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 acid4 (replica plates numbered 1 to 4) with the following results:

Identify the colonies that are prototrophic (wild type). What growth information leads to your answer? 

Key Concepts

Prototrophy vs. Auxotrophy

Prototrophic organisms can synthesize all essential compounds from basic nutrients, while auxotrophic organisms lack the ability to produce certain compounds and require them to be supplied in the medium. In the context of yeast, identifying whether a colony is prototrophic or auxotrophic is crucial for understanding its growth on minimal media versus complete media.

Ethyl Methanesulfonate (EMS) Mutagenesis

EMS is a chemical mutagen that induces mutations in DNA, often leading to changes in gene function. In yeast, exposure to EMS can result in a variety of mutations, some of which may affect the ability to synthesize essential amino acids, thus impacting the growth of the yeast on different media types.

Replica Plating Technique

Replica plating is a method used to transfer colonies from one agar plate to another, allowing for the assessment of growth under different conditions. This technique helps in identifying prototrophic colonies by comparing their growth on complete medium versus minimal medium, revealing which colonies can survive without additional nutrients.