Passage II

S. cerevisiae (baker's yeast) cells can form colonies (patches of genetically identical cells) on nutrient agar plates (NAPs), each of which contains the same 20 amino acids. The pattern of colonies on 1 NAP can be reproduced on another NAP using the technique of replica plating. A velvet cloth is pressed onto the first NAP so that the cloth collects cells from each colony. The cloth is then pressed onto a new NAP to transfer the cells, which, when incu­bated, grow to form colonies (see Figure 1).

Experiment 1

Students determined if 10 strains (Strains 1-10) of S. cerevisiae could grow in the absence of certain amino acids. Cells from Strain 1 were spread on an NAP, which was then incubated at 30°C for 72 hr to allow colonies to form. Cells from each colony were replica plated to each of 4 experimental plates (EPs). The EPs were identical to the NAPs except that each EP lacked 1 amino acid—either arginine (Arg), histidine (His), lysine (Lys), or leucine (Leu)—while still containing the remaining 19 amino acids. This process was also followed with Strains 2-10. All the EPs were incubated at 30°C for 72 hr and then examined for colonies (see Table 1).

Experiment 2

The students tested 3 mixed cultures (Mixed Cul­tures E, F, and G), each containing cells from 2 of the strains tested in Experiment 1. Each mixed culture was incubated on a separate NAP until colonies formed. The colonies were then replica plated and incubated as in Experiment 1 (see Figure 2).