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).
Main yeast lab page
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