The mighty yeast, Saccharomyces cerevisiae, or baker’s yeast, is a single-celled member of the kingdom Fungi. For 10,000 years or more, humans have been working with and benefiting from this simple, yet amazing organism.
Yeast are capable of growth using aerobic respiration or anaerobic fermentation, resulting in production of CO2 gas and ethanol. These fermentation byproducts have been contributing to leavened bread and the production of beer and wine for thousands of years, but baker’s yeast has a more modern contribution to human technology as well. Some forms of ethanol biofuel are produced using these yeast, and S. cerevisiae has been a star model eukaryote to study central processes in cells. When it comes to basic cell biology, researchers have learned much about human diseases by studying these simple eukaryotes.
Yeast have a somewhat unusual ability to reproduce either sexually using meiosis, or asexually using mitosis, depending on the environment in which they are living.
Preparation Task 1:
Describe the mechanisms by which meiosis would introduce genetic variability in a population. If one diploid yeast cell was heterozygous for an allele with a Dominant mutation on it, what proportion of its offspring could be expected to carry that dominant mutation after mating? (Assume it mates with a cell that is homozygous for the normal, recessive allele.)
Interestingly, yeast switch to sexual reproduction when they are under conditions of stress.
For the purposes of this task, assume yeast populations follow this simplified rule: When yeast are reproducing sexually, they will be found as diploid cells that can go through meiosis (or mitosis), while populations that are producing asexually will include only haploid cells undergoing mitosis. Remember that chromosomes can be counted using karyotypes.
Preparation Task 2:
Explain why sexual reproduction could be advantageous to a population under stressful conditions. Relate this to the process of natural selection.
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