The yeast life cycle is usually dominated by haploid cells, each with a single
set of unpaired chromosomes. The cell propagates asexually, and the genetic
material is replicated through mitosis. Cell division occurs every 2鈥�4 hours,
leading to 60鈥�100 generations in a single day. Yeast also reproduce sexually,
particularly under adverse environmental conditions. When two haploid
cells鈥攚ith DNA containing complementary mating-type alleles鈥攃onjugate, a
diploid zygote results. The diploid zygote can then complete the sexual
segment of the life cycle through meiosis. After meiosis, four haploid spores
are produced, which can germinate.
Researchers can grow yeast easily on nutrient-containing plates. Because both
asexual and sexual reproduction is rapid, yeast has become an important
organism for the experimental investigation of mutagenesis and evolution among
eukaryotes. Environmental factors, such as chemicals or radiation, induce
mutations. High-energy UV-c radiation of less than 1 minute in duration will
result in many mutated yeast cells. UV-c can be used to mutate a strain of
yeast in which the synthesis of adenine is blocked. This mutation is
observable because the \(ade-2\) mutant has a red color when cultured on
nutrientcontaining plates. Exposure to uv-c also can result in additional
mutations. In particular, one mutant, \(ade-7\), changes the color of the ade-2
mutant to white.
A. You have a uv-c lamp, culture plates, and growth chambers at \(23^{\circ}
\mathrm{C}\) and \(37^{\circ} \mathrm{C}\). You also have available known haploid
strains that are \((ade-2,+,+)\), where + denotes the wild type. Design a plan
to determine the rate of uv-c-induced mutations in nutrient-containing plates
inoculated with yeast.
Earth's ozone layer removes high-energy ultraviolet radiation, uv-c, from the
solar radiation received at the surface. Lower-energy ultraviolet radiation,
uv-b, strikes Earth鈥檚 surface. Damage to DNA induced by ultraviolet radiation
occurs with the formation of bonds between an adjacent pair of pyrimidine
nucleotides, thymine and cytosine, on the same strand of DNA. A repair enzyme,
photolyase, which is activated by visible light, is present in plants and most
animals, but not in humans. In characterizing the relationship between
environmental mutagens and cell damage, a useful assumption is often made and
referred to as the linear hypothesis. This assumption states that the extent
of damage is proportional to the amount of radiation received.
Mutation rates for a strain (preac) that does not produce photolyase and a
wild-type (+) strain were studied. Cultures of the two strains of yeast were
diluted, and nutrient-containing plates were inoculated in triplicate at 23
掳C. The plates were exposed to bright sunlight for varying time intervals.
After exposure, the plates were incubated in the dark at 23 掳C. After
incubation between 1 and 8 hours, data shown in the table below were collected
by counting the density of living cells relative to the control, and averaging
these among replicates.
B. Using the data table below, graph the average survival fraction, relative
to the wild-type control. Predict the number of mutations in a sample of 1,000
cells of the preac type that are exposed to bright sunlight for 15 seconds.
C. Based on these data, evaluate the merits of the alternative theories of the
adaptive advantage provided by sexual reproduction.
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