A wild-type culture of haploid yeast is exposed to ethyl methanesulfonate (EMS). Yeast cells are plated on a complete medium, and 6 colonies (colonies numbered 1 to 6) are transferred to a new complete medium plate for further study. Four replica plates are made from the complete medium plate to plates containing minimal medium or minimal medium plus one amino acid (replica plates numbered 1 to 4) with the following results:

Identify any colonies that are his⁻, arg⁻, leu⁻.

Verified step by step guidance

1. Step 1: Understand the experimental setup. The yeast cells are haploid, meaning they have only one copy of each gene. Ethyl methanesulfonate (EMS) is a mutagen that induces random mutations in the DNA. The goal is to identify colonies with specific auxotrophic mutations (his⁻, arg⁻, leu⁻), which means they cannot synthesize histidine, arginine, or leucine, respectively, and require these amino acids to grow.
2. Step 2: Analyze the replica plating results. Replica plating involves transferring colonies from a master plate to plates with different growth conditions. In this case, there are four plates: (1) minimal medium (MM), (2) MM + histidine, (3) MM + arginine, and (4) MM + leucine. Colonies that fail to grow on MM but grow on MM supplemented with a specific amino acid indicate a mutation in the corresponding biosynthetic pathway.
3. Step 3: Compare the growth patterns of each colony across the replica plates. For each colony (1 to 6), check whether it grows on MM. If it does not grow on MM but grows on MM + histidine, it is his⁻. Similarly, if it grows only on MM + arginine, it is arg⁻, and if it grows only on MM + leucine, it is leu⁻.
4. Step 4: Record the results for each colony. Based on the growth patterns, assign the appropriate auxotrophic designation (his⁻, arg⁻, leu⁻) to each colony. Colonies that grow on MM are prototrophic (wild-type) and do not require supplementation.
5. Step 5: Summarize the findings. List the colonies that are his⁻, arg⁻, and leu⁻ based on their growth requirements. This information will help identify the specific mutations induced by EMS in the yeast cells.

Key Concepts

Haploid Organisms

Haploid organisms, like yeast, have a single set of chromosomes, which means they carry only one allele for each gene. This simplifies genetic analysis because any mutation will directly manifest in the phenotype. In the context of the question, identifying mutations in haploid yeast allows researchers to easily determine the effects of specific genetic changes, such as those induced by EMS.

Mutagenesis

Mutagenesis is the process by which the genetic information of an organism is changed, resulting in mutations. Ethyl methanesulfonate (EMS) is a chemical mutagen that can induce point mutations, leading to changes in amino acid sequences of proteins. Understanding mutagenesis is crucial for interpreting the results of the yeast colonies, as it helps explain the potential loss of function in genes related to histidine (his), arginine (arg), and leucine (leu) biosynthesis.

Amino Acid Auxotrophy

Amino acid auxotrophy occurs when an organism cannot synthesize a particular amino acid and thus requires it to be supplied in the growth medium. In the experiment, colonies that are his⁻, arg⁻, or leu⁻ cannot grow on minimal medium unless the corresponding amino acid is added. Identifying these auxotrophic mutants is essential for understanding the genetic basis of metabolic pathways and the effects of mutations induced by EMS.