Baker’s yeast has two almost identical mating types, MATa and MATɑ, which can sexually reproduce with each other and asexually reproduce themselves. If the environment they are in is nutrient poor, the yeast cells can exist in a haploid form of MATa or MATɑ. A colony of haploid cells can be maintained by asexual budding. If the environment they are in is nutrient-rich, the different mating types will become shmoos, a nodule of the original cell that the cells use to join together. Once they become an a/ɑ diploid, they can bud to asexually reproduce two yeast cells, the new cell being exactly identical to the first. If a diploid cell is starved of nitrogen and also on a carbon-poor source, it will sporulate to form four ascospores within an ascus. Those spores can be released from the ascus membrane and become four haploid yeast cells, two a and two ɑ cells.
Mutations come about by mutagenesis, which is a relatively rare event in nature. DNA replication is a highly regulated event that rarely lets imperfections slip by. Even when a mutation occurs in DNA, it does not always lead to a change in phenotype. Mutagens such as UV light, as used in this experiment, X-Rays, and chemicals are often used to increase the frequency of mutations for scientific study. In order to successfully study mutations, the cells must live and be able to reproduce through the mutagen exposure and contain a non-lethal mutation.
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