Yeast Genetics

We used the labs as outlined in Tom Mauney's A Classroom Guide to Yeast Genetics, which can be purchased from Carolina Biological Company. We conducted the lab A Simple Cross as a class.

 

A Simple Cross

Objective: To observe the life cycle of bread yeast, Saccharomyces cerevisiae and to cross two strains of yeast; HA2 (a mutant strain that is red in color) with HBT ( a cream-colored strain). To follow the life cycle of the resultant cross.

 

Materials: Yeast strains: HA2 and HBT, petri plates with:YED (yeast dextrose agar), MV agar, YEKAC agar, incubator, alcohol for sterilizing, sterile toothpicks, fine-point marking pens, microscopes, slides, cover-slips. (Yeast strains, agar, petri plates, sterile toothpicks were purchased from Carolina Biological)

We also found it very helpful to watch the video tapes, that Carolina Biological has made which outline all the procedures and techniques for these labs.

Sterilizing Media

Procedure: (This is a rough procedure, you will need to purchase the kit for exact detailed directions)

1) On a YED agar plate make a streak of HA2 and HBT strains. Then incubate plates overnight.

2) Cross the two strains in the middle of the plate.Then incubate overnight.

(color of image not accurate,HA2 is red)

3) Make a replica of this plate on the MV agar plate, incubate overnight! Only the mating mixture will grow on this media because of a lack of enzymes in each of the parent strains.

4) To force the mating mixture to sporulate, inoculate a YEKAC plate with the mating mixture.

5) To separate the colonies to get back to the original haploid colonies make several streaks on a new YEKAC plate. Then observe under the resulting colonies under the microscope.

Results

Students should keep a record of their results in their lab notebook. They should make drawings of the petri plates each day and drawings of their microscopic observations as they document the lifecycle of the bread yeast.

 

Discussion/Conclusion

Students will be able to observe the complete life cycle of the bread yeast. They first observe two different haploid strains (HA2 and HBT). They will observe the shmoos as the two strains are mated. They will be able to see the diploid on the YEKAC agar. They will be able to observe the ascus with spores as they force the diploid to sporulate. The life cycle is then completed as the students separate the colonies and they see the reappearance of the two haploid colonies, the red HA2 and the cream-colord HBT.

The entire lab takes 10 days!