Dr Liu did a genetic screen for yeast cells that fail to syn

Dr. Liu did a genetic screen for yeast cells that fail to synthesize leucine (ie. leu^-). She first exposed the yeast cells to X-ray and then she plated the cells on to a plate containing complete medium. Then she replicated (using replicate plating technique) the colonies from the complete medium onto two new plates (plate A and B). Indicate medium type in A and B plate and how will she find the colony that is Leu- by comparing the colonies on A and B plates?

Solution

Replica plating technique identifies the nutritional auxotrophs by growing the microbes on complete and minimal media. The complete media contain all the nutrients required for growth and development of microbe uder study. The minimal media contain all but one nutrient for which the microbe is supposed to be auxotroph. In the given question, Dr grows the colonies on plate A (containing complete media) and plate B (containing media that lack leucine). The auxotrophs which fail to synthesize leucine can grow on the plate A having complete media only. The colonies of these auxotrophs are absent on the plate B with minimal media which does not supplement the requirement of leucine by the auxotrophs. This is how, she can find out the Leu- colonies on the plate A.