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′

Assuming there are no other mutations in the genome, will this double-mutant yeast strain be able to grow on minimal medium? If growth will occur, characterize the nature of growth relative to wild type.

Verified step by step guidance

1. Identify the nature of the first mutation: The nonsense mutation in the ade-1 gene changes a tryptophan codon (UGG) to a stop codon (UGA), which would typically result in premature termination of protein synthesis, preventing adenine synthesis.
2. Examine the second mutation: The mutation in the tRNA gene changes the anticodon from 3′−ACC−5′ to 3′−ACU−5′. This new anticodon can pair with the UGA stop codon, potentially allowing the insertion of tryptophan at this position instead of terminating translation.
3. Consider the effect of the tRNA mutation: If the modified tRNA can successfully insert tryptophan at the UGA stop codon, it may suppress the nonsense mutation, allowing the full-length protein to be synthesized and adenine to be produced.
4. Evaluate the growth on minimal medium: If the suppression is effective, the yeast strain may grow on minimal medium as it can synthesize adenine, similar to the wild type. However, the efficiency of suppression and potential competition with release factors at the stop codon should be considered.
5. Characterize the nature of growth: If growth occurs, it may not be as robust as the wild type due to potential inefficiencies in suppression or competition at the stop codon, leading to variable levels of adenine synthesis.

Key Concepts

Nonsense Mutation

A nonsense mutation is a type of point mutation where a single nucleotide change results in the formation of a premature stop codon in the mRNA sequence. This leads to the truncation of the protein, often rendering it nonfunctional. In the context of the ade-1 gene mutation in yeast, the conversion of a tryptophan codon to a stop codon prevents the synthesis of adenine, which is essential for growth on minimal medium.

tRNA and Anticodon Function

Transfer RNA (tRNA) molecules are crucial for translating mRNA into proteins, as they carry specific amino acids to the ribosome during protein synthesis. Each tRNA has an anticodon that pairs with the corresponding codon on the mRNA. The mutation affecting the tRNA's anticodon sequence can alter its ability to recognize and bind to specific codons, potentially impacting the translation of proteins and the overall growth of the yeast strain.

Minimal Medium Growth

Minimal medium is a growth medium that contains only the essential nutrients required for the growth of microorganisms, such as carbon, nitrogen, and salts, but lacks additional growth factors. For the yeast strain in question, the ability to grow on minimal medium depends on the presence of adenine, which is synthesized by the wild-type ade-1 gene. The double mutation's impact on adenine synthesis and tRNA function will determine whether the yeast can grow and how its growth compares to the wild type.