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Yeast Life Cycle and Basic Genetics

Radiation Effects on Yeast

Red and White Yeast: An Introduction to Science as a Process

Transformation of Yeast

Yeast Life Cycle and Basic Genetics

A simple experiment to explore the yeast life cycle makes use of colony color and nutritional mutations to distinguish haploids and diploids. The experiment can be performed in real time in about a week but can also be compressed to about an hour by using plates previously prepared with the appropriate yeast strains. Morphological changes involved in mating and sporulation can be detected by light microscopic examination. Complete genetic analysis is possible if a micromanipulator for ascus dissection is available.

Observation of the Yeast Life Cycle

Day One: Provide students with YED plates with small patches of two haploid strains of yeast, HA2 (red colony color) and HBT (white colony color). These strains are a and mating types, respectively. Mix a portion of each strain with the other in a separate location on the plate. At intervals (2-4 hours) after mixing the strains, make wet mount slides using a small amount of the mating mixture and examine under high power (400X) on a light microscope. The vegetatively growing haploids will appear oval-shaped, but in addition, at early times there should be "shmoos", the pear-shaped mating forms of yeast, and at later times, zygotes (dumbell-shaped or after the first bud has appeared, trefoil-shaped) can be observed. To select the diploid product of mating, streak from the mating mixture after a minimum of four hours of mating to a plate containing minimal medium (MV) on which neither haploid can grow. For a direct demonstration that the diploids can grow and the haploids cannot, replica plate from the mating plate using sterile velveteen or cheesecloth to a minimal medium plate. Incubate plates at 30 C. or room temperature.

Day Three: On minimal medium, white colonies from a streak or a white patch from the mating mixture should appear. These are diploid cells. The fact that the diploid is white shows that the red color is recessive. Replica plate the diploid to sporulation medium (YEKAc), which causes it to initiate meiosis and sporulation. Incubate sporulation plates.

Day Five: Make a wet mount slide from the sporulation plate and observe under the light microscope. Most of the diploids should have formed asci containing two, three, or four spores. The four-spored asci have a tetrahedral shape that gives them either a diamond or triangle appearance, depending upon the plane at which they are viewed. Streak for single colony isolation from the sporulated diploid onto rich medium (a YED plate) and incubate.

Day Seven: The streak plate should have white as well as red colonies. The reappearance of red colonies indicates a return to the haploid state.

Examples of possible student research projects

  • isolation and genetic analysis of mutants
  • use of red colony color to examine environmental effects on its production
  • genetic exploration of biochemical pathways (isolation of nutritional mutants)
  • analysis of the cell cycle using temperature-sensitive mutants and/or time-lapse photomicroscopy
  • analysis of mating by isolation of mutants or manipulation of physical conditions
  • analysis of sporulation (meiosis) by isolation of mutants or manipulation of physical conditions
  • genetic transformation by exogenous DNA (see transformation lab)

For additional information, see the GENE background pages Baker's yeast and its life cycle, Genetics of baker's yeast, and the experiment A simple cross and other genetics experiments (list of genetics experiments).

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March 28, 2001