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.

Which colonies are prototrophic and which are auxotrophic? What growth information is used to make these determinations?

Verified step by step guidance

1. Understand the definitions: Prototrophic organisms can grow on minimal medium because they can synthesize all necessary compounds from basic nutrients. Auxotrophic organisms require additional nutrients (e.g., adenine) to grow because they have mutations that prevent them from synthesizing certain compounds.
2. Analyze the growth data: Examine the growth results of the colonies on the minimal medium plates at both 25°C and 37°C. Colonies that grow on minimal medium at both temperatures are prototrophic, as they do not require additional nutrients. Colonies that fail to grow on minimal medium but grow on minimal medium supplemented with adenine are auxotrophic for adenine.
3. Consider the temperature sensitivity: If a colony grows on complete medium at 25°C but not at 37°C, it may have a temperature-sensitive mutation. This information helps identify whether the growth defect is related to temperature or nutrient requirements.
4. Correlate the growth patterns: Use the growth results on minimal medium, minimal plus adenine, and complete medium at both temperatures to classify each colony. For example, if a colony grows on minimal plus adenine but not on minimal medium, it is auxotrophic for adenine. If it grows on minimal medium at both temperatures, it is prototrophic.
5. Summarize the findings: Based on the growth data, classify each colony (1 to 6) as either prototrophic or auxotrophic. Clearly state which colonies are auxotrophic for adenine and which are prototrophic, and explain how the growth data supports these conclusions.

Key Concepts

Prototrophy and Auxotrophy

Prototrophy refers to the ability of an organism to synthesize all essential compounds needed for growth from basic nutrients, while auxotrophy indicates a mutation that prevents the organism from synthesizing a specific compound, requiring it to be supplied in the growth medium. In yeast, prototrophic strains can grow on minimal media, whereas auxotrophic strains need additional nutrients, such as adenine, to thrive.

Mutagenesis

Mutagenesis is the process of inducing mutations in an organism's DNA, often using chemical agents like ethyl methanesulfonate (EMS). This technique is commonly used in genetics to create variations in traits, allowing researchers to study gene function and identify mutations that affect growth and metabolism, such as those leading to prototrophy or auxotrophy.

Temperature Effects on Yeast Growth

Temperature significantly influences yeast growth and metabolic activity. Saccharomyces cerevisiae typically grows well at 37°C but can also grow at lower temperatures, such as 25°C. Understanding how temperature affects growth is crucial for interpreting experimental results, as it can impact the expression of mutations and the overall viability of yeast colonies on different media.