Common baker's yeast (Saccharomyces cerevisiae) is normally grown at 37°C, but it will grow actively at temperatures down to approximately 25°C. A haploid culture of wild-type yeast is mutagenized with EMS. Cells from the mutagenized culture are spread on a complete-medium plate and grown at 25°C. Six colonies (1 to 6) are selected from the original complete-medium plate and transferred to two fresh complete-medium plates. The new complete plates (shown) are grown at 25°C and 37°C. Four replica plates are made onto minimal medium or minimal plus adenine from the 25°C complete-medium plate. The new plates are grown at either 25°C or 37°C and the growth results are shown. Classify the nature of the mutations in colonies 1, 2, and 5.

Key Concepts

Mutagenesis

Mutagenesis is the process by which the genetic information of an organism is changed, resulting in a mutation. In this context, the wild-type yeast is treated with EMS (ethyl methanesulfonate), a chemical mutagen that induces random mutations in the DNA. Understanding mutagenesis is crucial for analyzing the resulting phenotypic changes in the yeast colonies, as it helps to identify whether the mutations are beneficial, neutral, or harmful.

Haploid Organisms

Haploid organisms, like the yeast Saccharomyces cerevisiae, have only one set of chromosomes, making them particularly useful for genetic studies. In haploids, any mutation will be expressed directly in the phenotype, allowing for straightforward observation of the effects of mutations. This is important for classifying the nature of mutations in the selected colonies, as it simplifies the analysis of genetic changes.

Phenotypic Analysis

Phenotypic analysis involves examining the observable traits or characteristics of an organism, which can be influenced by genetic mutations. In this experiment, the growth of yeast colonies at different temperatures and on various media provides insights into the functional consequences of the mutations. By comparing the growth patterns of colonies 1, 2, and 5, one can classify the mutations based on their effects on the yeast's ability to thrive under specific conditions.