to correctly pair with the original mRNA codon, it could be classified as a reversion, but this depends on whether it restores the original function.

tRNA and Codon-Anticodon Interaction

tRNA molecules play a crucial role in translation by carrying amino acids to the ribosome and matching their anticodons with the corresponding mRNA codons. The interaction between codons and anticodons is essential for accurate protein synthesis. In the question, the mutation in the tRNA's anticodon affects its ability to recognize specific mRNA codons, which can lead to changes in the protein produced, influencing whether the mutation is classified as forward or reversion.